Nutrient Cycling in Agroecosystems 69: 167–184, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
167
Assessment of phosphorus leaching losses from a free draining grassland soil Gurpal S. Toor1,*, Leo M. Condron2, Hong J. Di2, Keith C. Cameron2 and J. Thomas Sims1 1
Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716, USA; 2Centre for Soil and Environmental Quality, P.O. Box 84, Lincoln University, Canterbury 8150, New Zealand; *Author for correspondence (fax: ++ 1-302-831-0605; e-mail:
[email protected]) Received 14 July 2003; accepted in revised form 16 February 2004
Key words: Lysimetry, P balance, P leaching, Reactive P, Unreactive P
Abstract Intact soil monoliths 共70 cm deep, 50 cm diameter兲, collected from a free draining Lismore silt loam soil 共Udic Haplustept兲 under grassland, were used to evaluate phosphorus 共P兲 leaching for two years. The objective of the study was to investigate the effect of the application of mineral P fertiliser 共at 45 or 90 kg P ha–1 y–1兲 and/or farm dairy effluent 共FDE兲 共30 to 60 kg P ha–1 y–1兲 on P losses by leaching. Annual mean total P 共TP兲 concentrations and losses were higher from the treatments that received both FDE and P fertiliser 共203–429 g L–1; 1.4–2.5 kg ha–1兲 compared with P fertiliser alone 共77–151 g L–1; 0.6–1.3 kg ha–1兲. The form of applied P influenced the pattern of P forms leached. For example, significantly higher P losses in different P forms were observed for the combined mineral P fertiliser and FDE treatment 共P45/FDE200兲 than fertiliser alone 共P90/N200/U兲. This is due to the inclusion of liquid FDE in the former treatment although the total P inputs were similar for both treatments. This illustrates the potential of these soils to adsorb soluble inorganic P applied from mineral P fertiliser, while FDE contained unreactive P forms that were mobile in the soil profile. There was a distinct pattern of P forms leached in the following order: particulate unreactive P 共PUP: 40–70%兲 ⬎ dissolved unreactive P 共DUP: 14–53%兲 ⬎ particulate reactive P 共PRP: 5–12%兲 ⬎ dissolved reactive P 共DRP: 1–11%兲. Results also suggest that changing the irrigation method from flood to spray may be the most effective means to reduce P loss in these stony, free-draining soils. Abbreviations: P – phosphorus; TP – total P; DRP – dissolved reactive P; PRP – particulate reactive P; DUP – dissolved unreactive P; PUP – particulate unreactive P; FDE – farm dairy effluent; N – nitrogen.
Introduction Phosphorus 共P兲 accumulation in soils and the associated contamination of water bodies has become an environmental concern in many regions of the world 共Sims et al. 1998; Sharpley et al. 2001; Withers et al. 2001; Toor et al. 2003; Condron 2004兲. In the past, point-source pollution from sewage and industry was considered as the major reason for rising P concentrations in surface waters 共Heathwaite et al. 2000兲. However, with the reduction of point-source P by im-
proved water treatment, the relative contribution of diffuse P losses from agricultural land has become increasingly apparent 共Sharpley et al. 2001兲. In most intensive livestock areas, the continued inputs of fertiliser and manure P exceed output, resulting in P accumulation in the soil, and this increases the potential for water contamination 共Heathwaite et al. 1999; Hooda et al. 2000; Withers et al. 2001兲. There is an increasing body of knowledge that indicates P losses from agricultural land cause accelerated algae and aquatic plant growth in lakes, rivers, and streams
168 共Sharpley et al. 1999; Toor et al. 2003; Condron 2004兲. Pastoral agriculture is the predominant form of land use in New Zealand. In the last decade there has been a major shift in this land use, with significant numbers of sheep and mixed cropping farms being converted to more profitable and nutrient intensive dairy farms. The change in land use pattern has resulted in application of increased P inputs which are required to achieve and maintain higher levels of pasture production. For example, the annual maintenance P fertiliser requirement for sheep production under border dyke 共flood兲 irrigation in Canterbury, New Zealand, is 15–20 kg P ha–1, compared with 35–50 kg P ha–1 for dairying. In addition to higher inputs of mineral P fertiliser, excreta from dairy animals is returned to land, which includes farm dairy effluent 共FDE兲. Farm dairy effluent is a mixture of feces, urine and water generated from the washing of the milking parlor. There is a particular concern that the addition of high levels of mineral fertiliser, together with returns of nutrients in animal excreta and FDE, has the potential to increase soil nutrient 共mainly N and P兲 status and contaminate ground water, as many of the new dairy farms in Canterbury are sited on shallow, free-draining soils under flood or spray irrigation. Although knowledge of the region’s water resources in terms of N pollution has improved greatly in recent years, the understanding of P transfer from landscapes is limited 共Cameron et al. 2002; Toor et al. 2003, 2004兲. While there is some information of P loss by overland flow 共runoff兲 from pastures in New Zealand 共Gillingham and Thorrold 2000兲, little data is available on the impacts of FDE and/or P fertiliser on the amounts and forms of P in leachate from grassland soils under irrigation. The environmentally sustainable management of dairy farming systems requires detailed information about the effect of increased P inputs associated with dairy farming on the amounts, forms and mobility of P, so that compatible production and water quality goals can be met. The main objective of this study was to assess the effect of applying mineral P fertiliser and FDE on the concentrations, amounts, and physico-chemical forms of P in leachate from a free-draining irrigated pasture soil.
