Phosphorus from Farmland to Water
- Status, Flows and Preventive Measures in a Nordic Perspective
Report Food 21 no. 4/2007 Authors: Lars Bergström, Faruk Djodjic, Holger Kirchmann, Ingvar Nilsson, Barbro Ulén
Phosphorus from Farmland to Water Status, Flows and Preventive Measures in a Nordic Perspective
Lars Bergströma, Faruk Djodjicb, Holger Kirchmanna, Ingvar Nilssona, Barbro Uléna a
Dept of Soil Sciences, Swedish University of Agricultural Sciences, Box 7014, SE-750 07 Uppsala, Sweden. b Dept of Environmental Assessment, Swedish University of Agricultural Sciences, Box 7050, SE-750 07 Uppsala, Sweden.
Foreword The Baltic Sea is suffering from an unacceptably high nutrient load, leading to severe outbreaks of algal bloom. There is some disagreement among marine scientists as to whether nitrogen or phosphorus is causing this eutrophication. During the spring of 2006, a small assessment group comprised of international experts reported their analysis of the issue in Eutrophication of Swedish Seas (Swedish EPA Report 5509, 2006). There was general agreement that in addition to current preventive measures to reduce nitrogen inputs, phosphorus inputs must also decrease if algal bloom and, in particular, outbreaks of cyanobacter are to be controlled. A large proportion of the phosphorus released into the Baltic Sea has its origins in inadequately treated wastewater from households, industries and other point sources. In addition to these sources, leakage from stored manure and diffuse leaching from agricultural land within the Baltic catchment area are estimated to contribute as much phosphorus as all the other sources combined. For a long time, research devoted to nitrogen leaching has been considerably more comprehensive than that into phosphorus turnover, losses and management in agricultural land. The fact is that in the case of phosphorus there is a lack of knowledge not only about the efficiency of preventive measures, but also about the reasons that lie behind phosphorus losses. Against this background, the need for a collective research effort has been raised in various contexts, with the aim of further reducing the contribution of agriculture to the eutrophication of both inland waters and the Baltic Sea. This report reviews the current status of knowledge based on research and to some extent also on practical experiences in the Nordic countries, with the focus on comparable conditions, mainly in North America. The report was prepared by Professor Lars Bergström in collaboration with Dr. Barbro Ulén, Professor Holger Kirchmann and Professor Ingvar Nilsson at the Department of Soil Science, and Dr. Faruk Djodjic at the Department of Environmental Assessment. Both these Departments form part of SLU in Uppsala. This study was commissioned and funded by the Swedish Environmental Protection Agency through the initiative of Dr. Ingrid Rydberg, Principal administrative officer at the Agency’s Natural Resources Department. Rune Andersson, Programme Leader for the synthesis platform MAT 21 at SLU in Uppsala, was the administrator and coordinator at SLU.
Uppsala 15 December 2006
Rune Andersson Programme Leader MAT 21
Björn Risinger Director of Natural Resources Department, NV
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Contents Foreword
3
Summary
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Introduction Phosphorus losses to watercourses Soil phosphorus loads and phosphorus status Phosphorus losses and phosphorus forms in water from agricultural land Management of the phosphorus problem in Sweden and abroad Trends in agricultural rivers 1993-2004 Trends in agricultural rivers 1975-2004 The National Environmental Objective
8 9 9
Agronomic aspects of phosphorus Forms and quantities of phosphorus in agricultural soil Phosphorus in agricultural crops Amounts and concentrations in wheat, ley crops, potatoes and oilseed crops Concentrations in harvest residues Phosphorus fertilisation in agriculture Phosphorus in artificial fertiliser Phosphorus in manure Reactions of phosphorus from artificial fertiliser and manure in soil Phosphorus flows in the soil/plant system Deposition Leaching Surface runoff Phosphorus analyses Release of phosphorus in soil Inorganic phosphorus in the soil solution Dissolved organic phosphorus in the soil solution Turnover and leaching of organic forms of phosphorus in agricultural soil Organic and inorganic phosphorus Organic phosphorus compounds found in the soil and soil solution Leaching of organic phosphorus Factors regulating the occurrence of dissolved organic phosphorus (DOP) Mechanisms in leaching of organic phosphorus The carrier model Effect of properties of the constituent phosphorus compounds on phosphorus leaching Inositol phosphate fraction in leached organic phosphorus and importance of particulate transport of organic phosphorus Preventive measures to minimise phosphorus losses from agriculture Limiting the release of phosphorus Fertiliser placement
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11 12 14 15 15 17 17 17 17 18 19 19 19 19 20 20 21 21 22 22 22 22 23 23 24 24 25 25 25 26 27 27 28 29
Manure spreading Manure additives Impact on phosphorus transport Surface runoff Flow through large soil pores Flow through the soil matrix Measures on the farm Feed additives Calculation of phosphorus balances Manure storage Models and other tools Phosphorus models and tools used in Sweden Lag between model development and existing knowledge and research results Adjusting descriptions of phosphorus forms in the soil to analytical methods used in Sweden Phosphorus binding capacity and release Physical description of surface runoff/erosion Model development hampered by inadequate and unsuitable indata Soil texture and soil classification Drainage Phosphorus status in the soil Sorption parameters Model development hampered by inadequate/unsuitable calibration and validation data
30 31 31 31 35 36 38 38 39 40 41 41 44 44 45 45 45 45 46 46 46 46 47 47
Future research needs Availability and solubility of phosphorus in the soil Studies on availability of organic phosphorus compounds for transport and breakdown Laboratory studies of physical and chemical processes Phosphorus fertilising using sludge Long- and short-term changes in bioavailability and solubility of soil phosphorus Precision cropping Crop impact as a phosphorus filter Environmental monitoring of agricultural land Transport of phosphorus from arable land to watercourses Water flows in soil and in the landscape Effects of climate Private wastewater systems and source apportionment Field drains Phosphorus traps Lime filters
48 48 49 49 49 49 50 50 50 51 52
Concluding comments References
52 53
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47 48 48
Summary Phosphorus is lost from agricultural land through being mobilised and transported away with running water. There are great variations in the amounts of phosphorus lost from agricultural land in different European countries as a result of differences in soils, soil hydrology and agricultural production. There are also large regional differences in the form in which the losses occur. Dissolved phosphorus can constitute 9-90% of total phosphorus in water. Drainage losses can make up between 12-60% or more of phosphorus transport, while erosion can make up 40-90%. Long-term transport of total phosphorus in small agricultural streams in Scandinavia usually varies between 0.1-4 kg ha-1 yr-1, with the highest losses in Norway. All countries are focusing on phosphorus balances and control of phosphorus applications to the soil from slurry and artificial fertiliser. In the Scandinavian countries, control at source is combined with control of phosphorus transport from agricultural land. In Norway the focus is on reducing erosion. In southern Sweden, phosphorus concentrations have been declining in recent years at a rate of around 2% per year (1993-2004). In Norway, a decreasing trend has also been observed in agricultural streams. In Ireland, the length of waterways classified as non-polluted increased by a total of 3% between 1995-97 and 1998-2000 and has remained constant since then. There can be a number of factors and combinations of factors involved in these improvements. A characteristic of phosphorus losses from catchment areas is that 90% of the losses can occur from 10% of the area and during 1% of the time, which will have a strong impact on any prevention strategies introduced. It means that measures to reduce losses should be site-specific and operational during the times of the year when phosphorus flows are higher. Phosphorus can be transported off in different forms, from large aggregates and organic compounds to fine clay particles and colloids or in completely dissolved form as orthophosphates. For more information on these aspects and on the bioavailability of the different fractions, see Swedish EPA Report 5507. This report describes the factors controlling losses of phosphorus, including both inorganic and organic phosphorus, to sur face waters and groundwater. Agronomic aspects affecting the behaviour of phosphorus in the soil/plant system are also dealt with. The processes that control phosphorus losses are controlled by soil physical, soil chemical and soil hydrological conditions, but the losses can be decreased through cultivation practices. This means that a broad interdisciplinary approach must be adopted when dealing with this issue. We present a range of cultivation measures for decreasing phosphorus losses, including ways to apply and bind the phosphorus in the soil so that it is utilised by the crop instead of being mobilised and ways to allow water transport in the soil without channel flow. We also present tools and models for calculating phosphorus losses on different scales and discuss how the accuracy of these can be improved. This study is limited to diffuse phosphorus losses from agricultural land before they reach recipient waters. It is our opinion that it is easiest to introduce measures that produce good results on agricultural land at as early a stage as possible, i.e. to prevent phosphorus mobilisation from the soil to the greatest extent possible.
