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TECHNICAL REPORT

Surface Water Quality

Phosphorus Leaching in Relation to Soil Type and Soil Phosphorus Content

Department of Soil Sciences, Swedish University of Agricultural Sciences, Box 7072, S-750 07 Uppsala, Sweden

^* Corresponding author (Faruk.Djodjic{at}mv.slu.se).

Received for publication July 22, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Phosphorus losses from arable soils contribute to eutrophication of freshwater systems. In addition to losses through surface runoff, leaching has lately gained increased attention as an important P transport pathway. Increased P levels in arable soils have highlighted the necessity of establishing a relationship between actual P leaching and soil P levels. In this study, we measured leaching of total phosphorus (TP) and dissolved reactive phosphorus (DRP) during three years in undisturbed soil columns of five soils. The soils were collected at sites, established between 1957 and 1966, included in a long-term Swedish fertility experiment with four P fertilization levels at each site. Total P losses varied between 0.03 and 1.09 kg ha^–1 yr^–1, but no general correlation could be found between P concentrations and soil test P (Olsen P and phosphorus content in ammonium lactate extract [P-AL]) or P sorption indices (single-point phosphorus sorption index [PSI] and P sorption saturation) of the topsoil. Instead, water transport mechanism through the soil and subsoil properties seemed to be more important for P leaching than soil test P value in the topsoil. In one soil, where preferential flow was the dominant water transport pathway, water and P bypassed the high sorption capacity of the subsoil, resulting in high losses. On the other hand, P leaching from some soils was low in spite of high P applications due to high P sorption capacity in the subsoil. Therefore, site-specific factors may serve as indicators for P leaching losses, but a single, general indicator for all soil types was not found in this study.

Abbreviations: DRP, dissolved reactive phosphorus • P-AL, phosphorus content in ammonium lactate extract • PSI, single-point phosphorus sorption index • TP, total phosphorus • UP, unreactive phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
PHOSPHORUS is the main limiting nutrient in fresh water bodies (Schindler, 1977). Phosphorus losses from agricultural fields significantly contribute to enhanced eutrophication and must be reduced to improve or maintain surface water quality. Although surface runoff is assumed to be the main pathway for P transport from arable fields (Sharpley and Rekolainen, 1997), leaching losses of P have also attracted increased attention lately (Culley et al., 1983; Turtola and Paajanen, 1995; Stamm et al., 1998; Djodjic et al., 1999, 2000; Simard et al., 2000).

In most developed countries, excessive P fertilization has resulted in an increased P content in agricultural soils. For example, in Sweden, 50% of all arable fields have soil P contents higher than 8 mg 100 g^–1 soil (Eriksson et al., 1997) measured in ammonium lactate solution (P-AL; Egnér et al., 1960), which corresponds to high or very high soil P content according to the recommendations issued by the Swedish Board of Agriculture (2002). This buildup of the soil P pool has led to an increased risk for P losses. A great number of studies have been conducted on the relationship between soil P status and P losses to water. There is a reasonably consistent pattern whereby P losses increase significantly as the soil test P values increase beyond agronomically optimum ranges (Sims et al., 2000), either in a nonlinear (Heckrath et al., 1995; Hesketh and Brookes, 2000) or linear manner. In a laboratory experiment, Hooda et al. (2000) concluded that the amount of desorbed P had no relationship to either total soil P or P sorption capacity. The most important property affecting desorption, according to their study, was the extent of P saturation, but the authors warned that further research was needed to test the relationship between desorbable P and P transfer into leaching and runoff. However, a number of limitations are often connected with studies of the relationship between soil P content and P losses. For instance, studies are often conducted for several soil P levels but only for one soil. Furthermore, at field scale, measurements of P concentrations in water or losses are often infrequent and sporadic, and the effects of recent P applications may overshadow the effects of soil P content. Laboratory studies may, on the other hand, neglect the effects of transport mechanisms and field hydrology. For these reasons, it is important to study several different soils with a range of different soil P contents in each soil. At the same time, different soil P contents should not be a result of sporadic P fertilizer and/or manure applications, but rather the result of a continuous buildup. Furthermore, proper quantification of losses under controlled conditions must be comparable with natural field conditions to enable interpretation of results in terms of larger scales. Finally, a longer observation period is preferable to capture the temporal variations.

