Journal of Environmental Quality 31:1294-1299 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Surface Water Quality
Effect of Mixing Soil Aggregates on the Phosphorus Concentration in Surface Waters
R.O. Maguire*,a,
A.C. Edwardsb,
J.T. Simsa,
P.J.A. Kleinmanc and
A.N. Sharpleyc
a Dep. of Plant and Soil Sci., Univ. of Delaware, Newark, DE 19717-1303
b Macaulay Land Use Research Inst., Craigiebuckler, Aberdeen, AB15 8QH, UK
c USDA-ARS, Pasture Systems and Watershed Research Unit, Curtin Road, University Park, PA 16802-3702
* Corresponding author (rmaguire{at}udel.edu)
Received for publication October 16, 2001.
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ABSTRACT
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At any time, the phosphorus (P) concentration in surface waters is determined by a complex interaction of inputs of soluble P and sorption–desorption reactions of P with sediments. This study investigated what factors control P in solution when various soil aggregates were mixed, seen as being analogous to selective soil erosion events, transport, and mixing within river systems. Fifteen soils with widely differing properties were each separated into three aggregate size fractions (2–52 µm, 53–150 µm, and 151–2000 µm). Resin P, water-soluble phosphorus (WSP), and the phosphorus buffer capacity (PBC = resin P/WSP) were measured for each aggregate size fraction and WSP was also measured for 11 mixes of the aggregate fractions. The smallest aggregates tended to be enriched with resin P relative to the larger aggregates and the whole soils, while the opposite was true for WSP. As the PBC was a function of resin P and WSP, the PBC was greatest in the 2- to 52-µm aggregate size fraction in most cases. When two aggregate size fractions were mixed, the measured WSP was always lower than the predicted WSP (i.e., the average of the WSP in the two individual aggregates), indicating that WSP released by one aggregate fraction could be resorbed by another aggregate fraction. This resorption of P may result in lower than expected solution P concentration in some surface waters. The strength with which an eroded aggregate can release or resorb P to or from solution is in part determined by that aggregate's PBC.
Abbreviations: FPI, final percentage increase • PBC, phosphorus buffer capacity • resin P, phosphorus extracted with mixed cation and ion resins • WSP, water-soluble phosphorus
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INTRODUCTION
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NITROGEN and P from nonpoint agricultural sources have now been identified as the major nutrients having a detrimental effect on surface water quality in the USA (USEPA, 2000). As discharges of P from point sources (such as municipal sewage treatment works) have come under increasing regulatory control, the relative contributions of agricultural nonpoint sources of P have risen (Foy and Lennox, 2000; Daniel et al., 1998). Recent research effort has focused on quantifying the transfer of P from terrestrially derived sources to surface waters. For example, the National P Project in the USA is currently developing the P Site Index, by considering many of the factors that influence the diffuse loss of P and developing a field-scale risk assessment of P loss (Gburek et al., 2000). Research has shown that in general, the majority of diffuse P loss from agricultural fields occurs in surface runoff (Coale, 2000). Runoff water can erode soil material and transport it to surface waters. However, the size-selective nature of erosion events means that all soil aggregates are not transported equally and, in particular, there is preferential erosion of smaller-sized soil aggregates (Sharpley and Smith, 1990). Pierzynski et al. (1990) found that total P concentrations were generally highest in the clay-sized fractions in heavily fertilized soils and always highest in the "lowest-density separates," which are at increased risk of erosion compared with relatively dense silt- and sand-sized soil aggregates. Maguire et al. (1998) found that total P and plant-available P increased, but water-soluble P decreased, with decreasing soil aggregate size. This potential for enrichment of P in eroded soil aggregates, relative to the P content of the whole soil, can be expressed as the enrichment ratio. For six soils of varying physical and chemical composition, Sharpley (1985) measured average enrichment ratios of 2.45 for Bray I-P and 1.48 for other P forms, in sediments generated under simulated rainfall. Maguire et al. (1998) found enrichment ratios of up to 1.7 for resin P in silt-sized aggregate separated from Scottish soils.