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Introduction In all the countries of Northern Europe, agriculture is estimated to be responsible for the greatest contribution of phosphorus to inland waters and coastal waters. In Norway, despite the fact that only 3% of the total land area is farmed (Table 1), most of the phosphorus here also comes from agriculture (Borgvang et al., 2002; Kollerud, 2005). The gross load of phosphorus from agricultural land is estimated to contribute 40% of the total load to the Baltic Sea (Brandt & Ejhed, 2003), a contribution that has an exceedingly high negative impact on this brackish sea (Boesch et al., 2005). In the United Kingdom, around 50% of the flows of phosphorus loads are estimated to derive from agriculture (DEFRA, 2004). Phosphorus flows from the soil are complex and difficult to predict. The relative importance of the mechanisms involved must be known or at least conceptually recognised before appropriate preventive measures can be selected. To focus prevention efforts, a so-called risk index for phosphorus has been drawn up in which each individual field is assessed (Djodjic & Bergström, 2005). The strategy can be to reduce the problem by either controlling the sources of the losses or the actual transport. These two strategies can also be combined. For example, frequent attempts have been made to adjust fertilisation so that the phosphorus supplied corresponds to that removed with the harvested crop (control of sources), while other attempts have focused on reducing the cause of erosion or on establishing buffer zones along waterways (primarily control of transport). In areas of the USA, there are examples of strategies where the emphasis has been on one or the other of these strategies (Baker & Richards, 2002). However, the concept of a risk index for phosphorus losses represents a combination of both strategies (Bechmann, 2005; Djodjic & Bergström, 2005). In Europe, many countries have focused on controlling the source of the phosphorus losses but also attempt to reduce the actual transport to some extent. In addition to having good general knowledge of phosphorus mobilisation and transport, it is important to know the potential time taken to obtain a response to any agricultural and environmental policies introduced, through monitoring conditions in watercourses. Information on trends in watercourses, together with changes in agriculture, make it possible for authorities and politicians to set realistic targets. It is therefore important to monitor preventive measures through analysing water quality in both small and large watercourses and not just settling for an endpoint in a catchment area. Environmental monitoring of individual fields, sub-catchment areas, agricultural streams and agricultural rivers are all important and should be evaluated statistically using models and trend determinations. Such calculations must take the phosphorus contribution from private wastewater units into account. Scattered households are a feature of the agricultural landscape and emissions from such individual households still affect the phosphorus concentrations in watercourses to an exceedingly high degree, at least during the summer.
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Phosphorus losses to watercourses Soil phosphorus load and phosphorus status From the beginning of the 1950s and up until the middle of the 1970s, large amounts of artificial fertiliser were applied to farmland to increase yields. In addition, the soil received farmyard manure relatively often without any consideration being given to its value as a phosphorus fertiliser. After 1975, the amount of artificial fertiliser used in Sweden decreased rapidly. In recent years the use of manure has also decreased with the decline in livestock farming. Total phosphorus fertilisation is now down at the same level as it was a hundred years ago. In nearly all countries, there are production areas with intensive livestock farming. In Sweden, these are found e.g. in south-west Blekinge, south-west Halland and in certain parts of Småland. Current phosphorus fertilisation in Sweden is generating a surplus of on average 2 kg P ha-1. The value is highest in livestock-intensive areas, whereas in cereal growing areas without livestock there is often a deficit. However, net accumulation is small in relation to other European countries (Table 1), where accumulation has admittedly been reduced but is still increasing soil phosphorus reserves by amounts in the region of 4-8 kg P ha-1 yr-1.
Rönneå in Skåne (photo: Barbro Ulén)
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Table 1. Proportion of agricultural land in relation to total area of the country (%), amount of phosphorus supplied (mean of phosphorus (P) to agricultural land in the form of manure, artificial fertiliser, sewage sludge and atmospheric deposition), mean soil balance for all agricultural land and for areas with intensive livestock farming (IA areas) in recent years, phosphorus losses, annual climate, number of days with snow cover and typical runoff in Sweden and in four other countries Country
Sweden Norway
Agricultural area Total agricultural area (%) Ploughed agricultural area (%) Soil balances and concentrations Farmyard manure (kg P ha-1 yr-1) Artificial fertiliser (kg P ha-1 yr-1) Sewage sludge (kg P ha-1 yr-1) Atmospheric deposition (kg P ha-1 yr-1)
7 5 0.2 0.3
Total supply (kg P ha-1 yr-1)
13
Balance agricultural land (kg P ha-1 yr-1) Balance agricultural land in IA areas (kg P ha-1 yr-1) Conc. in topsoil (xmg P kg-1 or +mg P L-1) Phosphorus losses Total losses to water from agricultural soil or the agricultural landscape (kg P ha-1 yr-1) Proportion of dissolved phosphorusd in water (%) Climate/region ‘Hardiness zone’ (scale 1-11) No. of days with snow cover (%) Runoff from agricultural land (mm yr-1)
8 6
3 1
United Republic Germany Kingdom of Ireland 77 19
59 6
70 -
12 13 1.2b 0.3
9 17 0.4 0.3
20a 10 0.4 0.5
13 9 1.7 >> DOP > DOC, the difference between PO4 and DOP as regards adsorption capacity being considerable (Lilienfein et al., 2004). In this case, the composition of DOP was unknown. However, it was not of such a character that adsorption of organic phosphorus exceeded adsorption of orthophosphate. Similar observations have been made in other studies. It is difficult to draw any full general conclusions regarding the transport and immobilisation of dissolved organic phosphorus. One of the problems is that the phosphorus content in dissolved organic material is extremely variable, as illustrated
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by an often very variable DOC/DOP ratio. At the same time, there is often a lack of knowledge about the constituent components (Qualls & Haines, 1991). Inositol phosphate fraction of leached organic phosphorus and importance of particulate transport of organic phosphorus As mentioned above, inositol phosphate is the quantitatively dominant component of the soil organic phosphorus reserves. It is therefore important to clarify the role of inositol phosphate both in adsorption and in leaching of waterborne organic phosphorus. The question is whether leaching of inositol phosphate occurs in dissolved or particulate form. Due to its strong adsorption to oxide surfaces, there is reason to assume that the release and leaching occur in particulate form (Turner et al., 2002b). The laboratory experiments with enzyme tests cited previously (Turner et al., 2002 a) also indicate this. This would mean that the carrier model provides a rather incomplete picture of the total leaching of organic phosphorus. Inositol phosphate in particulate form would not be captured by vacuum lysimeters due to the fact that the pore size of the lysimeter materials would not permit this. The conclusion is that there is a risk of obtaining a skewed picture of the relative difference between total leaching of inorganic and organic phosphorus unless the difference in the amount and composition of both dissolved and particulate phosphorus is taken into account. Particulate phosphorus in water is often analysed in routine testing. However, dissolved and particulate phosphorus are very seldom divided up into an inorganic and an organic fraction, which is essential for creating a better understanding of e.g. the importance of phosphorus in a eutrophication context. This is underlined by the results of an investigation on watercourses emptying into the Baltic Sea carried out by Stepanauskas et al. (2002), which found that dissolved phosphate, DOP and particulate phosphorus comprised on average 46, 18 and 36% of the total phosphorus content. All particulate phosphorus was judged to be organic and thus almost 70% of all waterborne organic phosphorus occurred in particulate form.
Preventive measures to minimise phosphorus losses from agriculture A precondition in reducing phosphorus losses from agricultural land by appropriate preventive measures is that people at all levels (farmers, officials, politicians, etc) are aware of the problem and accept that poor utilisation of phosphorus within agriculture is a strong contributing factor in the eutrophication of surface waters. Laws, regulations and economic instruments are used to reduce losses, together with information campaigns and individual advice. To date, most European countries (incl. Sweden) have concentrated on preventing nitrogen leaching. Only Norway and Finland have concentrated more on the phosphorus problem, and Denmark has begun to do so recently. In additional to national regulations and international conventions (e.g. the EU Framework Directive for Water), there are a number of measures that can be adopted at farm and field level to decrease losses of phosphorus. However, to achieve good results, certain factors must be known. It must be possible to identify areas within a field or within a catchment area where the risk of losses is great. Furthermore, the mechanisms by which the phosphorus is transported must be known, e.g. whether this occurs by surface runoff or mainly through the soil to the drainage system. Such
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knowledge is completely decisive in determining the preventive measures that can be introduced. Cultivation practices to optimise phosphorus utilisation in agriculture (Best Management Practices, BMPs) should aim to achieve efficient and safe use of any phosphorus applied in order to maintain satisfactory levels of yield, while keeping the phosphorus levels in the soil within acceptable limits. In other words, soil depletion or unacceptable accumulation should be avoided and the aim should instead be to achieve a balance between inflows and outflows in the system. It is also obvious that BMPs should contribute to restricting the transport of phosphorus to surface waters and groundwater. A soundly-based and functional package of practices should not only have a positive effect on the environment but also be economically sustainable for the farmer adopting these practices. Figure 4 shows a flow diagram describing preventive measures aimed at minimising phosphorus losses at field level. These measures have been divided into those that decrease the release of phosphorus from soil and fertilisers and those that affect the actual transport of phosphorus on the soil surface or in the soil. The effects of the measures adopted are also described, as is more fundamental research at process level (Figure 4). When a more complete picture has been prepared, the measures expected to have the greatest effect on the magnitude of losses in each individual case can then be prioritised. Limiting the release of phosphorus Even if the supply of phosphorus to agricultural land is in balance with its removal, high losses can still occur due to the fact that they are often concentrated to short episodes (Haygarth & Sharpley, 2000), as discussed in several other sections of this report. To decrease the effect of these episodic losses, great attention should be given to the time of phosphorus application and the method of fertiliser application. In the case of manure, there are a number of regulations in Sweden governing the timing of application (SJVFS 1999:79). These regulations concern application within the year, the requirements on incorporation to avoid surface runoff losses and specific requirements on specific techniques to be used in certain counties. For example, it is forbidden to apply manure to frozen ground during the winter (1 January - 15 February) in sensitive areas (coastal areas of southern Sweden and parts of the agricultural region around lakes Mälaren, Hjälmaren, Vättern and Vänern) where the risk of phosphorus and other nutrients being transported from frozen ground at snowmelt is great. However, the regulations have recently (2005) been revised and application is permitted on thawed bare soil if the manure is then incorporated.