Here we present the results of a P leaching study performed in undisturbed soil columns collected at five sites in southern and central Sweden, where long-term fertility experiments with four different P application strategies have been conducted during the past 35 to 40 yr. The objectives were to (i) assess the influence of different soil P contents on P leaching in different soils (i.e., to assess whether the long-term buildup of soil P content through the overfertilization leads to increased P leaching); (ii) study the most important factors governing P losses in the selected soils (i.e., to identify one or a few most important factors influencing P leaching losses in different soils); and (iii) determine different forms of P in the leachate.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Description, Soil Sampling, and Analytical Procedures
Long-term soil fertility experiments at 12 sites in southern and central Sweden were started between 1957 and 1969. At these sites, applications of P are based on the principles of replacement, with four P input levels at each site. As well as a control without P application, referred here to as Treatment A, and a treatment in which P removed by the crops is replaced, Treatment B, two levels of higher P additions are included. These two levels are intended to achieve slow (Treatment C) and rapid (Treatment D) increase of the soil P status (Carlgren and Mattsson, 2001). The average annual P applications are presented in Table 1. Phosphorus fertilizer was applied in the form of superphosphate and incorporated into the soils immediately after application either at sowing or plowing. The fields in southern Sweden were included in 4-yr crop rotations with cereals, oilseeds, and sugar beet, whereas the fields in a central Sweden had 6-yr crop rotation with cereals and oilseeds. Crop residues were incorporated into the soil after harvest.

The last P fertilization before collection of the soil columns occurred in autumn 1998 at Högåsa and Klostergården, autumn 1996 at Kungsängen, and spring 1997 at Fjärdingslöv and Ekebo. On these occasions, half of the P fertilizer allocation for a whole 6-yr crop rotation (sites in central Sweden), or for a whole 4-yr crop rotation (sites in southern Sweden), was applied.

The undisturbed soil columns were collected in plastic (PVC) standard sewage pipes (0.295 m in diameter and 1.18 m long) using a drilling technique by which the pipe is gently lowered down into the soil (Persson and Bergström, 1991). After collection, about 0.07 m of the bottom part of each soil column was replaced with nonsorptive filter materials to provide gravity drainage. The lysimeters were then placed below ground at a lysimeter station in Uppsala, Sweden (59°49' N, 17°39' E), where they were exposed to similar soil temperature conditions as the surrounding soil. This lysimeter station has been described in detail by Bergström and Johansson (1991). Eight soil columns were collected at each site, giving two replicates of each P fertilization treatment. An exception was the soil at Ekebo, where lysimeters could not be collected from Treatment B due to the presence of large stones in the profile. Therefore, 38 lysimeters were collected in total. Each of the lysimeters was fertilized with NH₄NO₃ and K₂SO₄ in spring 2000, 2001, and 2002 at rates of 100 kg N ha^–1 and 30 kg K ha^–1, respectively, whereas no P fertilizer was applied. Spring barley (Hordeum vulgare L.) was grown in all soil columns in 2000 and 2001, while oat (Avena sativa L.) was grown in 2002. The crops were harvested on 6 Sept. 2000, 10 Sept. 2001, and 27 Aug. 2002 by cutting the aboveground biomass with a pair of scissors at ground level.

On the same occasion as the collection of lysimeters, soil samples were collected at a depth of 0 to 20 cm, then air-dried and sieved through a 2-mm sieve before analyses. Soil P content was determined in all soil samples by extraction with 0.5 M NaHCO₃ at pH 8.5 (Olsen P; Olsen and Sommers, 1982), and in ammonium lactate–acetic acid at pH 3.75 (P-AL; Egnér et al., 1960). To obtain an estimate of P sorption capacity in the soils, topsoils were equilibrated with a 0.01 M CaCl₂ solution containing 19.4 mmol P kg^–1 soil, at a soil to solution ratio of 1:10 (w/v) for 20 h. A single-point phosphorus sorption index (PSI₁) was determined as the amount of P sorbed by the soil (X) divided by the logarithm of the concentration C (in µM) in the equilibrium solution (X/log C) as suggested by Bache and Williams (1971). Measured values for Olsen P, P-AL, and PSI₁ are given in Table 2. As an estimate of P sorption saturation, the ratio of Olsen P or P-AL (recalculated in mmol P kg^–1 soil) to PSI₁ was calculated for all topsoils, as suggested by Börling et al. (2004). In Treatments A and D, a single-point phosphorus sorption index (PSI₂) had previously been determined using the same method as for PSI₁ but with an addition of 50 mmol P kg^–1 to soil samples from the entire profiles (0–100 cm) taken on the same occasion as the lysimeters (K. Börling, E. Barberis, and E. Otabbong, unpublished data, 2003). An estimate of P sorption saturation had been calculated as the ratio of Olsen P or P-AL and PSI₂ (K. Börling, E. Barberis, and E. Otabbong, unpublished data, 2003).