Materials and methods Lysimeter collection Intact monolith lysimeters 共50 cm diameter, 70 cm depth兲 of a free-draining Lismore silt loam soil 共Udic Haplustept兲 were taken from a permanent grassland site in Canterbury, New Zealand 共171°40⬘ E, 44°45⬘ S兲 which had been managed under flood irrigation with P fertiliser inputs over 50 years 共0–7.5 cm: pH 5.9, 36.5 g C kg–1, 3.53 g N kg–1, 1097 mg TP kg–1, 53 mg Olsen P kg–1兲. The lysimeters were collected according to the method described by Cameron et al. 共1992兲. In brief, each lysimeter consisted of a steel cylindrical casing which was pushed into the soil to collect an undisturbed soil monolith. A cutting ring at the base of the cylinder created a 0.5-cm annular gap between the soil monolith and the casing, which was filled with liquefied petroleum jelly. Once the jelly solidified, it formed an effective seal to prevent edge flow. The bottom 4 cm of the soil profile was removed from the cylinder and replaced with a washed gravel and sand mixture to create a free-draining situation similar to that of the field soil. Treatment application and management Eight treatments were established in a completely randomized block design with four replicates 共Table 1兲. The lysimeters received P fertiliser, FDE, N fertiliser 共urea兲, cow urine, and a carbon-rich organic amendment 共sawdust, SD兲 in different combinations. Phosphorus fertiliser was applied at 45 kg P ha–1 共superphosphate兲 and 90 kg P ha–1 共superphosphate ⫹ triple superphosphate兲 in two split applications during April and November each year. Urea 共at 200 kg N ha–1兲 and FDE 共at 200 or 400 kg N ha–1; 30 to 60 kg P ha–1兲 were applied in four split applications in February, May, August and November each year. The control treatment received P fertiliser at 45 kg P ha–1 y–1, as this is a common standard practice followed by farmers in this region. The combination of P fertiliser at 90 kg P ha–1 y–1 with N fertiliser or P fertiliser at 45 kg P ha–1 y–1 with FDE at 400 kg N ha–1 y–1 represents a worst case scenario for grazed pasture in terms of excessive N and P inputs. The maximum recommended annual N input in New Zealand through effluent irrigation is 150–200 kg N ha–1 y–1 共Selvarajah 1996兲. As this study was part of a large experiment on N losses from dairy pastures, some amendments 共such as cow urine兲 were included
86 82 104 82 From 10 November 2000, some of the above treatments were changed to: 1 P90/N200/U/N inhibitor; 2P45/N200/U 共spray irrigation兲; 3P45/FDE200/U; 4P45/N200/U/glucose.
– – – – 45 45 45 45
– – – 30 200 400 400 400 – – 1000 1000
75 105 105 105
45 45 90 90 – 200 200 200
Non-FDE treatments P45 P45/N200/U P90/N200/U P90/N200/U/SD1 FDE treatments P45/FDE200 P45/FDE4002 P45/FDE400/U3 P45/FDE400/U/SD4
45 45 90 90
– – – 30 – – – – – 1000 1000 1000
45 45 90 90
2000–2001 1999–2000
FDE 共kg N ha–1兲 Sawdust 共t ha–1兲 Urine 共kg N ha–1兲 N fertiliser 共kg N ha–1兲 P fertiliser 共kg P ha–1兲 Treatment
Table 1. Annual treatments included in the lysimeter experiment.
Total P inputs from fertiliser and FDE 共kg P ha–1兲
169 mainly to study N loss following their application, as cow urine contains only traces of P 共Haynes and Williams 1993兲. Urine was applied at 1000 kg N ha–1 only in April each year to simulate a urine patch. Sawdust was applied as a means of improving soil quality against pugging and to increase the C:N:P ratio, thereby increasing N and P immobilization and reducing N and P leaching. During the first year, sawdust was applied at 30 t ha–1 on March 19, 1999. The following application was made on March 27, 2000 at a reduced rate of 20 t ha–1. However, no sawdust was applied thereafter as results from the previous years indicated no effect of sawdust on P leaching. Fresh representative samples of the FDE and urine were collected from the Lincoln University dairy farm prior to each application. This farm has Holstein Friesian milking cows that were fed on a diet of fresh ryegrass-white clover mixed pasture and maize silage. The FDE and urine samples were then analyzed for total N 共Ebina et al. 1983兲 and the calculated amount was applied to respective lysimeters. An equivalent volume of water was applied to the non-FDE and non-urine treatments to maintain similar levels of water addition. From November 2000 共cumulative drainage: 1200 mm兲, some of the treatments in the experiment were changed 共see Table 1兲. However, for the ease of the reader the previous treatment names are used throughout the study period. Leachate collection and analyses Leachate was collected from the lysimeters following irrigation and/or significant rainfall on 51 occasions that occurred during two years 共1999–2000: 26; 2000–2001: 25兲. Leachate volume was recorded and a sub-sample was retained for analyses of different P fractions. Leachate was analysed for dissolved reactive P 共DRP兲 and total dissolved P 共TDP兲 in a filtered 共 ⬍ 0.45 m兲 sample, and total reactive P 共TRP兲 and total P 共TP兲 in an unfiltered sample within 48 of collection using malachite green colorimetry 共van Veldhoven and Mannaerts 1987; Ohno and Zibilski 1991兲. For the TDP and TP analysis, samples were digested with a mixture of K2S2O8 and NaOH 共Ebina et al. 1983兲. The difference in P concentration between TP and TRP was taken to be the concentration of total unreactive P 共TUP兲. Similarly, the other P fractions were calculated as: dissolved unreactive P 共DUP兲 ⫽ TDP ⫺ DRP; particulate unreactive P 共PUP兲 ⫽