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Fig. 4. Flow diagram showing the relationships between measures for decreasing phosphorus losses from agriculture (source: Djodjic et al., 2005).
Infiltration of water into frozen soil is mainly governed by the soil structure and the water content of the soil at the time of freezing (Zuzel & Pikul, 1987). When the soil freezes at a high water content or at saturation, it becomes practically impermeable to water, which means that meltwater or rain falling during the winter often give rises to severe surface runoff (Stähli, 1997), which in turn increases the risk of high phosphorus losses. This risk is particularly great if manure is applied in late autumn or during the winter (Sharpley et al., 1994). However, air-filled macropores in frozen soil can also pose a risk of phosphorus losses, since water with its content of dissolved or particle-bound phosphorus can be rapidly transported downwards in the soil (Pikul et al., 1996). Ulén (1995) demonstrated that considerable leaching losses can occur in such conditions. Fertiliser placement What can be done to improve the use efficiency of applied phosphorus and thereby decrease the risks of losses? A well-proven method is to apply fertiliser phosphorus in bands in the soil instead of broadcasting it on the soil surface. This applies equally to
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artificial fertiliser and manure. This decreases the risk e.g. of surface runoff losses and for manure also the risk of ammonia losses to the atmosphere. It has also been shown that a marked reduction in leaching losses of phosphorus can be achieved by incorporating fertiliser at the time of application (Djodjic et al., 2002). As mentioned previously, in soils with a high pH the phosphorus reacts with calcium and magnesium and forms compounds with low solubility (Sample et al., 1980). These compounds are considerably less available to crops than the fertiliser phosphorus applied and the availability often decreases over time. Similarly, in acid soils phosphorus forms poorly soluble compounds with iron and aluminium oxides. Placing the fertiliser in bands decreases the contact area between the soil material and the phosphorus applied and thereby prevents immobilisation (Tisdale et al., 1993). In soils with a low phosphorus content and a high phosphorus binding capacity, good crop growth is generally obtained if artificial fertiliser phosphorus is placed in bands beside the seed through combi-drilling (starter P). This also allows the fertiliser dose to be decreased. The degree of use efficiency can also be increased by keeping the fertiliser phosphorus applied in plant-available form for longer periods. Since roots cannot take up nutrients in dry soil, band placement of fertiliser granules in the soil is better than spreading them on the soil surface, which dries out rapidly in spring. However, the disadvantage of band placement is that the root volume coming into contact with the phosphorus applied is often smaller (Barber, 1977). It is also important to bear in mind that there are distinct differences between different crops regarding their response to band placement of fertilisers. For example, it has been shown that maize (Richards et al., 1985; Swaider & Schoemaker, 1998), soyabean (Randall & Hoeft, 1988), linseed and rape (Nyborg and Henning, 1969) can even be damaged by placement in seed rows of amounts of phosphorus fertiliser granules corresponding to the optimal dose. In such cases it is important that the fertiliser granules are placed somewhat below the seed. Manure spreading In the case of manure, there are several measures that can be adopted to decrease phosphorus losses and increase phosphorus use efficiency, of which some concern time of application and incorporation requirements as mentioned previously. The first measure is of course to analyse the nutrient content of the manure so that the correct amount of phosphorus is applied. Simply relying on standard values given in various books and other literature is not good, since the composition of fertiliser varies greatly depending on animal diet and manure management system. For solid manure, which varies most, it is recommended that samples be taken from the manure spreader, while slurry samples can be taken from the storage tank. There are a range of chemical methods used to determine the total phosphorus content of manure (Peters et al., 2003). There are also examples of cases where the phosphorus content is stated in terms of water-soluble phosphorus in order to emphasise that it is the amount of phosphorus that can either run off the soil surface or be leached out that is important, i.e. to give an indication of the potential environmental load. Another important factor for good phosphorus use efficiency is that the correct amount of manure is spread on fields, which requires a reliable manure spreader that is calibrated at regular intervals. If the spreader is not calibrated there is a risk of excessive amounts being applied, which in the long term can lead to unnecessary
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phosphorus losses to watercourses. When calibrating a manure spreader it is important to check both the amount emitted and the area over which this amount is applied. There are a number of methods used for calibration, from those based on individual loads of manure to those based on the contents of the slurry tank on the farm (Koelsch, 1995; Jokela, 2003). It is important to bear in mind that regardless of the calibration method used, the spreader must be re-calibrated after every effective change in the composition of the manure. In comparison with many other methods to decrease phosphorus losses from agricultural soil, it is cost-effective to have a wellcalibrated manure spreader. It ensures that the manure is applied in amounts appropriate for the crop and thus avoids poor use efficiency of a valuable plant nutrient resource. Manure additives In the USA, there is frequent mention of the possibility of decreasing the solubility of phosphorus in manure by the use of various additives (Shreve et al., 1995; Dou et al., 2003). These include e.g. aluminium sulphate (alum, Al2(SO4)3·14H20), which contributes to phosphorus binding and thus decreases losses when the manure is eventually applied (Sims & Luka-McCafferty, 2002). Aluminium sulphate is mainly used for nutrient-rich poultry manure (Moore et al., 2000). The amount of aluminium sulphate used should be 5-10% by weight of the manure mass, i.e. around 1-2 ton per 20 000 broilers. Such an addition rate gives incredibly efficient binding. A 90% decrease in surface runoff losses of phosphorus has been observed in individual field plots (Shreve et al., 1995), while the reduction from small catchments can be up to 75% (Moore et al., 2000). A positive side-effect is that losses of ammonia (NH3) also decrease when the manure is applied (Moore et al., 2000). Apart from aluminium sulphate, additives of aluminium chloride (AlCl3) and ash from incineration processes can also contribute to losses of phosphorus being significantly decreased (Smith et al., 2001), without any accompanying decrease in the amount of plant-available phosphorus. For aluminium chloride the addition rate must be 1:1 as regards aluminium:phosphorus (on a molar basis). However, there is reason to believe that this type of technical solution for temporarily binding phosphorus and thereby rendering it less accessible to dissemination in nature will not achieve any great breakthrough force in Sweden, where solutions based on avoiding excess balances of phosphorus and improved use efficiency have considerably greater acceptance. Impact on phosphorus transport As mentioned above, measures to reduce phosphorus losses must take account of the dominant transport pathway in phosphorus flow, which can occur through surface runoff or flow through the soil matrix (so-called piston flow) or macropores in the soil. The type of flow that dominates is dependent on a number of factors such as soil sorption capacity, rain intensity, etc. For example, leaching of phosphorus through the soil profile by piston flow is strongly affected by the sorption capacity of the soil, while surface runoff is completely unaffected by this parameter. When suitable preventive measures are being sought to decrease phosphorus losses in an area or from a field, it is therefore essential to identify the dominant transport pathway. Surface runoff A large proportion of the phosphorus losses from agricultural soil occur through surface runoff. In many countries such losses are considered to be completely
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dominant, which means that remediation strategies to decrease phosphorus emissions have been linked to a greater extent to methods designed to prevent and reduce erosion. Surface runoff losses can occur both in the form of dissolved reactive phosphorus and phosphorus bound to soil particles. In general, phosphorus losses decrease with increasing soil infiltration capacity and thus decreasing runoff (Turtola & Jaakola, 1995; Gillingham & Thorrold, 2000; Simard et al., 2000). Knowledge based on research into the dependency of erosion on rain intensity, soil characteristics, topography and soil tillage have been used to relieve problems relating to surface runoff losses of phosphorus. Methods have since been developed to improve soil infiltration capacity and decrease the release of particles from soil, while various measures have been designed to control the transport of phosphorus (Figure 4). The prevention of erosion demands systematic and often comprehensive efforts. However, it is important to take action in fields where erosion causes phosphorus losses, not just where erosion occurs, since erosion is not necessarily associated with losses of phosphorus (Sharpley et al., 1994). In actual fact, large phosphorus losses can occur even during periods with low rainfall intensity and small erosion losses. However, knowledge within this area needs to be improved and to encompass not only particle-bound phosphorus but also phosphorus bound to colloidal material (Ulén, 2003). A number of soil tillage strategies to decrease the velocity of water flow during surface runoff events, and thereby the transport of soil particles and any phosphorus bound to these, have been developed over the years. These include carrying out tillage operations perpendicular to the slope of the field, contour ploughing and construction of terraces, which are methods applied in a number of countries where sloping fields are a commonly occurring feature. However, there is little experience of these in Sweden. Reduced soil tillage is also used to decrease surface runoff losses of phosphorus. Leaving harvest residues on the soil surface increases infiltration and decreases soil drying and thus more water is retained in the soil for the following crop. By leaving harvest residues on the soil surface and omitting soil tillage after harvest, erosion losses can be reduced by 30-90% depending on the crop (Lemunyon, 2006a), which markedly decreases phosphorus losses. For each ton of soil particles prevented from leaving the field with runoff, phosphorus losses can be reduced by at least 50 g (Lemunyon, 2006a). However, it is important to bear in mind that general recommendations seldom produce good results and that erosion control measures should be adapted to local conditions. In line with what has been discussed above, it is also important to consider that not all erosion control measures decrease phosphorus losses. For example, plant residues left on the soil surface can act as a phosphorus source (Wendt & Corey, 1980; Gaynor & Findlay, 1995) and increase the losses of dissolved reactive phosphorus. Dissolved phosphorus creates considerably greater problems in most water ecosystems than particle-bound phosphorus due to its high bioavailability. Spring tillage, which is normally better than autumn tillage as regards reducing phosphorus losses, can destroy the soil structure and thereby decrease infiltration capacity and increase runoff if carried out when the soil has a high degree of hydraulic saturation. If soil tillage operations are not carried out, the macropore flow in certain soils can increase, which in turn often contributes to greater phosphorus losses (Petersen et al., 1997; Persson, 2001). McDowell & McGregor (1984) also found that even though the losses of total phosphorus were considerably reduced when no soil tillage was performed, the losses of dissolved phosphorus were
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eight-fold higher compared with after conventional tillage operations. This type of goal conflict must be considered when designing measures to minimise leaching of phosphorus to the surrounding environment. Other measures that have been proven to significantly decrease surface runoff losses of phosphorus in the agricultural landscape include vegetation filters along watercourses and wetlands, often referred to as buffer zones (Leinweber et al., 2002). The efficiency of vegetation filters along watercourses is strongly linked to filter width. For example, a Canadian study found that phosphorus losses were reduced by 31% in a 2 m wide filter and by as much as 89% when the width of the filter was increased to 15 m (Abu-Zreig et al., 2003). Flow velocity, vegetation type and density of the vegetation cover have been shown to be of secondary importance for the magnitude of the losses. In a Scandinavian study, total phosphorus content was reduced by between 27 and 97% depending on filter width, which is equivalent to 0.24-0.67 kg P ha-1 yr-1 (Uusi-Kämppä et al., 2000).