Surface runoff can transport soil aggregates from agricultural fields to surface waters where they often settle out of suspension. The precise time when eroded material stops being "soil" and becomes "sediment" is often difficult to identify. However, it is possible to assume that once in the surface drainage network, any similarities between the soil in situ and the material within the river system become reduced with time and distance transported. These eroded soil aggregates–sediments either sorb or desorb P depending on the prevailing conditions they encounter in the water course (House et al., 1995, 1998; Sallade and Sims, 1997; Taylor and Kunishi, 1971).
It is more than likely that sediment will be derived from soil material eroded from more than one source or field within a watershed. Within this scenario there is a wide potential for re-equilibration of P to occur between aggregates that vary in chemical and mineralogical properties. The extent to which individual soil aggregates influence solution P concentrations will partly depend on that soil aggregate's phosphorus buffering capacity (PBC). The soil PBC represents a means of quantifying the relative strength with which soils can influence their surrounding solution and is derived by dividing the quantity (Q) of P that a given amount of soil can sorb by the intensity (I) of P that the soil can maintain in solution (Moughli et al., 1993; Salmon, 1973; Barrow, 1967). The quantity of sorbed soil P (Q) can be measured by desorption or isotherm techniques and I is the concentration of P in solution at any particular Q (Bowman and Olsen, 1985). The sorption factor Q, which is often referred to as labile P, is only a small proportion of total P (Holford, 1997). The PBC can also be measured by a single-point isotherm, which was first suggested by Bache and Williams (1971).
In this paper, we investigated the effect of mixing soil aggregates from different sources on solution P concentration. The soil aggregates used ranged in size, chemical composition, P content, and PBC. The control of the solution P concentration in the mixes of the soil aggregates is linked to the soil aggregates' chemical properties, and the implications for influencing P concentrations in freshwater bodies containing eroded soil aggregates are discussed.
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MATERIALS AND METHODS
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Soil Selection and Soil Aggregate Size Separation
Teams in four European countries selected three soils each, which were representative of soils used in local agriculture, and had a known P fertilizer history. The basic properties of these soils and the sample numbering used in this paper are as reported by Barberis et al. (1996). The team from Great Britain sampled uncultivated (unfertilized) soils of the same soil series as their three fertilized soils (G2, G5, and G8 are the uncultivated counterparts to G3, G6, and G9, respectively). The other three countries involved contributed the cultivated (fertilized) soils: D1, D2, and D3 from Germany; E1, E2, and E3 from Spain; and I1, I2, and I3 from Italy. After collection, the soils were air-dried and sieved to pass through a 2-mm screen.
The soils were wet-sieved into the aggregate size fractions 2 to 52 µm, 53 to 150 µm, and 151 to 2000 µm using minimal dispersion as described by Maguire et al. (1998). This involved soaking 600 g of air-dry <2-mm soil in deionized water for 1 h, then gently washing the soil through a stack of sieves with deionized water, with mechanical shaking for two 5-min periods, until the washings were clear. The 53- to 150-µm and 151- to 2000-µm size fractions were removed from the sieves, while the 2- to 52-µm fraction was sedimented from the washings, before being dried. All results given are corrected for oven-dry soil or aggregates at 105°C.
Chemical Analysis
The pH was determined in deionized water at a soil to solution ratio of 1:1. All of the following extracts were carried out in triplicate and analyzed for P using the molybdate method of Murphy and Riley (1962). Water-soluble phosphorus (WSP) was measured by shaking 5 g of soil or aggregate size fraction with 15 mL deionized water for 1 h, followed by centrifuging at 2500 x g for 1 h. The supernatant was decanted and filtered through a 0.22-µm Millipore (Bedford, MA) membrane. Resin-extractable P (resin P) was determined on the 5-g soil sample residue from the WSP extraction, which was washed into a 250-mL polypropylene extraction bottle using 100 mL of deionized water. Mixed resin was added in mesh bags (2.8 mL of the cation resin Duolite 255 (British Drug Houses, Poole, UK) in the NH+4 form, and 4.0 mL of the anion resin Amberlite IRA 420 (BDH) in the Cl- form) to the extraction bottle and shaken overnight (16 h). After shaking, the resin bags were washed with deionized water and the P was eluted by shaking with 100 mL of 1 M NH4Cl at pH 2.0 (Somasiri and Edwards, 1992).