170 TUP ⫺ DUP; particulate reactive P 共PRP兲 ⫽ TRP ⫺ DRP. Total P concentrations in FDE were analysed by digesting samples with a mixture of concentrated HNO3-HClO4 prior to P analysis with the molybdate blue method 共Murphy and Riley 1962兲. Herbage analysis The lysimeters were managed by cutting the pasture to cattle grazing height 共20–30 mm兲 with electric grass shears when the pasture height was about 250– 300 mm. Over two years, herbage was collected and analysed for 16 times for total P concentration using HNO3-HClO4 digestion followed by vanadomolybdate colorimetry 共Olsen and Sommers 1982兲, and P uptake was calculated by multiplying the P concentration with the dry matter yield at each sampling time, and an average value was generated for both years. Statistical analysis Descriptive statistics and one-way ANOVA were carried out using Genstat 4.2, 5th Edition 共Lawes Agricultural Trust, Rothamsted, UK兲 to calculate means and standard errors, and to test for significant differences between means.
Results Water inputs and output for the lysimeters The amount of irrigation water applied was very similar 共736 mm兲 during both years; however, rainfall was higher during 1999–2000 共826 mm兲 compared with 2000–2001 共650 mm兲. Rainfall was evenly distributed during 1999–2000, except for the high rainfall recorded during June–July 1999 共287 mm兲 and January–April 2000 共258 mm兲 共Figure 1兲. On the other hand, low rainfall occurred during June–July 2000 共57 mm兲 and January–April 2001 共62 mm兲, although higher rainfall 共295 mm兲 was recorded between September and November compared with 1999–2000 共164 mm兲. A total of 190 mm of FDE was added during 2000–2001 compared with only 65 mm of FDE during 1999–2000. The drainage pattern from lysimeters was consistent with the water input. Generally greater than 50% of the applied water was discharged in drainage from all treatments, except for the P45/FDE400 treatment
where only 5% of applied water was collected from lysimeters during 2000–2001 共November to April兲, due to the conversion of this treatment from flood to spray irrigation. Change of irrigation method from flood to spray irrigation was attempted to reduce the volume of drainage and to investigate its impact on P leaching losses. Concentrations of phosphorus in leachate during individual drainage events Concentrations of TP varied from 200 to 300 g L–1 at the beginning of the experiment 共0–200 mm cumulative drainage兲 共Figure 2兲. Thereafter, TP was consistently lower than 100 g L–1 for the non-FDE treatments and between 100 and 300 g L–1 for the FDE treatments. An exception to this was higher TP during two drainage events, 300 mm 共400–500 g L–1兲 and 1200 mm 共400–1000 g L–1兲 for the nonFDE treatments. This is due to the coincidence of P fertiliser application prior to irrigation events and highlights the susceptibility of this soil to incidental P losses. For FDE treatments, concentrations ranging from 400 to 2500 g TP L–1 were frequently observed following FDE applications. The variation in TP between drainage events was more pronounced in leachate from FDE than non-FDE treatments. Discontinuing FDE application to P45/FDE400 and P45/ FDE400/U/SD treatments after 1200 mm cumulative drainage resulted in lower TP concentrations. Most of the TP in the leachate was present in unreactive 共DUP, PUP兲 rather than reactive forms 共DRP, PRP兲. Reactive P is thought to consist of orthophosphate, while unreactive P may contain organic and some condensed forms of P 共Ron Vaz et al. 1993兲. In our previous report, we observed that most of the unreactive P in leachate from Lismore soil is composed of mono- and di-ester forms 共Toor et al. 2003兲. Among reactive P forms, a dominance of PRP over DRP was noted, particularly for the FDE treatments 共Figures 3 and 4兲. Concentrations of PRP and DRP were consistently lower than 20 g L–1 for the P45 treatment, while for the P90 treatments these concentrations were slightly higher 共up to 40 g L–1兲. For FDE treatments, PRP concentrations were consistently higher 共50–400 g L–1兲 following FDE applications compared with DRP concentrations which were only higher after 1200 mm drainage 共Figure 3兲. In the unreactive P pool, most of the P in leachate was present as PUP rather than DUP 共Figures 5 and 6兲. At the beginning of the trial 共0–200 mm cumula-
171
Figure 1. Water inputs and drainage from lysimeters 共average of all treatments兲 during 1999–2000 and 2000–2001.
172
Figure 2. Mean concentrations 共g L–1兲 of TP determined in leachate collected from different treatments during individual drainage events over two years 共May 1999–April 2001兲 共standard errors of the means are shown by vertical bars兲.
173
Figure 3. Mean concentrations 共g L–1兲 of DRP determined in leachate collected from different treatments during individual drainage events over two years 共May 1999–April 2001兲 共standard errors of the means are shown by vertical bars兲.