Buffer zone along an open drain in Uppland (photo: Faruk Djodjic)
In the same study it was found that phosphorus retention in wetland was 17% of added phosphorus. The mechanisms that control phosphorus retention in vegetation filters are sediment deposition, infiltration capacity and uptake of phosphorus by the vegetation (Abu-Zreig et al., 2003). There is often a greater accumulation of particles in a vegetation filter than of phosphorus. For example, Magette et al. (1989) found phosphorus retention equivalent to 46% in a 9.1 m wide filter, while retention of particles in the same filter was 82%. Similar results have emerged in other studies (Dillaha et al., 1987; Patty et al., 1997). Vegetation filters have also been used to decrease phosphorus losses from different types of wastewater generated in agriculture, e.g. wastewater from dairy units. Such wastewater, which contains large amounts of phosphorus, can be spread on a vegetated area that captures up to 90% of the phosphorus (Schwer & Clausen, 1989). Several studies mention that with vegetation filters in cold regions there is a risk of the plant material in the filter freezing. This can lead to increased losses of phosphorus since freezing bursts the cell
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membranes and releases the phosphorus in the plant cells, which is then carried away by runoff water (e.g. Timmons et al., 1970; Miller et al., 1994). Increased phosphorus losses as a result of freezing of plant material has also been demonstrated in Swedish studies (e.g. Torstensson et al., 2006). It can be assumed that introduction of a catch crop, which is a proven method of decreasing nitrogen leaching from lighter soils in southern Sweden (Aronsson, 2000), would also have a decreasing effect on phosphorus losses (Hargrove, 1991). This would occur through decreased surface runoff and erosion, phosphorus uptake by the catch crop and improved infiltration. Studies have shown that a catch crop that is allowed to grow for several months can bind 10-30 kg P ha-1 in aboveground biomass (Lemunyon, 2006b). However, freezing out of the phosphorus in plant cells, and thus the potential for increased losses, is also a risk with catch crops under Swedish climatic conditions.
Wetland (photo: Helena Aronsson)
Wetlands have been established in the agricultural landscape in recent years to reduce the transport of nitrogen and phosphorus to the sea. However, there are rather few quantitative measurements showing how efficient wetlands are as phosphorus traps under Swedish conditions. In a compilation of data from 17 wetlands situated in Scandinavia, Switzerland and Illinois (USA), it was found that factors such as wetland area in relation to area of the catchment, wetland age, etc. were very important for phosphorus retention, which varied between 1 and 88% in the case of total phosphorus (Braskerud et al., 2005). The variation was even greater for dissolved reactive phosphorus (-19 to 89%). In wetlands with an area >1% of the catchment area and with a relatively high proportion of dissolved reactive phosphorus in inflow water, retention of total phosphorus was around 20%, which was equivalent to 4-11 kg P ha-1 wetland and yr-1. When the proportion of particle-bound phosphorus was large, the retention was considerably higher. These results indicate that wetlands can make a potential contribution to decreasing the losses of particle-bound phosphorus from agricultural land in regions with a cold climate. At the same time, wetlands designed to reduce phosphorus transport in summertime have demonstrated the ability
34
to contribute dissolved reactive phosphorus during the summer due to contributions from sediment (Tonderski et al., 2005). In Sweden, great economic inputs are currently being devoted to the establishment of wetlands, particularly with regard to nitrogen retention. However, for phosphorus this method must be regarded as relatively uncertain, despite the positive effects described above. This is mainly because the wetlands must be designed and maintained with the utmost care to have any effect whatsoever on diffuse losses of phosphorus from agricultural land. The water flows are commonly underestimated and the sedimentation rates are overestimated for the long transport colloidal clay particles that are responsible for much of the phosphorus transport to the Baltic Sea. Flow through large soil pores Large losses of phosphorus occasionally occur through water, and the dissolved or particle-bound phosphorus contained therein, being transported through large pores in the soil profile. During such transport, the phosphorus usually does not have time to react with the soil material, but bypasses sorption surfaces in the soil and continues to deeper layers in the profile. The effect on phosphorus losses is then similar to that described above for surface runoff. This type of loss is often referred to as internal erosion, since particles and their bound phosphorus are ripped from the pore walls when the water rushes past. Soil tillage is one way to break the continuity of macropores in the topsoil and thus decrease phosphorus transport (Thomas & Phillips, 1979). A number of studies have also shown that phosphorus losses are lower in tilled soil compared with undisturbed (McDowell & Monaghan, 2002). However, there are also studies showing that soil tillage does not have this effect. In a Swedish study, this was explained by macropores in the topsoil being recreated by repeated freezing/thawing and the resultant fracturing of soil aggregates (Djodjic et al., 2002). Another explanation may be that soil tillage contributes to percolating water having a longer retention time in the phosphorus-rich topsoil layer, which means that leaching increases. However, in the subsoil the structure is relatively unaffected and the downward transport of phosphorus through macropores to the drain system can be rapid. One way to decrease leaching of phosphorus would be to promote rapid flow in the topsoil through e.g. breaking up the plough pan, while another would be to decrease the flow velocity in the subsoil through e.g. deep ploughing. Some studies have shown that phosphorus leaching decreases substantially after ploughing to greater depth in certain conditions (Calvert, 1975), although this has not been tested in Sweden. More research is needed within this area, especially as regards structured clay soils. The disturbed backfill soil over a tile drain represents a good pathway for generating rapid phosphorus transport in the soil similar to the flow through macropores, especially in the first few years after drain installation. Based on the assumption that 2.5% of the soil volume in a newly tile-drained clay soil is made up of backfill (0.5 m wide pipe trench and 20 m drain spacing), it is obvious that the backfilling method has a great impact on e.g. phosphorus losses from drained arable soil. A method developed for clayey soils in Finland (the FOSTOP method, Nordkalk Oy Ab)
35
Establishment of a lime filter drain (photo: Barbro Ulén)
involves incorporating burnt (i.e. unslaked) lime (CaO) with the backfill material in drains. The result is a stable and porous backfill that efficiently binds the phosphorus in percolating water. The lime filter drain, as the method is often known, thus acts as a mini chemical treatment plant. The lime requirement has been determined in trials to be 3-8% of soil wet weight. The method has been tested in a number of experiments and has been found to reduce the phosphorus concentrations in running water by more than 80% in most cases. In addition to phosphorus removal, the lime filter drain can also lead to improved drainage in impervious clay soils and can thus contribute towards decreasing erosion. The average lifetime for the lime filter drain has been shown to exceed 10 years without any loss in treatment effect. In Sweden, the method has only been tested at one experimental site (Lindström & Ulén, 2003) and the longterm effects have not been monitored. Therefore more research is needed on the effects of lime filter drains in decreasing phosphorus losses from agricultural land under Swedish conditions before the method can achieve any breakthrough force. Flow through the soil matrix Large leaching losses of phosphorus have been measured from sandy soils with a low sorption capacity for phosphorus, particularly in combination with large phosphorus doses in the form of manure or artificial fertiliser. In a Swedish environmental monitoring programme (Observation Fields on Arable Land) that included 16 fields, it was found that the highest phosphorus losses were from a sandy soil with the distinctly lowest sorption capacity and the highest degree of phosphorus saturation (Djodjic & Bergström, 2005). There can be no doubt that such soils are very susceptible to high phosphorus leaching and will probably give rise to considerable leaching losses over a long period even if they are not fertilised, an issue that requires investigation. In other words, the possibility of restricting leaching from this type of soil is limited. However, attempts have been made to prevent phosphorus saturation of such soils through the introduction of legislation limiting the livestock density. An alternative proposed in recent years is to limit drain runoff from tile-drained fields through controlled drainage, i.e. by raising the water level in the field (Gilliam et al., 36
1999; Wesström, 2002). A condition for this is that the field is relatively flat. However, the resulting reducing conditions that develop in the soil can lead to the release of iron phosphate compounds, which in turn leads to greater losses of dissolved phosphorus (Sims et al., 1998). On the other hand, some studies have shown that phosphorus losses can be decreased by around 35% with the help of controlled drainage (Maguire et al., 2006). Another option to reduce the risk of phosphorus leaching is to grow crops such as lucerne, which due to their deep root system have the capacity to take up large amounts of phosphorus from the soil without any being added, a process usually referred to as mining. At harvest, the phosphorus is then removed from the field. However, the difficulties in establishing a dense lucerne crop can decrease the effect (Ulén & Mattsson, 2003). Perennial ley crops are generally better suited to mining than cereal crops and the practice works best on soils with high phosphorus levels. Maize has been demonstrated to decrease the phosphorus level in the soil by 150 kg P ha-1 during a period of 10 years (Eghball et al., 2003), which markedly lowers the risk of phosphorus leaching.