Using the resin P as a measure of the quantity of sorbed P (Q) and WSP as a measure of the intensity of solution P (I), the PBC is calculated by:
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Mixing Experiments
Pairs of aggregate size fractions (referred to as aggregates A and B for illustration) were mixed at two ratios. For the ratio 1A to 3B, 0.75 g of Aggregate A was mixed with 2.25 g of Aggregate B, and for the ratio 3A to 1B, 2.25 g of Aggregate A was mixed with 0.75 g of Aggregate B. Water-soluble P was determined by adding 9 mL of deionized water to each mix of aggregate size fractions (the same soil to solution ratio as WSP on the unmixed soils and aggregates) and shaking for 1 h, followed by centrifuging at 2500 x g for 1 h. The supernatant was decanted and filtered through a 0.22-µm Millipore membrane.
Statistical Analysis
Calculation of standard errors and regression analysis was carried out using Microsoft Excel 2000 (Microsoft, 2000).
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RESULTS AND DISCUSSION
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Properties of the Whole Soils
The selected soils encompass a wide range of physical and chemical properties, including five soil orders (Table 1)
. Soil texture ranged from a clay loam to a sand and the pH from moderately acidic (lowest pH = 5.5) to alkaline (highest pH = 7.4). Three soils (E1, E2, and I3) were calcareous, as shown by their calcium carbonate equivalents (Barberis et al., 1996). Resin-extractable P ranged from 15 mg kg-1 in G2 to 304 mg kg-1 in D1. Among the soils from Great Britain, resin P, which is one measure of plant-available P (Somasiri and Edwards, 1992), was always greater in the fertilized (G3, G6, and G9) than the unfertilized (G2, G5, and G8) soils. Previous research has shown that long-term fertilization increases plant-available P in soils, as measured by extraction with ion exchange resins (Warren and Sahrawat, 1993). Water-soluble P ranged from 0.1 mg kg-1 in G2 to 20.8 in D3 mg kg-1. Differences in the sorption properties between soils are clearly evident; for example, the G soils contained little WSP relative to the other soils and relative to their resin-extractable P contents. This can possibly be explained by their higher contents of amorphous aluminum (Al) and iron (Fe), as reported by Barberis et al. (1996), resulting in their capacity to maintain relatively low solution P concentrations (Delgado and Torrent, 2000; Sharma et al., 1980).
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Table 1. Selected properties of the whole soils (numbers in parentheses represent the standard errors of the means).
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The greater PBC of the G soils indicates their greater ability to buffer change in the surrounding solution P concentration (Holford, 1997; Moughli et al., 1993; Salmon, 1973; Barrow, 1967). The converse is true for soil D3; its small PBC indicates a reduced ability to buffer solution P concentration.
Phosphorus Release Characteristics of the Aggregate Size Fractions
The range in resin P was wide and varied from 5 mg kg-1 in the 151- to 2000-µm aggregate fraction of E3 to 777 mg kg-1 in the 2- to 52-µm aggregate fraction of D3 (Table 2)
. The effect of long-term fertilization was to increase the resin P in all aggregate sizes of all the G soils. The most common trend in the noncalcareous aggregates was an inverse relationship between resin P and aggregate size. However, the trend was not as clear as that reported by other researchers who commonly used fully disaggregated soil (Maguire et al., 1998; Agbenin and Tiessen, 1995; Ogaard, 1996). This may reflect a combination of the wide range of soil types used together with the possibility of inclusion of smaller soil particles into the larger aggregate size fractions. This trend for greater amounts of resin P in the smaller soil aggregates may be explained by the inverse relationship between soil particle size and P sorption capacity (Guzel and Ibrikei, 1994; Syers et al., 1969). The calcareous E1, E2, and I3 aggregates exhibited lowest resin P in the medium-sized 53- to 150-µm fraction. The greater resin P in the larger aggregates may be explained by their calcareous nature, as Agbenin and Tiessen (1995) found Ca-P to be the most predominant form of P in sand-sized soil particles.
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Table 2. Resin-extractable P, water-soluble P (WSP), and the phosphate buffer capacity (PBC) of the aggregate size fractions (numbers in parentheses represent the standard errors of the means).
