Phosphorus removal from soil solution is mainly due to adsorption and precipitation. For calcareous soils, with a large reservoir of exchangeable calcium, ...
Nutrient Cycling in Agroecosystems 67: 67–74, 2003. 2003 Kluwer Academic Publishers. Printed in the Netherlands.
67
Combined effect of water and organic matter on phosphorus availability in calcareous soils Ilaria Braschi, Claudio Ciavatta, Camilla Giovannini and Carlo Gessa* Di.S.T. A. ‘‘ Dipartimento di Scienze e Tecnologie Agroambientali’’, Universita` degli Studi di Bologna, Via Filippo Re 8, I-40126 Bologna, Italy; * Author for correspondence (e-mail: ibraschi@ agrsi.unibo.it) Received 30 October 2001; accepted in revised form 1 October 2002
Key words: Calcareous soil, Olsen-P, Organic matter, P kinetics, Soil water content
Abstract Phosphorus removal from soil solution is mainly due to adsorption and precipitation. For calcareous soils, with a large reservoir of exchangeable calcium, precipitation of insoluble Ca-P phases is the predominant process that reduces P availability to plants. Soil water content positively affects P-precipitation, while the addition of organic matter (OM) has an opposite effect. Little information on the effect of soil organic matter on P-insolubilisation as a function of soil water contents has prompted this study of the variation of extractable P, after addition of mineral P fertiliser. Columns packed with a calcareous soil were enriched with different levels of OM, extracted from Irish peat, and subjected to different rainfall simulations. After 102 days of experimentation and 171 mm of accumulated rainfall, the Olsen-P was 53% of the initially applied amount in 6.2% OM-enriched soil, 37% in 4.1% OM-enriched soil, and 20% in untreated soil (1.9% of OM). While the curve describing Olsen-P decrease as a function of accumulated rainfall was clearly exponential for untreated soil, the curves for OM-enriched samples were flatter, evidence that OM addition modified P-insolubilisation. The P-insolubilisation, after P-fertilisation, at several constant values of soil moisture for (i) calcareous soil, (ii) calcareous soil after removing carbonates and saturating the exchange complex with Ca, and (iii) calcareous soil after addition of different levels of OM followed first-order kinetics. The K obs s followed the order: Ca-saturated soil . untreated soil . OM-enriched samples. Results from rainfall simulation experiments and kinetics of Olsen-P decrease at several constant soil moisture contents indicated that the soil water amount was the main factor in reducing extractable P after P fertilisation and that the soil OM content was the main factor in keeping P in extractable forms. On the other hand, the addition of OM to calcareous soil increased the extractable P at each soil moisture regime, decreasing P-insolubilisation more effectively at lower soil water contents. P-sorption isotherms of calcareous soil after addition of different levels of OM showed that the presence of OM mainly influences P-insolubilisation, but not the adsorption process. Introduction Inorganic and organic phosphorus (P) occurs in soil in several forms, mainly slightly soluble and thus of low availability to plants. In calcareous soils, Ca-carbonate surfaces specifically interact with phosphate anions (Griffin and Jurinak 1973), although CaCO 3 also controls Ca concentration in soil solution and in the soil exchange complex. The removal of P from soil solution is mainly due to adsorption and precipitation processes. At low P solution concentrations, up to the millimolar range, adsorption processes predominate,
while precipitation reactions dominate at higher P concentrations (Matar et al. 1992). Many studies have been done to clarify the contribution of soluble Caions to P-precipitation. In particular, Akinremi and Cho (1991a, 1991b) underline that the contribution of exchangeable Ca-ions to P precipitation is higher than that of CaCO 3 . A study on two calcareous soils demonstrated that the Ca-ion activity in the liquid phase is mainly responsible for the formation of insoluble Ca-phosphate mineral phases (Tunesi et al. 1999). These findings suggest that solution-mediated processes predominate in calcareous soils.