174
Figure 4. Mean concentrations 共g L–1兲 of PRP determined in leachate collected from different treatments during individual drainage events over two years 共May 1999–April 2001兲 共standard errors of the means are shown by vertical bars兲.
175
Figure 5. Mean concentrations 共g L–1兲 of DUP determined in leachate collected from different treatments during individual drainage events over two years 共May 1999–April 2001兲 共standard errors of the means are shown by vertical bars兲.
176
Figure 6. Mean concentrations 共g L–1兲 of PUP determined in leachate collected from different treatments during individual drainage events over two years 共May 1999–April 2001兲 共standard errors of the means are shown by vertical bars兲.
177 Table 2. Annual mean concentrations 共g L–1兲 of DRP, PRP, DUP, PUP and TP determined in leachate collected from different treatments during 1999–2000 and 2000–2001. Treatments
1999–2000
2000–2001
Reactive P
Unreactive P
DRP
PRP
DUP
5 共0.5兲 8 共0.7兲 11 共1.5兲 7 共0.8兲
53 73 63 60
TP
Reactive P
Unreactive P
DRP
PRP
DUP
PUP
111 共13.4兲 138 共10.5兲 146 共12.2兲 121 共9.5兲
7 共1.1兲 11 共4.5兲 8 共3.3兲 16 共6.4兲
5 共0.9兲 12 共1.6兲 12 共1.8兲 14 共2.4兲
21 共1.8兲 30 共6.8兲 27 共3.7兲 41 共11.3兲
44 53 72 80
14 共3.6兲b 6 共0.7兲c 24 共7.6兲b 11 共2.1兲c
37 共8.6兲
59 共7.5兲
261 共51.7兲
371 共67.1兲
13 共6.4兲 34 共6.0兲
29 共9.9兲 39 共5.9兲
105 共54.5兲 183 共36.5兲
153 共70.8兲 280 共48.5兲
30 共15.0兲 51 共12.2兲 13 共5.5兲 22 共3.9兲
38 共11.6兲 72 共20.0兲 19 共4.5兲 40 共4.1兲
203 共104.7兲 282 共131.3兲
24 共10.9兲
25 共6.4兲
PUP
Non-FDE treatments P45 P45/N200/U P90/N200/U P90/N200/U/SD
6 2 3 3
FDE treatments P45/FDE200
8 共0.7兲
10 共2.0兲
68 共8.3兲
117 共33.2兲
203 共37.4兲
P45/FDE400
10 共1.2兲
19 共4.6兲
76 共8.5兲
162 共37.7兲
267 共45.1兲
共0.6兲a 共0.2兲 共0.3兲 共0.3兲
共8.5兲 共9.2兲 共8.5兲 共7.7兲
47 55 69 51
共10.3兲 共6.7兲 共8.4兲 共6.4兲
P45/FDE400/U
4 共0.5兲
28 共6.4兲
83 共9.0兲
183 共38.6兲
298 共49.0兲
P45/FDE400/U/SD
4 共0.6兲
27 共5.3兲
81 共8.4兲
179 共35.8兲
291共43.6兲
23 共8.7兲b 2 共0.6兲c 15 共4.2兲b 5 共1.7兲c
TP
共11.3兲 共5.4兲 共16.5兲 共12.0兲
77 共13.2兲 106 共15.5兲 119 共24.0兲 151 共29.8兲
283 共52.0兲
429 共82.3兲
100 共40.2兲 178 共27.3兲
134 共50.2兲 255 共33.1兲
207 共85.4兲
261 共102.7兲
a Standard error of the means; bmeans of 25 drainage events; cmeans of 13 drainage events 共before discontinuing FDE application in November 2000兲.