Two-way analyses of variance (ANOVA) using the General Linear Model procedure and Duncan's multiple range test were computed with SAS 8.2 software (SAS Institute, 2003) to assess the differences between soils and treatments regarding leachate volume and the volume-weighted concentrations of TP, DRP, and UP.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Drainage Conditions
Total precipitation at the lysimeter site was 1782 mm between October 1999 and September 2002. Over this 3-yr period, water leachate from the lysimeters averaged 39.4% of total precipitation, which is higher than what was found in a similar lysimeter study under Swedish climatic conditions (Bergström and Jokela, 2001). This was mainly due to the high precipitation during the autumn and winter seasons in both 2000–2001 and 2001–2002. Figure 1 shows the total water leachate for each year within the 3-yr period (October 1999–September 2002). The annual amounts in Fig. 1 refer to 12-mo periods from 1 October to 30 September. One of the Ekebo loam lysimeters from Treatment A had much smaller amounts of leachate compared with the other lysimeters (Fig. 1). The drilling equipment may have pushed stones into the soil columns during collection, causing soil compaction and thereby restricting water percolation. The largest amount of leachate was recorded from the Fjärdingslöv sandy loam soil, whereas Högåsa loamy sand had the smallest amounts of leachate (Fig. 1). Results of the ANOVA showed that leachate volume was significantly influenced by soil type but not by P treatment (Table 3). Duncan's multiple range test showed that the mean leachate volume for all Fjärdingslöv sandy loam columns was significantly higher than the means of the others. In addition, the average leachate volumes of Kungsängen clay and Klostergården silty clay loam were significantly higher than the means of Ekebo loam and Högåsa loamy sand. The last two soils have very homogeneous profiles and old water is therefore replaced by new water through a "piston-flow" type of water displacement. Soil water in these columns might have been exposed to higher evapotranspiration during longer periods of time, resulting in smaller leachate volumes compared with the other soils. Large amounts of leachate from the Fjärdingslöv sandy loam are probably a consequence of the low water content of this soil at field capacity. According to Kirchmann et al. (1999), Fjärdingslöv sandy loam has the lowest water content at field capacity in the upper 1 m of the profile of all soils included in this study (Table 1), and its ability to keep percolating water in the profile is therefore limited. Finally, the soils with high clay content and high water holding capacity (Kungsängen clay and Klostergården silty clay loam; Table 1) had similar amounts of leachate and showed only small variations between different soil columns (Fig. 1).

View larger version (42K): [in this window] [in a new window]   Fig. 1. Annual amounts of leachate over three years. Each bar is the mean of two replicates and the range bars refer to leachate amounts during the whole 3-yr period.

View larger version (36K): [in this window] [in a new window]   Fig. 2. Volume-weighted concentrations of dissolved reactive phosphorus (DRP) and unreactive phosphorus (UP) during the whole 3-yr period in the different soils and treatments. Each bar is the mean of two replicates and the range bars refer to leached amounts of total phosphorus (TP = DRP + UP).

View larger version (48K): [in this window] [in a new window]   Fig. 3. Annual leaching of dissolved reactive phosphorus (DRP) and unreactive phosphorus (UP) during three years in the different soils and treatments. Each bar is the mean of two replicates and the range bars refer to leached amounts of total phosphorus (TP = DRP + UP).

Dissolved reactive P constituted the main proportion of TP losses (Fig. 2 and 3) and accounted for the following percentage of total P: Högåsa loamy sand = 79%, Ekebo loam = 48%, Fjärdingslöv sandy loam = 82%, Klostergården silty clay loam = 56%, and Kungsängen clay = 66%. The values are calculated as an average of all P treatments on the same soil. The mean DRP leaching from the different soils followed the order: Kungsängen clay soil > Fjärdingslöv sandy loam > Högåsa loamy sand > Klostergården silty clay loam > Ekebo loam. The soils with the smallest TP losses (Ekebo loam and Klostergården silty clay loam) had the highest proportions of UP. Slow water movement and efficient retention of DRP in these two loam soils may have increased the importance of the particle transport and thereby also the relative contribution of UP to the total losses. However, the Kungsängen clay soil had the largest absolute amounts of UP leached (Fig. 2), most likely due to the higher internal erosion through macropores in this soil (Kirchmann, 1991). The relative importance of the UP for this soil is lower, probably because preferential flow also transports DRP more efficiently.