68 In calcareous soils, as crop yield increases with increasing rainfall amounts and distribution, the level of crop response to the addition of P fertilisers decreases. In semi-arid regions, such as the Mediterranean area, the best response to P-fertilisation was found in dry and unproductive years, and there was a negative relationship between relative response to P and crop yield (Matar 1977). The organic matter (OM) of soil reduces P-insolubilisation. In a longterm study on P-availability in acidic soils characterised by different OM contents, equation parameters reflecting the significance of slow relative to fast P sorption were positively correlated with the OM / soil specific surface area ratio (Afif et al. 1995). In soils with different pH characteristics, a protective effect against P-fixation processes and losses by leaching was found by using peat-based fertilisers (Figliolia et al. 1996). The combined addition of poultry manure and P-fertiliser to different soils caused a continuous increase in extractable P and kept P in available form for a longer period than P alone (Toor and Bahl 1997). Several studies have been conducted on the protective effect of OM from different sources on P-insolubilisation in soils. However, little information exists on the effect of OM content in calcareous soils under different moisture regimes on P availability. The aim of this study was: (i) to investigate the effect of the addition of OM to a calcareous soil on the dynamics of extractable P after mineral P-fertilisation under different rainfall conditions; (ii) to study the kinetics of changes in Olsen-P, after P-fertilisation, at different percentages of soil moisture on a calcareous soil, on the same soil after removing carbonates and saturating the exchange complex with Ca, and also after the addition of different levels of OM; and (iii) to study the P-insolubilisation process and different OM-additions to calcareous soil by means of P-sorption isotherms.
Materials and methods Samples of a Typic Ustifluvent soil (Soil Survey Staff
1975) were taken from the ploughed profile (0–40 cm) of an unfertilised plot. Air-dried samples were ground, sieved to 2 mm, and analysed for texture (pipette method), pH (soil / water ratio 1 / 2.5), total calcium carbonate (volumetric calcimeter method with hydrochloric acid), active calcium carbonate (with ammonium oxalate; Loeppert and Suarez 1996), OM (Walkley–Black method; Nelson and Sommers 1996), Fe and Al oxides (citrate-bicarbonate-dithionite extractable Fe and Al; Loeppert and Suarez 1996; Bertsch and Bloom 1996), cation exchange capacity (CEC) (with BaCl 2 ; Amhrein and Suarez 1990), total P (digestion with perchloric acid; Kuo 1996), and Olsen-P (with NaHCO 3 ; Kuo 1996). The physical and chemical properties of the soil are summarised in Table 1. Soil treatments On calcareous soil two different treatments were done: (i) removal of carbonates before saturation of soil exchange complex with Ca; and (ii) addition of different OM levels. Calcareous soil samples without carbonates and then Ca-saturated were prepared as follows. Soil carbonates were removed following the method proposed by Tunesi et al. (1999), by adding 1 M Naacetate buffer solution at pH 4.75 to the soil and heating for 1 h at 80 8C. The soil was then left to equilibrate in a dialysis membrane (cellu.Sep T3, MWCO 12,000–14,000) in the Na-acetate solution, until complete absence of effervescence was observed when HCl was added to the slurry. Soil samples without carbonates were Ca-saturated by using 1 M CaCl 2 solution. Calcareous soil samples were enriched with organic matter extracted from a Sphanium Irish peat, containing 52.3% of total organic carbon and 0.3 g kg 21 of total P (Francioso et al. 1996). The OM was extracted from air-dried peat with 0.5 M NaOH solution at 65 8C under N 2 for 24 h. The suspension was centrifuged at 7000 g and filtered through a 0.8 mm filter (Millipore-type AA). The supernatant,
Table 1. Physical and chemical properties of the soil. pH
8.3
Texture
sand
silt
clay
37
36
27
Total CaCO 3 (g kg 21 )
Active CaCO 3 (g kg 21 )
OM (g kg 21 )
Fe (g kg 21 )
Al (g kg 21 )
CEC (cmol c kg 21 )
Total P (mg kg 21 )
Olsen P (mg kg 21 )
131
70
19.0
7.4
0.67
16
563
4.1