tive drainage兲, concentrations of DUP were higher 共175–250 g L–1兲 than those of PUP 共 ⬍ 100 g L–1兲 for all treatments. Thereafter, DUP concentrations were generally lower than 50 g L–1 for non-FDE treatments. The concentrations of DUP were similar for FDE200 and FDE400 treatments 共 ⬍ 250 g L–1兲 except for P45/FDE400/U, where DUP concentrations were greater than 600 g L–1 during one drainage event 共1250 mm兲. For FDE treatments, PUP concentrations up to 1900 g L–1 were noted following FDE application. The discontinuation of FDE in P45/FDE400 and P45/FDE400/U/SD treatments in November 2000 共cumulative drainage: 1200 mm兲 reduced PUP concentrations to less than 250 g L–1. For FDE treatments, concentrations of PUP and DUP were higher and the variation between drainage events was also much more pronounced than for nonFDE treatments. Annual average phosphorus concentrations A summary of mean annual TP concentrations during 1999–2000 and 2000–2001 for different treatments is
presented in Table 2. The data following change of treatments during 2000–2001 are also shown in Table 2 to compare the relative P losses in different P forms. During both years, TP concentrations were lowest from the P45 treatment 共111 g L–1 in 1999–2000, 77 g L–1 in 2000–2001兲 and highest from the P45/ FDE400/U treatment 共298 g L–1 in 1999–2000, 429 g L–1 in 2000–2001兲. Concentrations of TP were significantly 共P ⬍ 0.05兲 higher for the P90 treatments compared to the P45 treatment only during 2000– 2001, while TP was significantly 共P ⬍ 0.05兲 higher for all FDE treatments compared with the P45 treatment during both years. For FDE treatments, during 1999–2000 PUP concentrations were 117–183 g L–1, followed by DUP 共68–83 g L–1兲, PRP 共10–28 g L–1兲 and DRP 共4–10 g L–1兲. Similarly, for 2000–2001, PUP concentrations were highest at 178–283 g L–1, followed by DUP, PRP, and DRP. Non-FDE treatments showed a similar pattern of P loss during both years. Within each year, concentrations of PUP and PRP were significantly higher for the FDE than for P45 treatment, while DUP concentrations were significantly
178 Table 3. Percent of total P concentration distributed among different P forms during 1999–2000 and 2000–2001. Treatments
1999–2000
2000–2001
Reactive P DRP Non-FDE treatments P45 P45/N200/U P90/N200/U P90/N200/U/SD FDE treatments P45/FDE200 P45/FDE400 P45/FDE400/U P45/FDE400/U/SD
PRP
Unreactive P
Reactive P
Unreactive P
DUP
PUP
DRP
PRP
DUP
PUP
5 1 2 2
5 6 8 6
48 53 43 50
42 40 47 42
9 11 7 11
7 11 10 9
27 28 23 27
57 50 60 53
4 4 1 1
5 7 10 9
33 28 28 28
58 61 61 62
4 9 5 6
10 12 12 9
16 14 17 15
70 65 66 70
共P ⬍ 0.05兲 higher for FDE than P45 treatment only during 2000–2001. Unreactive P 共DUP, PUP兲 represented 78–93% of TP, compared with 7–28% as reactive P 共DRP, PRP兲 共Table 3兲. Most of the TP was present as PUP rather than DUP during both years. However, for the nonFDE treatments during 1999–2000, DUP concentrations 共43–53%兲 were similar to those of PUP 共40–47%兲, while for FDE treatments, PUP concentrations were two-fold higher than those of DUP. In 2000–2001, PUP concentrations were two-fold higher than those of DUP for non-FDE and four-fold higher for FDE treatments. Phosphorus concentrations as DRP were higher during 2000–2001 共4–11%兲 than 1999–2000 共 ⬍ 5 %兲.
ment, where PUP loss was significantly 共P ⬍ 0.05兲 higher during 2000–2001 共1.8 kg ha–1兲 compared with 1999–2000 共0.7 kg ha–1兲. After PUP, most of the P loss occurred as DUP. For non-FDE treatments, DUP losses were significantly 共P ⬍ 0.05兲 higher during 1999–2000 共0.4–0.5 kg ha–1兲 than during 2000–2001 共0.2–0.3 kg ha–1兲. For FDE treatments, PRP loss was higher during 2000– 2001 共 ⬎ 0.2 kg ha–1兲 compared with 1999–2000 共0.1–0.20 kg ha–1兲. Within each year, PUP, DUP and PRP losses were significantly 共P ⬍ 0.05兲 higher for P90 and FDE treatments compared with the P45 treatment. However, DRP losses were only significant for the P45/FDE200 treatment during 2000–2001. Phosphorus uptake and budget
Annual amounts of phosphorus leached Annual mean cumulative TP losses in leachate for the non-FDE treatments were 0.9–1.3 kg ha–1 during 1999–2000 compared with 0.6–1.0 kg ha–1 during 2000–2001. The year to year variation of TP losses between the various treatments was only significant for the P90/N200/U and P45/FDE200 treatments. For both years, TP losses were significantly 共P ⬍ 0.05兲 higher for the FDE treatments 共1.4–2.3 kg ha–1 in 1999–2000, 1.5–2.5 kg ha–1 in 2000–2001兲 compared with P45 treatment 共0.9 kg ha–1 in 1999–2000, 0.6 kg ha–1 in 2000–2001兲. Consistent with the concentrations, a similar pattern of P loss was observed for different P forms 共PUP ⬎ DUP ⬎ PRP ⬎ DRP兲 共Figure 7兲. Interestingly, for FDE treatments PUP loss was very similar during 1999–2000 共1.1–1.5 kg ha–1兲 and 2000–2001 共1.1–1.4 kg ha–1兲, except for the P45/FDE200 treat-
Annual P inputs, offtake and net balances for 1999– 2000 and 2000–2001 are presented in Table 4. For each year, FDE400 treatments received the highest P inputs followed by P90, FDE200 and P45 treatments. During both years, P uptake was generally lowest in the P45 treatment compared to other treatments. The highest P surplus was recorded for the FDE400 treatments, followed by P90 and FDE200. In some P45 treatments, P deficit was recorded. As the pasture was damaged in the P45 treatment during 2000–2001, it resulted in lower grass growth and hence lower P uptake 共42 kg P ha–1兲 compared with 1999–2000 共54 kg P ha–1兲, accordingly the P surplus was 2.4 kg P ha–1 in 2000–2001. The lower P uptake from the P90 treatments during 2000–2001 resulted in a higher P surplus 共43–47 kg P ha–1兲. For the FDE200 treatment, P surplus was very similar during both years 共16–20 kg P ha–1兲.
179
Figure 7. Mean cumulative losses 共kg ha–1 y–1兲 of DRP, PRP, DUP and PUP determined in leachate collected from different treatments during 1999–2000 and 2000–2001 共standard errors of the means are shown by vertical bars兲.