Phosphorus Leaching in Relation to Soil Phosphorus Content and Phosphorus Sorption Saturation
The volume-weighted concentrations of TP, DRP, and UP did not show any clear or consistent pattern when comparing different treatments, that is, there was no evidence that an increasing P input, and consequent raise in soil P content caused an increase in the volume-weighted P concentrations (Fig. 2 and 3). When Olsen P in the topsoils (Table 2) were plotted against volume-weighted TP concentrations, the lack of any general pattern was further confirmed (Fig. 4) . Similar results were obtained when P-AL values were plotted against volume-weighted TP concentrations (data not shown).

View larger version (17K): [in this window] [in a new window]   Fig. 4. Olsen P plotted against volume-weighted concentrations of total phosphorus (TP). Each point is the mean of two replicates (±range).

View larger version (16K): [in this window] [in a new window]   Fig. 5. Phosphorus sorption saturation expressed as the ratio between Olsen P/single-point phosphorus sorption index (PSI1) plotted against volume-weighted concentrations of total phosphorus (TP). Each point is the mean of two replicates (±range).

Importance of Subsoil Properties and Water Transport Mechanisms
In this study, the average TP concentrations in the leachate for all treatments were: Ekebo loam = 0.06 mg L^–1, Klostergården silty clay loam = 0.07 mg L^–1, Högåsa loamy sand = 0.11 mg L^–1, Fjärdingslöv sandy loam = 0.13 mg L^–1, and Kungsängen clay = 0.23 mg L^–1. The discrepancy between high total P release in the topsoil (0.2 mg L^–1) measured with a weak extractor (0.01 M CaCl₂) resembling the soil solution (Börling et al., 2004) and the above-mentioned leaching concentrations emphasizes the importance of subsoil properties for P leaching. Hence, the ability of the subsoil to retain P, which is determined by the interplay between soil P sorption and desorption characteristics and water transport mechanism, can be more important for P leaching than P indices in the topsoil. If there is slow, uniform water transport through the whole pore volume (e.g., Ekebo loam, Klostergården silty clay loam, and Högåsa loamy sand), the subsoil may be regulating P concentrations in the percolating water. For instance, Högåsa loamy sand had the highest (0.11 mg L^–1) average volume-weighted TP concentration (irrespective of fertilization treatment) when compared with the corresponding values for Ekebo loam and Klostergården silty clay loam (0.06 and 0.07 mg L^–1, respectively). Högåsa loamy sand had also the lowest average PSI₂ value (2.5 mmol kg^–1) in the subsoil (60–100 cm), compared with corresponding values for Ekebo loam (5.5 mmol kg^–1) and Klostergården silty clay loam (9.5 mmol kg^–1). Additionally, very high P concentrations (0.18–0.28 mg L^–1) were extracted from the subsoil horizons of the Högåsa loamy sand soil with distilled water (P_w; K. Börling, E. Barberis, and E. Otabbong, unpublished data, 2003) compared with the corresponding values of P_w in Ekebo and Klostergården (0.02 mg L^–1; K. Börling, E. Barberis, and E. Otabbong, unpublished data, 2003). Therefore, it can be assumed that the high sorption capacity of the Ekebo and Klostergården subsoil reduces P concentrations in the percolating water and results in low P losses. The subsoil of Högåsa loamy sand has a low sorption capacity and a high ability to release P, serving as a P source rather than as a P sink, leading to higher overall P leaching. However, in both cases, the subsoil is crucial for P leaching.

On the other hand, Kungsängen clay and Fjärdingslöv sandy loam did not behave according to this pattern and had larger leaching losses than what would be expected from their subsoil (60–100 cm) P sorption capacities (PSI₂: 14.7 and 5.8 mmol kg^–1, respectively; K. Börling, E. Barberis, and E. Otabbong, unpublished data, 2003). Kirchmann (1991) found that continuous cracks in the Kungsängen clay soil enabled roots to penetrate to the bottom of the 1-m profile. This was emphasized by the fact that this clay soil has a high organic matter content throughout the profile (12.6–21.4 g organic C kg^–1 soil). Thus, water and solute transport can occur rapidly through macropores and the P sorption capacity of the soil remains unexploited because the contact time between percolating water and the bulk soil is short or missing. Furthermore, the average volume-weighted TP concentration in leachate of all eight Kungsängen clay soil columns of 0.23 mg L^–1 was similar to the average TP concentration in CaCl₂ extracts (0.24 mg L^–1) of topsoil samples from all treatments of this soil (Börling et al., 2004). Although these values are not directly comparable, they may indicate small changes in solution P concentration as water is transported through the soil profile. The high potential for P release in the topsoil, even when P was not added to the soil surface during several decades, in combination with the above-mentioned preferential flow, makes this soil vulnerable to P leaching.