69 dialysed against distilled water until reaching a neutral pH, contained an OM amount of 31.1 g l 21 measured according to the Walkley–Black method (Nelson and Sommers 1996), and less than 0.01 mg l 21 P, measured after digestion with perchloric acid (Kuo 1996). To increase the OM content of the calcareous soil from the initial value of 1.9% to 3.1%, 3.6%, 4.1%, 4.8% and 6.2%, aliquots of OM solution, diluted to 20 ml with distilled water, were added to 20 g of soil. The OM-enriched soil samples were placed on an end-over-end shaker at 100 rpm for 12 h at 25 8C, dried in a forced air oven at 30 8C overnight, ground and sieved to 2 mm. Afterwards, 20 ml of distilled water was added to the OM-enriched samples and the wetting / drying cycle described above was repeated four times to make the samples homogeneous. The same wetting / drying procedure was done on calcareous soil and on soil without carbonates and then Ca-saturated. Soil columns Soil columns were prepared by packing untreated calcareous soil, and 4.1% and 6.2% OM-enriched soil samples at 1.10 kg dm 23 of bulk density. Three sets of 20 columns were prepared by packing untreated soil in a metacrylate tube (3.4 cm internal diameter) to a height of 15 cm. The soil was placed in five layers of 20 g each (2 cm height), separated, through a glass wool septum, by a 1 cm layer of sand (White quartz, 250 170 mesh, Aldrich, USA) to ensure water drainage. The upper layer was obtained by mixing 0.725 mg of P as dry Ca(H 2 PO 4 ) 2 ?H 2 O (Carlo Erba, Italy) to 20 g of soil. A glass wool septum was placed at the top of the column. The columns were kept in a laboratory at 25 8C. The above-described experiment for untreated soil columns was repeated packing three sets of 20 columns with 4.1% OM-enriched soil, and three more sets of 20 columns, with 6.2% OM-enriched soil. P-insolubilisation under rainfall simulation on soil columns For untreated and OM-enriched soil, distilled water was added at a rate of 1 mm H 2 O min 21 to the three sets of columns, following the daily rainfall data for the period June–September 1985 of three sites in Italy: Cittadella (northeast), Bologna (east), and Bari (southeast), characterised by high (HR), medium
(MR), and low (LR) rainfall, respectively. The sets of columns, watered according to the rainfall data of Cittadella, Bologna and Bari, were called HR-soil, MR-soil, and LR-soil, respectively. A column from each set was regularly analysed and Olsen-P and Total-P were determined on three subsamples for each soil layer. The data of Olsen-P from soil column experiments were subjected to analysis of variance using ANOVA (SAS / STAT) applied to the randomised block design to test the effect of OM addition, rainfall condition, and time. Olsen-P means were submitted to the Duncan multiple range test. Kinetics of P-insolubilisation under constant soil moisture Kinetics of Olsen-P, after P addition, were run in triplicate for four weeks at 25 8C on: (i) untreated calcareous soil; (ii) calcareous soil without carbonates and then Ca-saturated; (iii) 4.1% OM-enriched; and (iv) 6.2% OM-enriched soil. The soil samples were incubated at 3%, 12%, 25%, and 43% (w / w) of soil water content. The first three moisture values were those of air-dried soil, measured by gravimetric determination at 105 8C, permanent wilting point and field moisture capacity of the soil (Klute 1986). For the air-dried samples, 0.725 mg of P as dry Ca(H 2 PO 4 ) 2 ?H 2 O was added to 20 g of soil samples and kinetics of Olsen-P were run. To increase the soil water content to 12%, 25% and 43%, an excess of distilled water was added to air-dried soil samples and left to evaporate slowly at room temperature until the required value was reached. The samples were divided into sub-samples of 20 g and, after addition of 0.725 g of P as dry Ca(H 2 PO 4 ) 2 ?H 2 O, stored in desiccators containing free water at the bottom. For each water content, a sub-sample was used as control for humidity over time, and the others were used for measurements of Olsen-P. Control sub-samples were checked weekly for moisture content by gravimetric determination at 105 8C. For 12%, 25% and 43% of soil water content the moisture decreased after one week to 11.8%, 24.6% and 42.4%, respectively, and to 11.3%, 23.5% and 40.4% after four weeks. P-sorption isotherms P-sorption was studied in duplicate at 25 8C on: (i) untreated soil; (ii) 3.1% OM-; (iii) 3.6% OM-; and (iv) 4.8% OM-enriched soil at a slurry concentration
70 of 12.5 g l 21 and in the 0.13 to 3.00 mM range of P concentration. Soil samples (0.5 g) were mixed with 38 ml of a 0.05 M NaCl solution in dark glass containers and allowed to equilibrate for 24 h under gentle shaking. P was added from a 0.01 M solution of Na 2 HPO 4 (Carlo Erba, Italy) in 0.05 M NaCl. The final volume was adjusted to 40 ml by adding aliquots of 0.05 M NaCl solution and the suspension was shaken at 25 8C. After 24 h, the suspension was centrifuged at 8000 g for 20 min and the supernatant was analysed for P in solution, according to the method by Murphy and Riley (1962), modified by Watanabe and Olsen (1965).