Discontinuation of FDE to P45/FDE400 and P45/ FDE400/U/SD treatments resulted in lower P surplus in 2000–2001 共27–30 kg P ha–1兲 compared with 1999–2000 共47–52 kg P ha–1兲. A higher P surplus of 65 kg P ha–1 was noted for the P45/FDE400/U treatment due to lower P uptake in the second year. The calculated TP loss by leaching to 70 cm depth as a percentage of P inputs was less than 3% for all the treatments during both years. During 2000–2001, %
P loss was higher for the FDE treatments 共1.8–2.9%兲 compared with non-FDE treatments 共1.0–1.8%兲. Effect of changing irrigation regime on phosphorus losses Two previously amended FDE treatments 共P45/ FDE400 and P45/FDE400/U/SD兲 were selected to investigate the impact of changing irrigation regime on
180 Table 4. Total P balance 共kg ha–1 y–1兲 calculated for the different treatments during 1999–2000 and 2000–2001. P inputs Fertiliser-P
P offtake FDE-P
Herbage-P
Net P
P loss as % of P inputs
balancea
Leachate-P
1999–2000 Non-FDE treatments P45 P45/N200/U P90/N200/U P90/N200/U/SD FDE treatments P45/FDE200 P45/FDE400 P45/FDE400/U P45/FDE400/U/SD Non-FDE treatments P45 P45/N200/U P90/N200/U P90/N200/U/SD FDE treatments P45/FDE200 P45/FDE400 P45/FDE400/U P45/FDE400/U/SD
45 45 90 90
–b – – –
54 68 65 60
0.9 1.2 1.3 1.1
⫺ 9.9 ⫺ 24.2 23.7 28.9
2.0 2.7 1.4 1.2
45 45 45 45
30 60 60 60
58 56 57 51
1.4 1.8 2.2 2.3 2000–2001
15.6 47.2 45.8 51.7
1.9 1.7 2.1 2.2
45 45 90 90
– – – –
42 57 46 42
0.6 0.8 0.9 1.0
2.4 ⫺ 12.8 43.1 47.0
1.3 1.8 1.0 1.1
45 45 45 45
41 37 59 37
64 51 37 53
2.5 1.5 2.2 2.0
19.5 29.5 64.8 27.0
2.9 1.8 2.1 2.4
Net P balance ⫽ P inputs 共fertiliser-P ⫹ FDE-P兲 – P offtake 共herbage-P ⫹ leachate P兲. bNo application.
a
Table 5. Mean concentrations 共g L–1兲 and losses 共kg ha–1兲 of different P fractions in flood and spray irrigated FDE residual treatments, and P45 treatment during the 2000–2001 irrigation season 共November 2000–April 2001兲. Flood
Spray
P45
Drainage 共mm兲 DRP PRP DUP PUP TP
P45/FDE400/U/SD
a
P45/FDE400a
g L–1
kg ha–1
g L–1
kg ha–1
g L–1
kg ha–1
456 共49兲b 8 5 28 54 95
0.038 0.023 0.132 0.246 0.440
413 共45兲 17 14c 56c 118 205
0.050d 0.058d 0.259d 0.480d 0.847d
45 共5兲 13 22 30 104 169
0.005 0.013 0.009 0.042 0.070
a No FDE was applied during this period 共November 2000–April 2001兲. bFigures in parentheses are % of applied water. cDifferences in concentration 共g L–1兲 between flood 共P45/FDE400/U/SD兲 and spray 共P45/FDE400兲 FDE residual treatments were significant. dDifferences in loss 共kg ha–1兲 between flood 共P45/FDE400/U/SD兲 and spray 共P45/FDE400兲 FDE residual treatments were significant at P ⬍ 0.05.
P leaching. No fresh addition of FDE was made during this period 共November 2000–April 2001兲. The irrigation regime was changed from flood to spray irrigation in the P45/FDE400 treatment, while P45/ FDE400/U/SD remained under flood irrigation. A total of 8 artificial irrigations were applied to the flood irrigated treatment while the spray irrigated treatment received 16 applications, although both treatments received an equal amount of water during this period 共736 mm兲.
A change from flood to spray irrigation resulted in significantly lower losses as reactive and unreactive P in the P45/FDE400 treatment 共Table 5兲. Concentrations of TP were not significantly different between flood 共205 g L–1兲 and spray 共169 g L–1兲 irrigated treatments, but, TP losses were 12-fold higher from the flood 共0.847 kg ha–1兲 than from the spray irrigated treatment 共0.070 kg ha–1兲. This is because only 5% of the applied water was collected from spray irrigated lysimeters compared with flood irrigated lysimeters,
181 where 45% of applied water was received. Interestingly, TP losses were much higher with the flood irrigated P45 treatment 共0.440 kg ha–1兲 than the spray irrigated FDE residual treatment.