Fjärdingslöv sandy loam had the lowest P sorption capacity of the 10 topsoils included in the Swedish long-term fertility experiment studied by Börling et al. (2001). However, Olsen P and P-AL decreased while the P sorption capacity increased with depth, resulting in low ratios of Olsen P/PSI₂ and P-AL/PSI₂ in the subsoil (0.005 and 0.07 respectively; K. Börling, E. Barberis, and E. Otabbong, unpublished data, 2003). Nevertheless, Kirchmann et al. (1999) observed development of cracks in the subsoil of the Fjärdingslöv sandy loam due to shrinkage caused by the smectite-like minerals. This could reduce the effects of P sorption in this part of the profile and lead to higher leaching, especially in the overfertilized Treatments C and D. However, high P leaching was measured from Treatment D but not from Treatment C (Fig. 2). We do not have a good explanation for this behavior. Nevertheless, water in this soil was most likely transported rather slowly in the upper part of the profile and rapidly through the subsoil. Such water transport emphasizes the importance of P sorption capacity of soil layers between the P enriched topsoil and the preferential flow dominated subsoil. High P fertilizer applications may have exceeded the P sorption capacity of that part of the profile in Treatment D but not in the other treatments, resulting in better P retention and lower leaching losses in Treatments A, B, and C.

It should be stressed that the focus of this lysimeter study was on leaching. Under natural field conditions, part of the water leaving agricultural fields reaches rivers and lakes via surface runoff, subsurface lateral flow, and man-made shortcuts (such as surface water intake wells). Under such conditions, the soil P content and P sorption saturation in the topsoil may be better correlated to P losses, because buffering effects of the subsoil are then short-circuited. Nevertheless, even though the correlation between soil test P values and actual P leaching loads was weak, an increase in soil P levels beyond the agronomic optimum level is highly undesirable and nonsustainable over the long-term, considering the limited sources of P. Continuous surplus fertilization can eventually overcome the ability of the soil to keep P within the profile, especially for soils with low P sorption capacity. High P leaching from soils and treatments where P was not added for several decades (Kungsängen clay, Treatment A) and low P leaching from highly fertilized treatments of some soils (Ekebo loam and Klostergården silty clay loam, Treatments C and D) emphasize the importance of site- and soil-specific risk assessment.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The results of this study suggest that agronomic soil P test values or P sorption saturation measured in the topsoil should not be used alone for risk assessment of P leaching. Taking a step from the laboratory to a larger scale (lysimeters, plots, and fields) introduces new important factors for the spread of P in the environment, which must be accounted for in reliable risk assessments. The potential of the soil for P release, the capacity of the subsoil to adsorb or release P and the water transport mechanisms seemed to be the most important factors influencing P leaching in this study.

In soils with slow water transport and prolonged contact time between soil particles and percolating water, P leaching is usually low and influenced by the P sorption saturation of the subsoil. However, if water transport through the subsoil occurs rapidly, the buffering effect of the subsoil may be reduced and result in larger leaching loads. In addition, in some cases subsoil can be a P source and lead to increased P leaching.

In one soil, high P leaching was observed from treatments in which fertilizer had not been applied during the past four decades. This indicates that some soils may be vulnerable to P leaching in spite of small external inputs. On the other hand, P leaching loads from some soils were small in spite of high P applications due to high P sorption capacity.

Finally, we should bear in mind that soil is a complex system and must be approached as such. Potential simplifications, generalizations, and scaling-up processes must undergo careful testing, cross-checking, and evaluation to avoid inaccurate conclusions.

	ACKNOWLEDGMENTS

 
This study was conducted within the multidisciplinary Swedish project, Food 21, with financial support from the Swedish Foundation for Strategic Environmental Research (MISTRA). We would like to thank Dr Lennart Mattsson for allowing us to sample the soil monoliths from the plots in the long-term fertility experiment and for his help with the statistics. We would also like to thank Göran Johansson for his valuable contributions and help when collecting the soil monoliths.

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