Results and discussion P-insolubilisation on soil columns under rainfall simulation In the experiment of rainfall simulation on soil columns, no variation of Total-P or Olsen-P was found in the layers below 3 cm, showing that there was no P-leaching through the columns (data not shown). Our results are consistent with the findings of Tunesi et al. (1999), where no P-leaching was observed through columns packed with the same soil as we used, placed in the field after P-fertilisation and exposed to natural rainfall. The variation in Olsen-P with time in the 0–3 cm layer, together with accumulated rainfall amounts, are reported in Figure 1. In untreated HR-soil, after 7 d and 57 mm of rain, Olsen-P had decreased to 61% of the initially applied P-amount and, even though no water was added in the following four days, the Pinsolubilisation process still proceeded. Only 41% of the initially added P amount was found on the 11th day. In the following period, between 11 and 88 days, from 57 to 171 mm of accumulated rainfall, Olsen-P dropped from 41% to 20% of the initial value. This value of 20%, corresponding to 0.23 mg kg 21 , was reached on the 78th day and remained constant for almost four weeks, although 24 mm of water was added during this period. This P value, mobilised with Olsen extractant, can mainly be ascribed to the adsorbed P fraction (Stevenson 1986). In both 4.1% and 6.2% OM-enriched HR-soil samples, Olsen-P was higher than in untreated HRsoil throughout the experiment. The two levels of OM started showing a different effect in preventing P-
Figure 1. Olsen-P variations with time in untreated and OMenriched soil columns under high (HR), medium (MR), and low rainfall (LR).
insolubilisation after 120 mm of H 2 O. At the end of the experiment, Olsen-P was 37% and 53% of the initially applied P-amount for 4.1% and 6.2% OMenriched soil samples, respectively. In the experiments on MR-soil, where the accumulated rainfall was lower than in HR-soil, the general decrease of Olsen-P was less. In untreated MR-soil at the end of the experiment and after 84 mm of rainfall, Olsen-P was 45% of the initially applied P-amount, twice the amount for untreated HR-soil. In the OMenriched MR-soil samples, decrease of Olsen-P was less than in OM-enriched HR-soil, but there was no significant difference between the two OM levels in
71 preventing P-insolubilisation; at the end of the experiment, Olsen-P was around 70% of the initial Pamount for both. In the experiments on LR-soil, the decrease of Olsen-P was even less. In untreated LR-soil, the Olsen-P remained above 90% of the initially applied P amount for 59 days due to very low rainfall (6 mm), and it decreased after the rainfalls became more consistent. The Olsen-P measured between 59 and 66 days and under 28 mm of rainfall, kept decreasing until the end of the experiment, when it was 56% of the initially applied P amount. Under the same rainfall conditions, the two levels of OM maintained the Olsen-P value above 90% of the initial amount. In general, in all soil column experiments, the decrease in Olsen-P started immediately after the first rainfall event simulation. The extent of the decrease was in relation to the applied water amount. In moist soil, Olsen-P kept decreasing for several days, even if no further rainfall occurred. The addition of OM to calcareous soil reduced the P-insolubilisation. OlsenP was always higher in OM-enriched samples than in untreated soil throughout the experiments and under every rainfall simulation. Under low (LR) and medium rainfall (MR) conditions, the two levels of OM added showed similar effects on Olsen-P insolubilisation. Under high rainfall (HR) conditions the prevention of P-insolubilisation due to 6.2% OM addition was markedly greater compared to the 4.1% OM addition. The Olsen-P data from soil column experiments subjected to analysis of variance are shown in Table 2. While Olsen-P means did not decrease significantly over time at the probability level ,0.01%, except in the first days of the experiment, the level of OM addition to soil and rainfall conditions significantly affected the extractable P content in the soil. To better understand the effect of OM addition to the soil on P-insolubilisation, for each level of OM addition the Olsen-P data from HR-soil, MR-soil and LR-soil column experiments were plotted together versus the accumulated rainfall (Figure 2). The decrease of Olsen-P was found to be strictly related to the rainfall volume and this dependence was greater in untreated soil than in OM-enriched samples. The interpolation of Olsen-P data versus rainfall amount, according to a linear and an exponential model, is listed in Table 3. While the equation describing OlsenP variation as a function of accumulated rainfall was clearly exponential for untreated soil, by comparing the correlation coefficients for linear and exponential
Table 2. Effect of time, soil treatments, and rainfall on Olsen-P trend in soil columns experiments. Time (d) 0 7 11 46 54 59 66 78 88 102 Soil treatment Untreated 4.1% OM 6.2% OM Rainfall HR MR LR
Olsen-P mean a (mg kg 21 ) 39.13 a 36.33 b 32.02 c 30.04 cd 28.51 de 27.18 ef 26.44 ef 25.04 fg 23.89 g 23.53 g Olsen-P mean a (mg kg 21 ) 23.49 c 30.83 b 33.31 a Olsen-P mean a (mg kg 21 ) 21.00 c 29.64 b 36.99 a
a
Means followed by the same letter are not significantly different at the 0.01% probability level.