Discussion Concentrations and amounts of phosphorus forms in leachate The mean TP concentrations were significantly higher from FDE treatments 共203–429 g L–1兲 than from non-FDE treatments 共77–151 g L–1兲. As critical TP concentrations required for eutrophication vary between 35 and 100 g TP L–1 共Crouzet et al. 1999兲, the TP concentrations observed in this study from all treatments were well above the upper limit suggested for eutrophication. Individual maximum TP concentrations were 1311–2651 g L–1 for FDE treatments compared with 349–1003 g L–1 for non-FDE treatments 共Figure 2兲. This suggests that there is a greater threat to groundwater contamination from the FDE amended soils if this leached P reaches ground or surface waters. These results support the findings of Turner and Haygarth 共2000兲, who found that TP concentrations in leachate from four contrasting soil types, amended with mineral P fertiliser, routinely exceeded 100 g L–1, with maximum concentrations of ⬎ 1000 g L–1. However, it must be noted that the potential for P movement to ground water depends upon the depth of the groundwater, the type of P form 共inorganic/organic兲 entering the water as inorganic P is readily available to aquatic organisms, confinement of ground water, and the amount of water percolating through the soil. Losses of P from the Lismore silt loam soil were equivalent to less than 3% of the applied P either as fertiliser or FDE. Annual losses of P by leaching were found to be greater from treatments receiving both FDE and P fertiliser 共1.4–2.5 kg ha–1兲 compared with P fertiliser alone 共0.6–1.3 kg ha–1兲. Higher TP losses from the FDE treatments were due to the repeated application of P from FDE at regular intervals compared with non-FDE treatments, where fertiliser P was applied at less frequent intervals. In the UK, Turner and Haygarth 共2000兲 reported lower TP losses of ⬍ 0.5 kg ha–1 y–1 in leachate from four grassland soils 共sand to silty clay兲 which received annual P fertiliser of 40 kg P ha–1 共annual rainfall: 1100 mm; drainage: 332–491 mm兲. The higher P losses observed
in the present study compared to Turner and Haygarth 共2000兲 were due to application of higher P inputs in FDE and P fertiliser. Even for the P45 treatment which received fertiliser P at 45 kg P ha–1 y–1, annual TP loss varied between 0.6 and 0.9 kg ha–1 y–1, which was also slightly higher than the losses observed by Turner and Haygarth 共2000兲. This is possibly due to the stony, free-draining nature of the Lismore soil under flood irrigation, which makes it prone to higher P leaching losses compared with the UK system, where there was no irrigation. These results support the previous observations that typical leaching losses of P from agricultural fields seldom exceed 2 kg ha–1 y–1 共Cogger and Duxbury 1984; Haygarth and Jarvis 1999兲. Combined TP losses for the two years were higher from the FDE400 treatments 共 ⬎ 4.3 kg ha–1兲, followed by FDE200 共 ⬎ 3.9 kg ha–1兲, P90 共 ⬎ 2.1 kg ha–1兲 and P45 共 ⬎ 1.5 kg ha–1兲 treatments. In this two-year study, a greater proportion of the TP in leachate was present in unreactive P forms 共PUP: 40-70%, DUP: 14–53%兲, compared with reactive P forms 共PRP: 5–12%, DRP: 1–11%兲. The predominance of unreactive P suggests that these forms of P are composed of organic P species, which are less strongly sorbed onto soil colloids than reactive P 共Frossard et al. 1989兲. We previously reported that the unreactive P forms in the leachate in this study were mainly comprised of orthophosphate monoesters 共67% of TP兲 and diesters 共20% of TP兲 共Toor et al. 2003兲. Concentrations of DRP were similar for the FDE and non-FDE treatments, despite the regular addition of DRP from FDE in the FDE treatments. On the other hand, PRP concentrations were higher from the FDE treatments than from the non-FDE treatments. Lower losses of DRP from FDE treatments confirm the relative ability of these soils to quickly attenuate P present as DRP 共Sinaj et al. 2002兲; however, PRP being in the particulate phase leached from the soil. The higher fixation of reactive P compared with unreactive P highlights the importance of P fixation in the mitigation of P loss 共without preferential flow兲 and the potential contribution of unreactive P forms to the overall P loss. Effect of applied phosphorus form on phosphorus leaching losses Concentrations of TP were very similar for the P45 and P90 treatments, although P90 treatments received twice as much fertiliser P 共90 kg P ha–1 y–1兲 compared
182 with P45 treatments 共45 kg P ha–1 y–1兲. This illustrates the potential of these soils to adsorb P present in inorganic forms. In our previous report 共Sinaj et al. 2002兲, we showed that these soils have a high capacity to adsorb P due to the presence of increased amounts of Fe and Al in the lower depths. However, over the long term, if P fixation sites are saturated, there is a potential for higher losses in inorganic forms of P. On the other hand, TP concentrations and losses were substantially higher from the P45/FDE200 treatment compared with P90 treatments, although P45/FDE200 treatment received similar P inputs as P90 treatments. This indicates that the form of the applied P is an important factor in determining P loss. Of the total P in FDE, 67% was in the reactive forms 共total dissolved P: 60%兲 共Toor et al. 2004兲 and was applied in a liquid form, compared with 100% inorganic P in fertiliser, which was applied in solid form. This difference in the forms of P applied was probably responsible for higher TP losses from P45/FDE200 treatment, as P was already present in the solution phase, than from P90 treatments. A higher P uptake and P loss from the combined N and P treatments 共mineral P fertiliser or FDE兲 compared to the P45 treatment was possibly due to the synergistic effect of N on the stimulation of microbial activity. This increased the mineralization of organic P, thereby resulting in higher P in soil solution which was