N leaching (kg N ha-1 yr-1)
Large leaching losses of phosphorus are often associated with large phosphorus doses applied with manure and artificial fertiliser. However, this is not necessarily always the case. In a Swedish lysimeter study measuring phosphorus leaching from five soils that had received increasing doses of artificial fertiliser phosphorus since the 1950s, it 60 50 40 30 20
P leaching (kg ha-1 yr-1)
0.12 0.10 0.08 0.06 0.04
0 0
50 100 150 200 N 40 80 120 160 P Application rates (kg ha-1 yr-1)
Fig. 5. Leaching of nitrogen and phosphorus with increasing doses of pig slurry on a sandy soil (source: Bergström & Kirchmann, 2006).
37
was found that in three of these soils leaching tended to decrease with increasing phosphorus supply (Djodjic et al., 2004). The explanation given was that the ability of these soils to release phosphorus varied and that the way phosphorus was transported through the profile was different in different soils. In another study, leaching of nitrogen and phosphorus was investigated in a sandy soil given increasing doses of manure over two years (Bergström & Kirchmann, 2006). As expected, nitrogen leaching increased as a result of higher doses of manure, but leaching of phosphorus decreased (Figure 5). A phosphorus dose of 320 kg P ha-1 during the two-year period gave lower leaching than when no phosphorus was supplied. An explanation for this unexpected result could be that application of manure increases the pH in the topsoil, leading to the formation of relatively insoluble calcium phosphates and a decrease in leaching (Sharpley et al., 2004). However, it is impossible to confirm whether two years of manuring were sufficient to alter the binding conditions for phosphorus in the soil in the example cited. Nevertheless, it is obvious that the studies described above indicate that there is no unequivocal relationship between phosphorus application and phosphorus leaching. Measures on the farm In addition to measures directed at influencing the release and transport of phosphorus in agricultural soil, there are a range of other measures that can be adopted to reduce losses of phosphorus. These include e.g. additives for animal feed that increase uptake of phosphorus, calculation of farm phosphorus balances and appropriate storage of manure. Feed additives It is a well-known fact that phosphorus utilisation in animal feed is poor due to the fact that 80-90% of the phosphorus in cereal grain is stored as phytate (inositol hexakisphosphate) (Jongbloed & Kemme, 1990). Phytate is stable and poorly digestible for most species of animals, particularly monogastrics (pigs and poultry) (Smith et al., 2004a), which do not possess the advantage of having a rumen containing microbes that can release phytate-P. Because e.g. pigs have a very low utilisation rate of phytate, inorganic phosphorus is often added to pig feed, which further increases the risk of phosphorus losses since the amount of phosphorus in the manure increases. There is namely a distinct correlation between intake and excretion of phosphorus (e.g. Ternouth, 1989). Studies have shown that the amount of phosphorus in pig manure increases to levels between 20 and 40 g P kg-1 DM with relatively high rates of phosphorus addition to the feed (Barnett, 1994), while the levels are generally below 20 g P kg-1 DM with low or no additives (Peperzak et al., 1959; Gerritse & Zugec, 1977). There are currently two ways to counteract the problem of low phosphorus utilisation in feed. One is simply to use feed that contains phosphorus in a more available form (HAP) (smaller amounts of phytate-P), while the other is to add the enzyme phytase (Baxter et al., 2003). Phytase is produced by microorganisms (e.g. Aspergillus niger), and the enzyme catalyses the hydrolysis of phytate in the digestive tract of animals to produce orthophosphate, which can be taken up (Kornegay, 1996; Figure 6). Addition of phytase means that the animals can utilise the phosphorus in the feed better and the phosphorus content can therefore be lowered. This in turn has been shown to decrease
38
P P P P
P P
Fig. 6. Phytase cleaves inorganic phosphorus from phytate to form orthophosphate, which can be taken up in the digestive tract of animals.
the amount of phosphorus in manure (e.g. Baxter et al., 2003; Smith et al., 2004b) and thereby the risk of losses to the environment. For example, addition of phytase in combination with HAP feed for poultry was shown to decrease runoff losses during heavy rainfall by 45% compared with feed without additives (Smith et al., 2004b), and there are a number of other examples of similar results. In addition, the cost of adding phytase is usually lower than that of adding extra phosphorus to the feed (Smith & Joern, 2006). In other words, phytase additives are a good measure to reduce phosphorus losses to surface waters and groundwater in a cost-effective way. Furthermore, phytase additives have a number of other positive effects, such as increased availability of nutrients such as Ca and Zn (Smith et al., 2004b). Calculation of phosphorus balances A good starting point for minimising phosphorus losses from agriculture is to have a system in balance at all levels (field, animal production unit, entire farm), i.e. to have inflows and outflows of phosphorus balancing each other. A positive balance indicates that there is a risk of phosphorus accumulation in the system, while a negative balance indicates a risk of depletion. Accumulation of phosphorus in a field is usually revealed when some form of soil test is performed. Therefore the risk of phosphorus losses from agricultural land is currently often based on soil test values determining the distribution between dissolved and bound phosphorus. A number of studies have shown that the risk of phosphorus losses is best predicted by determining the concentration of dissolved phosphorus in the soil (Leinweber et al., 1999; Schoumans & Groenendijk, 2000), which therefore becomes decisive for fertilisation recommendations aimed at avoiding overapplication of phosphorus. Figure 7 shows a matrix indicating the relationship between phosphorus balances and soil test values. The risk of large excesses within a system is naturally greatest when large quantities of feed are bought in for an animal production unit. However, in Sweden this is controlled by not allowing livestock density to exceed manure production equivalent to 22 kg P ha-1, which corresponds to 1.4 dairy cows. However, there is a degree of danger in using standard data for this. In certain regions a livestock density equivalent to 22 kg P ha-1 is too high to achieve acceptable phosphorus losses, while in other regions the opposite is true, i.e. a higher livestock density can be accepted. In other
39
P balance Annual supply – Annual removal
Soil test value
Low
0
–
Agronomic responsibility
+ Desirable
Optimal
OK
Ideal
OK
Excess
Desirable
OK
Potential environmental responsibility
Fig. 7. Matrix indicating the relationship between phosphorus balances and soil test values (source: Beegle & Lanyon, 2006).
words, a more flexible approach to livestock density can be justifiable but it is difficult to implement in practice. Manure storage There are a number of tried and tested measures to reduce phosphorus losses during storage and application of manure. The most important of these is probably to have sufficient storage capacity, which provides better opportunities for ensuring that the manure is spread when the risk of phosphorus losses to the environment is small. This also increases the value of manure as a source of nutrients. It requires a good manure container with a storage capacity corresponding to the amount of manure stored during periods when spreading is not permitted. The container dimensions should also be designed with regard to extreme weather conditions and other factors that can hamper manure spreading. In Sweden, the storage capacity for manure is controlled in different regions with respect to number of animals by the government directive Environmental Protection in Agriculture (SFS 1998:915). Farms with more than 100 livestock units situated in southern Sweden (Skåne, Halland, Blekinge, Gotland, Öland) or in other sensitive regions must have a storage capacity of 8-10 months depending on animal species. In practice, problems still arise in view of the fact that there are few opportunities during the year when conditions for spreading are good. This is particularly true in the case of slurry spreading on clay soil.