models, in OM-enriched samples the correlation coefficients were similar for both the linear and exponential model. The curves for OM-enriched soil samples were flatter, evidence that OM addition modified P-insolubilisation dynamics. Kinetics of P-insolubilisation under constant soil moisture Kinetics of Olsen-P decreasing after P-fertilisation
Figure 2. Olsen-P variation as a function of accumulated rainfall in untreated and OM-enriched soil samples.
72 Table 3. Models of Olsen-P data versus accumulated rainfall for untreated and OM-enriched soil samples. Soil sample
Linear equation Olsen P 5 aH 2 O (mm) 1 b
Corr. coeff.
Exponential equation ln Olsen P 5 aH 2 O (mm) 1 ln b
Corr. coeff.
Untreated 4.1% OM 6.2% OM
y 5 20.1992x 1 36.595 y 5 20.1602x 1 39.972 y 5 20.1202x 1 39.458
0.8987 0.9029 0.8996
y 5 20.0097x 1 3.6611 y 5 20.0062x 1 3.7298 y 5 20.0040x 1 3.6900
0.9663 0.8824 0.9096
were run on untreated, without carbonates and then Ca-saturated, 4.1% OM- and 6.2% OM-enriched soil samples at 3%, 12%, 25%, and 43% of soil water content. As shown in Table 4, the decrease in Olsen-P followed first-order kinetics with correlation coefficients between 0.986 and 0.830. Table 4 clearly shows that, for each soil sample, the higher the soil water percentage, the higher was the P-insolubilisation rate (K obs ). The K obs s followed the order: Casaturated soil . untreated soil . 4.1% OM-enriched soil . 6.2% OM enriched soil, and confirmed that OM addition inhibited the P-insolubilisation process. In an additional kinetics experiment of Olsen-P decreasing in the soil without carbonates and then Nasaturated, no variation of Olsen-P was measured during a four-week experiment (data not reported), confirming the role of Ca-ions in the P-insolubilisation process in calcareous soil. First order kinetics for the P-insolubilisation process in the presence of exchangeable Ca-ions can be explained by ascribing as constant the Ca-ions concentration in the soil solution. Untreated, Ca-satu-
Table 4. First-order kinetics constant (k obs ) of Olsen-P in untreated and treated soil samples. Soil treatment
Water content (%)
k obs 10 23 (day 21 )
Corr. coeff.
Untreated (1.9% OM)
3 12 25 43 3 12 25 43 3 12 25 43 3 12 25 43
1.610 5.590 10.35 14.75 4.030 6.730 18.73 34.66 0.9005 1.102 2.303 6.729 0.5011 1.002 1.402 5.682
0.954 0.851 0.853 0.830 0.948 0.923 0.864 0.945 0.985 0.847 0.830 0.925 0.935 0.986 0.844 0.932
Ca-saturated
4.1% OM-enriched
6.2% OM-enriched
rated, or OM-enriched soils seem to behave like a matrix capable of releasing a constant amount of Caions in the soil solution at the same rate as Ca-ions consumed by the formation of insoluble Ca-phosphate phases. The values of Olsen-P half-life (t 1 / 2 ) as a function of soil water content (Figure 3) show that, for all soil samples, the lower the soil water content, the higher the Olsen-P t 1 / 2 . In other words, P-insolubilisation is low (long t 1 / 2 ) at low soil water content not only for untreated soil and Ca-saturated soil, but also for OMenriched samples. At 43% water content, when Pinsolubilisation is fast, the Olsen-P t 1 / 2 for untreated, 4.1% and 6.2% OM-enriched soil samples were 47, 103, and 122 days, respectively. Even though these differences are not very accentuated in Figure 3, they can be considered important to plant response. In the untreated soil at 3% water content, a t 1 / 2 of 187 days was calculated, confirming the slow but active Pinsolubilisation process found in relatively dry soils of arid and semi-arid Mediterranean areas by Castro and Torrent (1993). In the untreated soil, the Olsen-P t 1 / 2 decreased from 124 days at 12% soil water content to 47 days at 43% water content. Within the same range of soil water contents, a more accentuated decrease of Olsen-P was found in soil without carbonates and then Ca-saturated, where t 1 / 2 decreased from 172 to 20 days. Similar results were obtained by Akinremi and Cho (1991a, 1991b), where the presence of the Cacarbonate influenced P-insolubilisation to a lesser extent than a Ca-saturated exchange resin. In the 4.1% OM-enriched soil, the Olsen-P t 1 / 2 decreased from 670 to 103 days as soil water content increased from 3% to 43%. A more marked Olsen-P t 1 / 2 variation was found in the 6.2% OM-enriched soil, where it decreased from 1386 to 122 days between 3% and 43% soil water content. The results of kinetics at constant moisture were found to be in agreement with the findings from rainfall simulation on soil columns and clearly highlighted the combined effect of OM and moisture regime on P-insolubilisation in a calcareous soil. If the water is supplied to calcareous soils by