available for P uptake and also susceptible to P leaching. However, no relationship was observed between P uptake and P loss for any of the treatments. This may be partly attributed to the fact that P uptake was in the range of 50–60 kg P ha–1 y–1, whereas TP losses were ⬍ 2 kg ha–1 y–1, and most of the TP loss 共 ⬎ 80%兲 occurred in the unreactive P forms. Effect of rainfall on phosphorus forms in leachate There was a difference in the P loss pattern in 1999– 2000 compared to 2000-2001, where the relative proportion of DUP was higher than PUP for non-FDE treatments. The higher P losses as DUP were probably caused by the continuous low intensity rainfall during the winter months of June–July 1999 共287 mm兲 共see Figure 1兲, which would have maintained higher concentrations of DUP in the soil solution. These wet weather conditions prevented the development of macropores, resulting in lower losses as PUP. While in the next year 共2000–2001兲, only 57 mm of rainfall was received in few rainfall events during June–July, causing greater wetting-drying conditions
in the soil, promoting the development of macropores which then facilitated higher losses as PUP than DUP. The corresponding higher PUP losses for the FDE treatments during both years are due to the regular inputs of the FDE in these treatments, which contained a significant amount 共 ⬎ 32%兲 of P as PUP 共Toor et al. 2004兲. Potential for phosphorus accumulation in soil In the present study, the accumulation of P was related to the level of the P input. The highest P accumulation was recorded for the FDE400 treatments, followed by P90, FDE200 and P45 treatments. Several researchers have calculated P balances for grazed pasture systems 共Haynes and Williams 1993; Sibbesen and Runge-Metzger 1995兲. For instance, Haygarth et al. 共1998兲 reported that 26 kg P ha–1 y–1 was accumulated in the plant-soil system under an intensive English dairy farm compared with an extensive Scottish hill sheep farm, where P accumulation was only 0.28 kg P ha–1 y–1. Edwards and Withers 共1998兲 reported that the majority of intensive-livestock farms in the UK have an annual P surplus ⬎ 20 kg P ha–1. Similarly, P accumulation in soil due to excessive P inputs has been reported in other parts of the world. In an irrigated grazing trial in New Zealand, Williams and Haynes 共1992兲 compared total gains and losses of P for unfertilised pasture with a pasture fertilised with 19 or 38 kg P ha–1 y–1 as superphosphate for 38 years. They found that only 35% of the fertiliser P applied could be accounted for in produce or in the different residual soil and plant pools, whereas the rest of the P may have been lost in surface runoff or leaching during flood irrigation in the shallow stony soil. On the other hand, Nguyen and Goh 共1992兲 used a mass-balance approach to examine the P balance on the same trial and concluded that 83 to 94% of the applied P was recovered in the soil-plant-animal system. They suggested that net P losses from the soilplant system were substantially less than those reported by Williams and Haynes 共1992兲. These contrasting results indicate that direct ‘in situ’ measurements of P losses, as have been conducted in the present study are more reliable than the P balance/ budget model when assessing the actual or potential environmental impacts associated with long-term application of P inputs. Higher P accumulation in the P90 treatments soil 共23–47 kg P ha–1兲 than the P45/FDE200 共16–20 kg P ha–1兲 may have been due to the greater supply of P in
183 inorganic form in the P90 treatments, a significant proportion of which was immediately converted to insoluble P forms in soil. In contrast FDE contained appreciable amounts of unreactive P 共33% of total P兲 共Toor et al. 2004兲 that may have slowly converted to inorganic P forms, resulting in higher plant P uptake from the FDE treatments. For example, during 2000– 2001 herbage P uptake was 64 kg P ha–1 from the FDE200 treatment compared with 44 kg P ha–1 from the P90 treatments.
Conclusions The findings of this study contribute to an improved understanding of the impacts of application of FDE and P fertiliser on the amounts and forms of diffuse P loss by leaching from a free-draining grassland soil under irrigation. Annual TP losses varied from 1.4 to 2.5 kg ha–1 for the FDE and P fertiliser treatments compared with 0.6–1.3 kg ha–1 for the P fertiliseralone treatments. Higher P losses from the FDE200 treatments compared with P90 treatments indicate that the form of the applied P determined the amount of P loss, as total P inputs in these treatments were similar. Increasing fertiliser P inputs from 45 to 90 kg P ha–1 had no apparent effect on P loss by leaching over two years. This illustrates the potential of these soils to adsorb soluble inorganic P, although this may diminish with time as P sorption sites are saturated. Results indicate that changing the irrigation method from flood to spray may reduce P loss by leaching from FDE amended soil. This suggests that changing the method of irrigation water may be the most significant means to reduce P losses in the short term. But there is potential for saturation of P fixation sites if this practice is continued over a longer period and once the capacity of soil to retain inorganic P is exceeded, it may lead to increased P loss. Accordingly, long-term patterns of P loss in fertiliser and FDE amended soils requires further investigation.
Acknowledgements This research was supported by the New Zealand Vice-Chancellors’ Committee through its Commonwealth Scholarship programme to G.S.T. Funding for the establishment of the lysimeter experiment was provided by Ravensdown Fertiliser Co-operative Limited and the New Zealand Fertiliser Manufactur-
ers’ Research Association. The authors would like to thank Trevor Hendry of Lincoln University for his help with lysimeter sampling and leachate collection.
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