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Water-soluble P did not follow the same trend as resin P among the aggregates. In fact the most common trend in WSP was opposite to that exhibited by resin P. The most common trend in WSP in both calcareous and noncalcareous soils was for increasing WSP with increasing aggregate size, although WSP distribution in D2, D3, and I2 did not follow this pattern. Water-soluble P was at least twofold greater in the aggregates of D1, D3, and E2 than in the aggregates from any of the other soils. It has been reported that fertilization increases P release from smaller soil particles to a greater extent than from larger particles (Ogaard, 1996). However, fertilization greatly increased both resin P and WSP in all aggregate sizes of the G soils. Again, this may be due to the weak dispersion and inclusion of smaller soil particles into the larger aggregate size fractions.
For 13 of the 15 soils studied, the PBC was greatest in the smallest (2–52 µm) aggregate size fraction, and for 11 of the 15 soils, the PBC of the aggregates decreased in the order 2 to 52 µm > 53 to 150 µm > 151 to 2000 µm. As smaller aggregates are preferentially eroded compared with larger aggregates, eroded aggregates will have a greater ability to influence the concentration of P in solution around them than would be predicted by measuring the PBC in the whole soil. Fertilization had no consistent effect on the PBC of any aggregate size fraction of the G soils. From Eq. [1] above, it would be expected that a high PBC in an aggregate could be the result of either a greater resin P or small WSP content. The relatively high PBC of the G soils is due to their low WSP rather than high resin P, whereas the relatively low PBC of D1, D3, and I2 is due to their great WSP content.
Phosphorus Release Characteristics When the Aggregate Size Fractions Are Mixed
If we consider mixing two aggregate size fractions (A and B) at gravimetric ratios of 3:1 and 1:3, then the transition in WSP release from only Aggregate A, through an increasing proportion of Aggregate B, to the WSP release of only B can be plotted. This is shown by the "measured" WSP curve in Fig. 1
, obtained from mixing the G5 (2–52 µm) (Aggregate A) with the G3 (151–2000 µm) (Aggregate B). Based on an arithmetic mean of the WSP from the two aggregates, a linear relationship for the release of WSP would be predicted, but such a predicted line assumes no interaction between the aggregates (Fig. 1). If we consider the proportional increase in WSP release from G5 (2–52 µm) to G3 (151–2000 µm), such that:
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where A is taken to be 100% of the increase in WSP between G5 (2–52 µm) and G3 (151–2000 µm), then a linear model could be used to predict WSP concentration following mixing, assuming a weight-based influence of individual samples. If this was the correct assumption, the increase in predicted WSP from pure G5 (2–52 µm) to the 3:1 mix of G5 (2–52 µm) and G3 (151–2000 µm) was 25%, the increase in predicted WSP from the 3:1 to 1:3 mix was 50%, and the final increase in predicted WSP from the 1:3 mix to pure G3 (151–2000 µm) was 25% (Fig. 1). We refer to this final increase in WSP, which was predicted to be 25%, as the final percentage increase (FPI). If the FPI is not equal to 25% for the observed WSP, we can infer that there was interaction between the aggregates. The greater the difference between observed and predicted FPI, the greater the differential effect of one of the aggregates on WSP.
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Fig. 1. Water-soluble phosphorus (WSP) release from pure G5 (2 to 52 µm) and G3 (151 to 2000 µm) with mixes of these aggregate size fractions at ratios of 1:3 and 3:1. Predicted versus measured WSP values are given, and the final percentage increase (FPI) is identified.
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The measured WSP was consistently lower than the predicted WSP for the mixes of G5 (2–52 µm) and G3 (151–2000 µm) (Fig. 1). The most likely explanation is that when the two aggregates were mixed, the G5 (2–52 µm) aggregates sorbed some of the WSP that was desorbed from the G3 (151–2000 µm) aggregates. House et al. (1995) observed sorption of soluble reactive P from solution to both suspended and bed sediments. Elsewhere, Sharpley et al. (1981) found the sorption capacity of suspended sediments in surface runoff could affect soluble P in runoff, with the suspended sediments capable of acting as a P sink by removing P from solution. In our study, the FPI was 66%, much larger than the 25% predicted, indicating that the G5 (2–52 µm) aggregates were able to disproportionally control P in solution after the mixing of the aggregates. A total of 11 mixes were carried out for both calcareous and noncalcareous aggregates and, in all cases, the WSP release characteristics were similar to that of Fig. 1, with observed WSP being less than WSP predicted by the linear model (Table 3
; for consistency during calculations, Aggregate A always had lower WSP than Aggregate B). Only the magnitude of the observed FPI varied. The observed FPIs ranged from 27%, which was close to the 25% predicted, to 75%. In 9 of the 11 mixes, the PBC was greater for the aggregate that had lower WSP, as lower WSP generally leads to greater PBC. Even in the two cases where the PBC was higher for the aggregate that had greater WSP [the mix of G8 (151–2000 µm) with G9 (151–2000 µm), and the mix of D2 (53–150 µm) with D1 (151–2000 µm)], the FPI was >25%, indicating that the aggregate with lower WSP, rather than lower PBC, exerted more influence on the WSP than the aggregate with greater WSP. However, the difference in PBC for these two cases was relatively small, and these mixes accounted for two of the three smallest FPIs. The WSP behavior in the mixes of the calcareous aggregates (aggregates of E1, E2, and I3) followed the same pattern as WSP in the mixes of noncalcareous aggregates, despite differences in the mechanisms of P retention between calcareous and noncalcareous soils (Stevenson, 1986).