73
Figure 3. Olsen-P half-lives (t 1 / 2 ) as a function of the soil water content in untreated and OM-enriched soil samples.
irrigation, the study of the P-insolubilisation process could be more complex. In a further soil column experiment, not described here, tap water containing 2.1 mM Ca-ions was added to simulate irrigation. The resulting Olsen-P decrease was much more marked than that found in the soil under rainfall simulation with distilled water. For this reason, under field irrigation, the Ca-ion supply from the groundwater should be considered as modifying the equilibria between natural CaCO 3 , Ca-ions and phosphate anions in the soil solution as well as the protective effect of OM addition on P-insolubilisation processes. P-sorption isotherms P-sorption isotherms on untreated and OM-enriched soil samples are shown in Figure 4. The trend of the isotherms in the range 0.0–1.0 mM of P in solution was similar, both with regard to slope and shape, for
all soil samples. In this range the presence of OM does not seem to influence the P-sorption process. Above this range of P in solution, different trends of isotherms were found and the slope in the untreated soil was steeper than those of OM-enriched soil samples. The decrease in slope from the 3.1% OM- to the 4.8% OM-enriched sample was related to the increase of OM%-addition. Since the P-sorption process in the soil corresponds to the first part of the isotherm while the P-insolubilisation processes are described by subsequent parts (Castro and Torrent 1998; Tunesi et al. 1999), and because in our experiments the OM addition influenced only the second part of the isotherms, we can suggest that the presence of OM mainly influences P-insolubilisation, but not the adsorption process.
Conclusions After P-fertilisation of calcareous soil, the extent of P-insolubilisation was a function of the rainfall volume and / or soil moisture content, as shown by the results from the rainfall simulation and kinetics experiments. The OM addition to soil reduced P-insolubilisation at every soil water content, but the lower the soil water content, the stronger was the OM effect on P-insolubilisation. The rate of P-insolubilisation at constant soil moisture followed first order kinetics. Since the P-insolubilisation process depends on P concentration and also on Ca-ions concentration in the soil solution, first order kinetics for this process means that the Ca-ion release in soil solution has to mantain a constant concentration over time. As shown by P-sorption isotherms on OM-enriched soil samples, the OM addition slowed down the Pinsolubilisation processes, but no clear effect was found in the part of the isotherms corresponding to adsorption processes. Spectroscopic analysis of CaOM links is underway to better understand the Pinsolubilisation process in calcareous soils.
Acknowledgements
Figure 4. P-sorption isotherms on untreated and OM-enriched soil samples.
This research was supported by grants from the Italian Ministry of Agricultural, Food, and Forestry Resources (project PANDA, ‘Agricultural Production in Environmental Protection’, subproject 3) and from the CNR-Consiglio Nazionale delle Ricerche (project
74 CNR-Agenzia 2000, cod. CNRC002BD3-002, ‘Agronomic and environmental aspects of phosphorus dynamics in Emilia-Romagna’s soils: availability to plants, insolubilisation and solubilisation processes and potential eutrophication risk of aquatic environments’). We acknowledge Dr. Anna Nastri of the Department of Agronomy of the University of Bologna for the statistical analysis of the data.
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