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Models and other tools Models and other tools have begun to be used to determine phosphorus losses from agricultural land. However, the complexity of the processes controlling the forms in which phosphorus occurs in the soil, their mobilisation and transport through the soil profile and the agricultural landscape, and the multitude of potential phosphorus sources (point and diffuse sources) render it necessary to use different tools and models to quantify losses, calculate source apportionment and determine the magnitude of various transport mechanisms. In addition, the EU Framework Directive for Water requires the implementation of more efficient environmental monitoring and protection programmes, and models are therefore becoming increasingly important tools in improving decision support for such measures. Models can be an aid in evaluating where measures can best be introduced. A number of models have been used in Sweden to calculate phosphorus transport. Several reviews of phosphorus models have been published in recent years (Arheimer & Olsson, 2003; Wallin et al., 2004; Brandt et al., 2006). To avoid repetition, here we merely provide a brief summary of these reviews and attempt to approach the subject with the objective of ‘quantifying phosphorus losses from arable land’ rather than the use of models/methods to achieve this. The focus is on identifying gaps in existing knowledge and deficiencies in application of existing information within modelling work, research needed to improve the calculations and decrease uncertainty, and deficiencies and limitations in existing data used as input in the models. Research into phosphorus losses from the soil in Sweden has often been carried out on a smaller scale (experiments in the laboratory, lysimeters, experimental plots, fields) in order to identify the most important processes involved under controlled conditions. However, models have mainly been applied to the larger scale (fields, small and large catchment areas, regional and national calculation systems), primarily because this scale is most interesting for different users, not least since the introduction of the EU Framework Directive for Water. Furthermore, the environmental monitoring data required for model calibration are often on this greater scale. The resulting problems with scaling-up of research results has to be solved in a satisfactory way in order to improve the reliability of the models. On the other hand, the models are good tools for applying research and environmental monitoring results and viewing their relevance in a wider context. The focus in this section is on modelling of phosphorus losses from arable land, which is a considerable source of eutrophication of recipient waters. Therefore modelling at field scale, the natural entity for arable land, and catchment area scale, the natural entity for water and thus nutrient transport, are considered here. Phosphorus models and tools used in Sweden An important aspect is that choice of the model must be determined by the aim of the modelling work. Previous modelling work has often been aimed at formulating and testing our understanding, i.e. models were to a high degree research tools for studying the controlling processes within a system. However, an increased need for decision support tools has led to the use of models as a support for status description, impact analysis and evaluation of the effects of remedial measures. Therefore models
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have increasingly become a bridge between environmental monitoring and management. As mentioned above, a number of reviews of phosphorus models have been produced in recent years. Most of the models used in Sweden and in Europe are described in these reviews, with short descriptions of the different models and lists of perceived strengths and weaknesses and experiences of their application under Swedish conditions. A short summary of these models is presented in Table 3. From Table 3, it is clear that the models have been constructed for different purposes (source apportionment calculations, evaluation of remedial action scenarios, quantification of losses from arable land) and scales (field, catchment area, regional and national calculation systems). There are also differences between different models and quantification tools as regards their complexity, need for input data, and resolution in time and space (Figure 8). A combination of different approaches is often used in a single model. Thus there may be an empirical description of water flow and a more conceptual process description of the phosphorus cycle (e.g. SWAT, ICECREAM). Other models have a detailed physical description of water flow and use type concentrations for calculating phosphorus transport (e.g. HBV-NP). Therefore it is difficult to simply place the models on a scale without a detailed description of the process descriptions involved in this placement.
Table 3. Summary of a number of models/tools used in Sweden Model/Tools
Scale
Description
Reference
AVGWLF
Catchment area
Empirical, dynamic
Evans et al., 2002
ICECREAM
Field
Dynamic, action-orientated
Larsson et al., 2003
Phosphorus Index
Field
Static, risk assessment tool
Djodjic & Bergström, 2005
Fyriså model
Catchment area
Source apportionment model
Kvarnäs, 1996
HBV NP
Catchment area
Source apportionment model
Bergström, 1995; Arheimer & Brandt, 1998; Andersson et al., 2005
Regression model
Field
Empirical model
Ulén et al., 2001
SWAT
Catchment area
Dynamic, action-orientated
Nietsch et al., 2002
WATSHMAN
Catchment area
Source apportionment model
Zakrisson et al., 2003
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Fig. 8. General relationships between model complexity (left), model type (right) and output generated (source: Schoumans & Silgram, 2003).
However, there are a number of common denominators for some of these models. A common denominator for the source apportionment models (Fyriså model, HBV-NP, WATSHMAN) is that they use type concentrations to calculate the contribution of arable land to total phosphorus losses. A type concentration is a mean concentration normalised for weather conditions and assumed to be representative for a combination of soil texture and crop. Type concentrations are calculated with the aid of other models (e.g. a regression model or ICECREAM). ICECREAM and SWAT are American models that describe hydrology and nutrient transport in similar ways. However, ICECREAM is a field model while SWAT is intended for catchment area scale. These two models, like WATSHMAN and AVGWLF, calculate water transport with the help of Soil Conservation Curve Number (SCS, 1972), which is an empirical method for apportioning runoff between surface runoff and infiltration. The regression model is based on measurements of total phosphorus from fields included in the environmental monitoring programme Observation Fields on Arable Land and relates phosphorus losses to a number of variables (livestock density, P-HCl content in topsoil, specific area of soil particles and duration of high flow periods). Phosphorus Index is a tool that makes a risk classification for individual fields or parts of fields based on an assessment of phosphorus sources (phosphorus concentration in the soil, fertilisation) and transport mechanisms (surface runoff, leaching). Experiences from application of the models referred to above are that satisfactory results can be obtained in comparisons of empirical data, but the uncertainties are great. This is often explained by a combination of inadequate data, weaknesses in the models and deficiencies in the data material available for model calibration. The uncertainty also tends to increase with distance from the calibration point. In other words, a simulated value that agrees well with measured data in the outlet of a field or a catchment area does not necessarily mean accurate transport calculation and source apportionment upstream from the calibration point. Furthermore, there is no accepted
43
statistical measure for evaluating model performance, although certain parameters occur frequently (e.g. Nash & Sutcliffe, 1970). Time-consuming set-up and calibration of models limits the number of model applications and makes model evaluation more difficult under different climatic and hydrological conditions. ‘Equifinality’ is another known phenomenon by which different combinations of model parameters lead to the same end-result, which further impairs evaluation of model performance. A distinct failing in source apportionment models when comparing calculations for nitrogen and phosphorus is the great difference in the way representative type concentrations are produced. Type concentrations for nitrogen calculated using the SOILNDB model (Johnsson et al., 2002) are regarded as being more representative and accurate than the corresponding values for phosphorus produced by the regression model. However, in the future these type concentrations will be replaced by type concentrations produced using the ICECREAM model. Thus improvement of the models that calculate type concentrations is a precondition for more accurate source apportionment models. Catchment area models should be improved to take account of the position of the field within the landscape, since this controls hydrology, nutrient transport and retention. In view of the fact that models are being increasingly used to evaluate environmental monitoring and protection programmes, it is a great disadvantage that modelling work does not have the same continuous character, i.e. while environmental monitoring and environmental protection work are regarded as continuous processes, modelling is still performed sporadically, without continued efforts. A more integrated approach is required for environmental monitoring, modelling and protection work, not least to link together different scales (field, catchment area), sources (point, diffuse) and nutrients (phosphorus and nitrogen). Based on the experiences and limitations cited above, it is important to identify areas in which model improvement is needed. The most important are the following: 1. Model development has not kept pace with existing knowledge and research results. 2. Model development is in great need of research. 3. Model development is hampered by insufficient and unsuitable indata. 4. Model development is hampered by insufficient/unsuitable data for calibration and validation. Lag between model development and existing knowledge and research results Adjusting descriptions of phosphorus forms in the soil to analytical methods used in Sweden The phosphorus concentration in the soil is analysed through total saturation with oxidising acids and by various extraction methods. In Sweden, the content of plantavailable phosphorus is measured by the P-AL method according to Egnér et al. (1960), where the soil is shaken with an acetic acid solution of ammonium lactate. In addition, extraction of soil phosphorus is often performed with HCl, which is regarded as being a better measure of the amount of reserve phosphorus, i.e. a large proportion of the total phosphorus pool in the soil is extracted. Other countries often use values based on different extraction solutions (Olsen P, Bray P, Mehlich P). The measured values are then used in models for field (e.g. ICECREAM) and catchment area (e.g.