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Table 3. Water-soluble P (WSP) release characteristics when two aggregate size fractions are mixed together, in relation to their P buffer capacities (PBC) (numbers in parentheses represent the standard errors of the mean).
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Predicting the Influence Each Aggregate Can Exert on Water-Soluble Phosphorus
There are two ways to characterize the differences between the PBCs of the Aggregates A and B used in each mix. The numeric difference can be calculated subtracting the PBCA from PBCB and the proportional difference can be calculated by dividing PBCA by PBCB. When the FPI of each of the 11 aggregate mixes was plotted against the numerical differences between the PBCs of the two aggregates mixed, a significant linear relationship was observed, with r2 = 0.53 (significant at the 0.05 probability level; Fig. 2a)
. However, when the FPI of each mix was plotted against the proportional difference in the PBCs of the two aggregate sizes mixed, a logarithmic relationship was seen, with r2 = 0.71 (significant at the 0.01 probability level). The implication of this relationship is that when soil aggregates are eroded from different sources and mixed, either in runoff or stream water, the resultant WSP in the solution surrounding them is not an arithmetic average of their individual WSP. Rather, the differences between the PBC of individual aggregates must be taken into consideration as well as their WSP content. As a result, aggregates with the greatest PBCs exert a disproportionate influence over WSP.
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Fig. 2. Relationship between the final percentage increase when two aggregates were mixed and (a) the numeric difference and (b) the proportional difference between their phosphorus buffer capacities (PBCs).
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CONCLUSIONS
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Elevated concentrations of P in surface waters are a major concern in many areas. Agriculture, specifically erosion of agricultural soils, has been identified as an important source of P. When soil is eroded, the smaller soil aggregates are preferentially removed. In this study, as in past research, the smaller soil aggregates tended to contain more plant-available P (resin P) than the bulk soil. However, WSP did not follow the same pattern and, in most cases, increased with increasing aggregate size. Past research has suggested that the PBC is a good method for estimating the ability of soils to influence the concentration of P in surrounding solutions. When we calculated the PBC for the three aggregate sizes studied here, the smallest aggregates tended to have the greatest PBCs. When soil aggregates were mixed, it was not possible to predict the resulting WSP from the arithmetic average of the WSP released by each aggregate. For most of the mixes of aggregates, the PBC was greater in aggregates with lower WSP. The WSP of all mixes was lower than would have been predicted by merely taking an average of the WSP release of each aggregate. This suggests that in situations where the P concentration in surface waters is important in influencing the extent of eutrophication, then for waters receiving eroded soil aggregates, some control may be exerted through the resorption of P by small soil aggregates that can maintain a comparatively low solution P concentration. The FPI was used as a way of measuring how much the measured WSP was lower than the WSP predicted from the average of the WSP in the individual aggregates. There was a good correlation between the FPI and the proportional difference between the PBCs of the two aggregates mixed. Therefore, when an aggregate had low WSP and a large PBC, it was able to resorb more P from solution than an aggregate with a low PBC. This was true for both the noncalcareous and calcareous soils studied. Conversely, an aggregate with a high content of WSP and a large PBC had a greater ability to maintain a relatively high solution P concentration in the surrounding solution.
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ACKNOWLEDGMENTS
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This work was funded in part by the European Community (AIR3 CT92-0303) and Scottish Executive Rural Affairs Department.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