44
SWAT) that have inbuilt descriptions of various phosphorus pools in the soil. This makes the use of Swedish measurements (P-AL, P-HCl) more difficult. A step forward has already been made in that ICECREAM is now using P-HCl measurements as indata, but further improvement is needed to take account of the more mobile plant- and leaching-available soluble P-AL fraction. Börling (2003) showed rather strong correlations between P-AL and CaCl2-extractable phosphorus (which is used as a measure of the amount of phosphorus in the soil solution). With individual in-depth studies in combination with existing long series of measurements (the environmental monitoring programmes Observation Fields on Arable Land and Type Regions on Arable Land, water quality measurements in rivers, long-running soil fertility trials), it should be possible to obtain sufficient background material to adjust description of phosphorus pools in the soil to the values measured in the field. Phosphorus binding capacity and release Phosphorus release in the soil is controlled by both the phosphorus content and the phosphorus binding capacity of the soil. In the models discussed above, the phosphorus binding capacity of the soil is predefined to certain assumed values or related to the clay content in the soil. A certain updating of these processes has already been proposed (Vadas et al., 2006). Degree of phosphorus saturation, which is a useful measure of the propensity of the soil to release phosphorus, has long been an accepted concept but is rarely used in modelling work carried out in Sweden. Börling (2003) and Ulén (2006) show that phosphorus sorption and release in Swedish soils are related to the amount of iron and aluminium extracted with ammonium oxalate and ammonium lactate respectively. The introduction of these terms into model equations should improve adjustment to Swedish conditions and increase model reliability. Physical description of surface runoff/erosion The existing description of the erosion process in phosphorus models is based on the Universal Soil Loss Equation (USLE) or one of its derivatives (Modified USLE (MUSLE); Revised USLE (RUSLE)). These methods have been developed and tested in the USA, where extensive measurements and calibration of the constituent parameters have been carried out. Corresponding data for Swedish soils and conditions are lacking. In addition, in Sweden erosion is of lower intensity in most cases and therefore difficult to describe using the above-mentioned methods, but it still has great significance for phosphorus transport and therefore a process description is required in the models that take this into account. There are already a number of European models that physically describe erosion and sediment transport (EUROSEM, LISEM) and experience of these models can be used to improve the description of erosion in phosphorus models. They have also provided new knowledge about the importance of colloid-bound phosphorus transport (Heathwaite et al., 2005). Model development hampered by insufficient and unsuitable indata Soil texture and soil classification In Sweden, indata on soil texture are mainly taken from two sources: the SGU soil texture map or national sampling of Swedish soils (Eriksson et al., 1997). Both these sources have rather low resolution. In addition, it is difficult to apply the SGU soil textural classification for modelling purposes. In other words, it is difficult to obtain
45
information on soil physical characteristics and texture solely from soil classification. In the long term, considerable improvement is required in this area. Drainage Nearly all readily accessible data regarding the proportion of drained soil are included/used in modelling. Drainage as a considerable intervention in the landscape plays a decisive role in water movement in the soil and the connectivity of fields in the landscape, and it is a distinct disadvantage to be unable to take this into account. Phosphorus status in the soil Farmers have often quite good information about phosphorus content (P-AL value) in the soil, even at field level. For modelling purposes it is often possible to obtain data with good resolution at field scale. However at a higher scale, where information is being sought about a catchment area, it is often difficult and laborious to collect data and there is a reliance on e.g. national sampling of Swedish soils (Eriksson et al., 1997). However, this involves rather rough generalisations and smoothing of results with little scope to take account of the existing spatial variation. By leaving the field scale, one also leaves the management scale and it is difficult to create scenarios that reflect the effects of remedial measures undertaken in agriculture. Sorption parameters In contrast to information on phosphorus status in the soil, there is often a lack of indata for parameters controlling phosphorus sorption in the soil (iron and aluminium concentrations). Here too, national sampling of Swedish soils (Eriksson et al., 1997) is an important source of indata for models. The question is whether these data can be improved through linking the results from the investigations with other information (e.g. soil texture maps or bedrock data) in order to achieve more reliable scaling-up of point information. Model development hampered by inadequate/unsuitable calibration and validation data A number of studies (Ulén, 1995b; Grant et al., 1996; Djodjic et al., 2000) have demonstrated the episodic character of phosphorus losses, which is commented on earlier in this report. In other words, the main proportion of phosphorus supply from arable land to watercourses occurs in a few short-duration events. Manual sampling for determination of water quality is often carried out once or at best a few times per month and can therefore miss these episodes, and thus phosphorus losses from arable land are often underestimated. This underestimation of phosphorus losses is so extreme that it can conceal other changes in a study area, e.g. the effects of any countermeasures introduced. In such cases it is clearly apparent that environmental monitoring and modelling can derive benefit from each other and that an integrated approach is required to achieve the best outcome. Another environmental monitoring strategy that is seldom used is synoptic sampling, where samples are collected simultaneously from various sampling points within a catchment area to obtain a picture of the spatial variation rather than the temporal variation. In other words, time is replaced by space to obtain a better insight into spatial variation in phosphorus losses.
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Future research needs Despite the fact that there are currently a number of countermeasures that can be introduced to decrease phosphorus losses from agriculture to surface waters and groundwater, as described above, there is still a great need to improve the situation as regards how the issue of future prevention strategies is managed. Much of the problem relates to the fact that we simply do not have a sufficient degree of knowledge about the effectiveness of various preventive measures under Swedish conditions. However, the main reason is that knowledge of phosphorus behaviour in the soil (the form in which it exists in different conditions, how it is transported in different soils and the landscape as a whole, etc.) is often too inadequate for the formulation of goal-orientated and cost-effective preventive measures. The gaps in knowledge have been highlighted previously in this report, but a number of complementary comments are presented here in concentrated form, together with a number of other research issues that we consider to be in urgent need of research in the near future. Although this summary is relatively comprehensive, there are probably further issues that need to be identified in order for us to achieve more efficient use of a finite resource and thereby lower phosphorus losses from agriculture. Future research needs on the behaviour of phosphorus in the soil/plant system are of course not only linked to the need to develop new measures to prevent phosphorus losses, but also to the question of phosphorus as an efficient production resource. In addition, in the future we will probably rely more on mathematical simulation models to make assessments regarding levels of losses. This also demands increased knowledge of phosphorus behaviour in the soil. The examples given below are therefore urgent research issues in many respects. Availability and solubility of phosphorus in the soil Studies on availability of organic phosphorus compounds for transport and breakdown Individual studies in the United Kingdom, Australia and Switzerland have provided an indication of the role of organic phosphorus in turnover and transport. Studies in Scandinavia, the USA and Canada have all yielded results indicating that phosphorus bound to plant material is an important factor in phosphorus mobilisation from agricultural soil. Fundamental research is needed to develop methods of using green manure, green fallow and ley without incurring high leaching. Such studies should also cover the maintenance of buffer zones (vegetation filters) along watercourses, since Finnish studies have shown that the plant material can increase phosphorus additions to watercourses in some cases. Determination of mineralisation potential, and thus bioavailability of both particulate and dissolved organic phosphorus in the soil liquid phase as a function of crop and harvest residues (humus quality), inorganic fertilisers (P and N content), and organic fertilisers (content of various P and N forms plus other quality variables), should be carried out using bioassays with different combinations of phosphatases. A research area that still remains largely uninvestigated is the conversion of organic phosphorus compounds and their role in water pollution. The phosphorus in crops and
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roots is bound in organic form and a considerable proportion of the phosphorus in the soil is associated with the organic material, as discussed above. Organic forms of phosphorus should be analysed simultaneously, e.g. with the aid of 31P NMR. Laboratory studies of physical and chemical processes By calculating the degree of phosphorus saturation in soil it is possible to get an indication of its potential to release dissolved orthophosphates. A high degree of phosphorus saturation indicates that there is a risk of high leaching and studies of subsoil samples have confirmed this relationship. However, further study is needed. The soil aggregates should also be studied to get an idea of the ability of phosphorus to bind to particles and the role of organic material as a cohesive agent. Phosphorus fertilisation using sludge In order to complete nutrient cycles, it is desirable to return the phosphorus in sewage sludge from wastewater treatment plants to arable land. Although the supply of sludge to Swedish arable land is modest (0.2 kg P ha-1), energy crops such as Salix are given very large doses in the form of bulk fertilisation for several years’ requirements and energy forest is regarded as a treatment filter. In future agriculture, energy production is predicted to become increasingly important and phosphorus fertilisation of energy crops and the resulting risk of leaching should be evaluated. As for other phosphorus leaching, such studies should be carried out on several scales – lysimeters, experimental plots, experimental fields and catchment areas. Long- and short-term changes in bioavailability and solubility of soil phosphorus Detailed studies of the dynamics of various forms of phosphorus in the soil are lacking, especially in relation to the aforementioned determination methods used in Sweden (P-AL, P-HCl, P-CaCl2). The bioavailable fraction of total phosphorus in the soil is considered to be quite stable, but there is little knowledge of how it is affected during or a short time after fertilisation and how it varies during the year. In view of the episodic character of phosphorus losses and the significance of this in designing appropriate preventive measures, it is of the utmost importance that the dynamics of phosphorus forms in both the short term (days, weeks) and the long term (years, decades) can be taken into account. This is also particularly important for modelling of phosphorus losses, since the models often attempt to describe long periods of time. Examples of other issues that can be linked to the solubility of phosphorus in the soil include how phosphorus mobilisation occurs during the winter, the effects of frost and freezing/thawing processes, and the differences between organic and mineral fertilisers. Precision cropping The existing legislation regarding livestock density is a good and farsighted foundation for preventing overfertilisation of arable land. However, the legislation does not govern the apportionment at farm level or in individual fields. By linking the results from laboratory experiments and soil characterisation, e.g. the degree of phosphorus saturation at field level and the application of phosphorus fertiliser with the help of maps and GPS, it is possible to fertilise with precision and on the basis of more accurate predicted yields. In the long term, more uniform phosphorus fertilisation will reduce phosphorus losses. More knowledge is needed in this area.
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Crop impact as a phosphorus filter Regarding suitable crops for mining, there is a lack of efficient catch crops for phosphorus that can rapidly establish a deep root system in the autumn and increase the capacity of the soil to act as a filter and minimise phosphorus leaching during the winter. Autumn-sown crops with deep roots can manage without any phosphorus fertiliser in many cases but do not have time to establish an efficient root filter. There is also a lack of knowledge on how individual crops affect leaching in relation to the entire crop rotation, and on the extent to which phosphorus can be released from plants damaged by freezing. We also lack knowledge on whether there are decisive differences between cut and newly emerged fresh plant material and how plants translocate phosphorus down to the roots as they mature. Environmental monitoring of agricultural land The current soil sample archives provide a resource for soil characterisation of properties important for phosphorus leaching. Using e.g. NIR (Near Infrared Spectroscopy) and improved statistical methods (PLS), it is possible to create statistical models for prediction of soil properties important for phosphorus mobilisation. Transport of phosphorus from arable land to watercourses Water flows in soil and in the landscape Phosphorus, which is a rather immobile element, is generally bound in the soil. However, rapid water flows in the soil profile (macropore flow), surface runoff, or in the landscape decrease the contact time between soil particles and phosphorus, and bypass the buffering capacity of the soil. The path of water in the soil and in the landscape is therefore also completely decisive for phosphorus transport and losses, and must be described in an accurate way in order to act as a basis for development of efficient countermeasures. As mentioned above, the models that describe phosphorus transport are often successful in producing transport values that agree well with measured values in the outlet from a field or catchment area, i.e. the models generally succeed in capturing variations in time in a satisfactory way. However, even if the total sum agrees well, the underlying sub-totals may be erroneous, with over- and under-compensations that balance each other out. Therefore the spatial variation that is very important in allowing countermeasures to be proposed, applied and evaluated is often not captured. Errors in apportionment between surface water and infiltration, which are not visible in the end results for either water transport or phosphorus losses, can e.g. be reproduced further in a recommendation for a countermeasure. Thus for example overestimated surface runoff can lead to a recommendation for buffer zones (vegetation filters) as an appropriate preventive measures. If in reality water infiltrates and is transported rapidly via macropores, a buffer zone will have a limited effect. There are a number of other important issues relating to this. How does soil infiltration capacity vary during the year, how can soil structure and structural stability be described in a more physically precise way and how are they affected by soil compaction and tillage? The answers to such questions are very important when producing recommendations for restricting phosphorus losses. Transport mechanisms and leaching dynamics of particulate and dissolved organic phosphorus in relation to total phosphorus in the soil solution should also be investigated. The main question is whether transport of organic phosphorus is
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controlled by the chemical properties of the dissolved organic carbon or whether it is linked more directly to the constituent forms of organic phosphorus. Particularly important issues as regards leaching of organic phosphorus include bioavailability, transport mechanisms and the leaching dynamics of inositol phosphates, partly due to the high quantitative contribution of these compounds to the organic phosphorus pool in the soil and partly because there are a range of different opinions concerning both bioavailability and transport mechanisms. Within hydrology, there has been comprehensive study of so-called variable source areas (VSA), i.e. parts of a soil profile/field/catchment area that are hydrologically active (Hewlett & Hibbert, 1967; Frankenberger, 1999). The majority of mobilised nutrients come from these areas. In the formulation of action plans, the areas lying closest to the watercourse have long been prioritised, but in modelling of phosphorus transport they have often been overlooked. Another important question is how drainage affects the scope of VSA and phosphorus transport in a landscape perspective. What is the role of groundwater as a carrier and/or dilution factor? In order to understand and describe these processes, there is a need for more spatially distributed environmental monitoring (water flow and water quality) and modelling. In other words, a stepwise scaling-up (field – sub-catchment area – catchment area) and studies of important processes within these ‘nested watersheds’ are necessary to interlink the hydrology, nutrient transport and retention from source to recipient. To achieve this, there will have to be a number of demonstration areas in which environmental monitoring occurs on several scales (lysimeter – experimental plot – field – sub-catchment area – catchment area). Effects of climate Sweden has a distinctive climate, particularly as regards temperature. At these latitudes, there are only five months of the year in which mean daily temperature exceeds 10 oC, and there are several months with temperatures below freezing. This is very important in phosphorus mobilisation and transport from fields. Thus Sweden cannot rely on field results produced in other parts of Europe and North America. We need to investigate more closely how climate change affects diffuse phosphorus losses to surface waters and groundwater. Private wastewater systems and source apportionment Many studies at catchment area scale identify private wastewater systems as a large source of phosphorus, especially in calculations of gross loads. However, the contributions from private wastewater outlets have seldom been measured and studied on a wider scale to distinguish their importance in relation to their position in the landscape and their connections with watercourses. Many studies show that phosphorus losses can be much less than those often used as standard values (Gold & Sims, 2000). Therefore it is important to test many of these assumptions used today in order to obtain a clearer picture of the importance of private wastewater systems. Catchment areas with many private wastewater systems and with low contributions from other sources should be studied to improve the quantification, dynamics and spatial variability in phosphorus losses from private wastewater systems. Field drains According to Danish studies of small watercourses (Laubel, 2004), erosion of drain banks contributed approximately tenfold more sediment (suspended soil material)
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Sedimentation tank
Sedimentation pond
Lime cassette
Lime cassette
Fig. 9. Diagram of a phosphorus trap in a runoff drain: Particle sedimentation followed by P-precipitation by lime cassette (upper diagram). Conversion of wetland to a phosphorus trap: Particle sedimentation followed by P-precipitation by lime cassette (lower diagram).
than erosion of arable land. In view of the importance of erosion for phosphorus transport, it is necessary to determine the importance of erosion from drains under Swedish conditions, which affect prevention strategies. In a modelling context, this source has hitherto been overlooked, even though a number of models include the potential to model certain processes controlling erosion in watercourses. However, there is little scope to calibrate models and validate results due to a lack of empirical data. Phosphorus traps As described above, wetlands can make a considerable contribution towards decreasing phosphorus losses from arable land, but there are a range of questions regarding their efficiency that have to be investigated. How large must the wetland be in relation to the drained area, what water flow is acceptable for sedimentation to function satisfactorily, etc.? These types of question need to be answered. One can also conceive of more technical solutions based on the same principle, namely installation of a number of sedimentation tanks in succession in a drain to which drainage water is conducted (Figure 9). These tanks can then be emptied when necessary. Before the purified water runs out into a recipient, it can be diverted through a lime filter that captures any dissolved phosphorus. This creates a treatment system for arable land.
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Lime filters A lime filter drain is described in a previous section as a method of decreasing phosphorus losses from arable land.
Liming along drains (photo: Barbro Ulén)
There are probably other situations where application of burnt lime can decrease losses of phosphorus, for example spreading around surface water drainage wells and along the edges of open drains. However, such applications have been poorly tested in Sweden. Liming in conjunction with installation of surface water drainage wells is particularly interesting since such installations are frequent. Lime sand filters on areas with high surface runoff containing high phosphorus concentrations should also be investigated, particularly on acid soils.
Concluding comments Phosphorus is involved in a range of inorganic and organic compounds and participates in biological, chemical and physical processes. The element occurs in dissolved form, as small and large colloids, and in larger particles and aggregates. More complete knowledge of all these forms and how they behave in the soil and water environment is needed to counteract undesirable environmental effects. In other words there is a need for a concerted effort at a relatively basic level to produce research results that can form the basis for efficient prevention work leading to decreased phosphorus losses from agriculture in the future. Furthermore, no direct comparisons can be made with nitrogen losses, for which there are already a number of effective and well-tested prevention strategies. Phosphorus behaviour in the soil/plant system is simply much more complicated, as discussed at length in this report. The recycling of phosphorus to agricultural soil currently occurs through manure and crop residues, i.e. as organic material and not mineral fertiliser. There is also evidence to show that the phosphorus in manure is more mobile than that in mineral fertiliser. Of the measures introduced to date to decrease phosphorus losses, a restriction of
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manure doses based on the phosphorus concentration in the soil has been the most effective, at least in the short term. Many of the problems with manure have decreased in recent years due to extensive regulation of storage and spreading. However, the problem will never be completely solved as long as manure continues to be applied to soils in the high P-AL classes IV and V. Research into manure treatment at farm level by both simple and more complicated methods should be prioritised so that the manure can be transported at a reasonable price, as should technology for spraying and mobile grazing (moving animals around). All phosphorus fertilisers should be applied in such a way that the phosphorus has as good soil contact as possible while at the same time being available to the crop. There is scope here for further technological development and for the use of precision cropping. Leaching of phosphorus through the soil profile seems to be the most important loss process in large areas of Sweden. This means that the factors with the greatest impact on phosphorus losses are the intrinsic characteristics of the soil: soil hydrology, soil texture and soil chemistry in the entire soil profile down to drainage depth. Soil hydrology can be affected by surface water management through good drainage and tillage to produce good, uniform infiltration into the soil, while all forms of channel flow must be avoided. However, more knowledge is needed here concerning e.g. the effects of liming, choice of crop and soil tillage on hydrology and soil chemistry. Appropriate preventive measures in future must be adapted to the local soil type and cropping system and to the sensitivity of the local recipient waters. Functioning strategies and sufficiently advanced tools for this local adaptation of preventive measures within agriculture are currently lacking but need to be developed.
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7KLVLV)22' FOOD 21 is an interdisciplinary Research Program dealing with issues of a sustainable food chain, from production to consumption. The most important goals are to provide suggestions for solutions concerning the weak links in Swedish agriculture and food production. The consumers should feel comfortable about food quality and production methods when purchasing food. A set of objectives for sustainability has been developed concerning crop production, animal husbandry, product quality, consumers and producers to encompass research and evaluation of new production methods and means. The Foundation for Strategic Environmental Research, MISTRA, is financing the Program over an eleven-year period starting in 1997. Some 75 senior researchers are involved and up to now twenty-six doctoral theses have been launched. The Swedish University of Agricultural Sciences is the centre of activities but research was also conducted at the universities in Gothenburg, Umeå, Lund and Uppsala. Over the last three years FOOD 21 is organised as a synthesis platform. Subjects of special interest for farmers or other food chain actors are analysed in a number of synthesis projects. Scientists and food chain stakeholders get together in subject oriented theme groups analyzing LVVXHVVXFKDV³VXVWDLQDEOHSHVWPDQDJHPHQWLQFURSSURGXFWLRQ´DQG³UHSODFLQJLPSRUWHG SURWHLQIHHGE\KRPHPDGH´HWF%XWDOVRRWKHUHYDOXDWLRQVDUHGHDOWZLWKZLWKLQWKLV synthesis platform. In this report is presented the current state of knowledge and research needs concerning preventive measures to reduce phosphorus loadings from agricultural production systems, with special emphasis on reducing eutrRSKLFDWLRQSUREOHPVLQWKH%DOWLF6HD
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