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Journal of Environmental Quality 30:1829-1835 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Surface Water Quality

Using Phosphorus Concentration in the Soil Solution to Predict Phosphorus Desorption to Water

José Torrent*,a and Antonio Delgadob

a Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain
b Departamento de Ciencias Agroforestales, Universidad de Sevilla, EUITA, Ctra. Utrera km 1, 41013 Sevilla, Spain

* Corresponding author (cr1tocaj{at}uco.es)

Received for publication December 4, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 THEORY
 SOILS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The growing concerns about water eutrophication have made it urgent to restrict losses of phosphorus (P) from agricultural soils and to develop methods for predicting such losses. In this work, we used the paradigm of P sorption–desorption curves to confirm the hypothesis that the amount of dissolved reactive phosphorus (DRP) released to a dilute electrolyte tends to be proportional to the concentration of DRP in the soil solution raised to a power that decreases with increasing solution to soil ratio (W). The hypothesis was tested for a group of 12 widely ranging European agricultural soils fertilized with P in excess of crop needs. Phosphorus desorption was studied under near-static and turbulent conditions in laboratory experiments. The concentration of DRP in the 1:1 soil to water extract (P1:1) was used as a proxy for the DRP concentration in the soil solution. The amount of desorbed P was found to be correlated with P1:1 raised to a power that decreased from 0.7 to 0.9 at W = 100 to 0.2 to 0.4 at W = 10000. Correlation was not improved by introducing additional variables related to P sorption–desorption properties. Olsen P was found to be of lower predictive value than P1:1. Also, the index of degree of soil saturation with phosphorus (DSSP) based on oxalate extraction failed to predict P desorption. The fact that P1:1 seemingly predicts P desorption accurately for a wide range of soils makes it potentially useful in areas of high soil diversity.

Abbreviations: DRP, dissolved reactive phosphorus • DSSP, degree of soil saturation with phosphorus • P1:1, concentration of dissolved reactive phoshorus in the 1:1 soil to water extract • PSC, phosphorus sorption capacity • STP, soil test phosphorus. Subscripts for Al, Fe, and P extracted by different solutions: d, citrate–bicarbonate–dithionite • ox, oxalate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
THEORY
 SOILS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE risk of eutrophication of streams, lakes, and reservoirs has made it urgent to restrict the transport of phosphorus (P) from soils in many agricultural areas to surface waters. Substantial P losses via surface runoff or subsurface flow can occur when soils rich in P are fertilized with inorganic P or animal manure (Sharpley and Withers, 1994). It is thus crucial to predict the partitioning of applied P into solution P and P bound to the solid phase. Partitioning of P is described in terms of P sorption isotherms, also known as quantity–intensity (Q/I) relationships or P sorption curves.

Generally, the equations of Langmuir and Freundlich have been used to describe the shape of the sorption curves. For a soil with a well-defined phosphorus sorption capacity (PSC), large increases in solution P concentration are expected in response to small additions of phosphate as the PSC is approached. It therefore seems pertinent for practical purposes to determine the degree of soil saturation with phosphorus (DSSP), which is defined as the ratio of the amount of P already sorbed to the PSC of the soil (van der Zee et al., 1988; Breeuwsma et al., 1989; Schoumans, 1995). Thus, the DSSP may help predict the potential of soil to release P to water (Sallade and Sims, 1997).

Under a specific set of conditions (e.g., the solution to soil ratio, reaction time, and temperature), any soil possesses a characteristic sorption curve. The maximum ordinate of this curve is the PSC of the soil and each point of the curve represents a value of the DSSP and the corresponding equilibrium solution P concentration. If the sorption–desorption of P were reversible and the P already sorbed and the PSC of the soil could be determined accurately, predicting P desorption or partitioning of P added to the soil would be an easy task. Lack of reversibility (Barrow, 1983) and difficulties in determining the PSC and the P already sorbed by the soil make it difficult in practice to find an index for the DDSP that is universally valid to predict P desorbability. One useful DSSP index is the Pox/q(Alox + Feox) ratio, where the subscript ox stands for acid oxalate-extractable and q is usually 1 or 0.5. This index, first proposed and used by authors from the Netherlands (van der Zee and van Riemsdijk, 1988; Breeuwsma et al., 1995; van der Zee et al., 1990a,b), is based on the assumption that oxalate extracts P sorbed by the soil and that (Alox + Feox) is a proxy for the PSC. This is in turn based on the fact that (Alox + Feox) provides a measure for the most active P-sorbing soil components (poorly crystalline Fe and Al oxides, organically complexed Al and Fe, and edges of silicate clays). Generally, this index exhibits good correlation with P dissolved in runoff or drainage water (Pote et al., 1999) and also with P released to various types of solutions in laboratory experiments (Beauchemin and Simard, 1999). However, in soils where P occurs in non–Fe and Al compounds (e.g., Ca phosphate), this index may not be effective in predicting P loss to water.

Other methods of assessing P loss to water are based on standard soil test phosphorus (STP) or on the ratio of STP to PSC. A number of STP methods, including Mehlich-3, Olsen, iron oxide–coated paper, and water-extraction P (Pote et al., 1996, 1999; Hooda et al., 2000) have been used.

While procedures based on DSSP (Pote et al., 1999) or STP (Pote et al., 1999; Sharpley, 1995) have correlated to P loss and/or release to water, current research suggests that these procedures may not be applicable to most soils (Beauchemin and Simard, 1999; Uusitalo and Tuhkanen, 2000). Consequently, lack of a universally applicable index has restricted the practical prediction of soil P loss to an empirical exercise. Ideally, we need a P loss assessing index that is based on simple analytical procedures and applicable to a wide range of soil types. The aim of the laboratory experiments presented here was to demonstrate that a simple index based on the concentration of P in the soil solution is useful to predict P loss to water.


   THEORY
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
SOILS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Many studies have shown that the Freundlich equation provides a simple description of P sorption curves over a wide concentration range (~10-6.5 to ~10-3.5 M) in the soil solution (Barrow, 1980; Torrent, 1987; Singh and Gilkes, 1991). The equation can be expressed as

[1]
where X is the amount of sorbed P, c is the equilibrium solution P concentration, A is the amount of P sorbed at unit solution P concentration, and b is a constant typical of each soil and experimental condition. The value of A varies widely depending on the PSC for the soil and b usually ranges from 0.25 to 0.65 (Barrow, 1980). Figure 1 shows the shape of a Freundlich equation with b = 0.4. This figure also includes the corresponding P desorption curve. Such a curve is assumed to conform to the Freundlich equation, consistent with the work of Le Mare (1982), Barrow (1983), and Okajima et al. (1983).



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  		Fig. 1. Phosphorus sorption and desorption curves (positive and negative branches, respectively, of the y axis) of a hypothetical soil conforming Freundlich equations with b = 0.4. The amount of total desorbable P for an initial dissolved reactive phosphorus (DRP) in the soil solution of 2 mg L-1 was one-half the amount of sorbed P.

 
When a mass of soil contacts a mass of runoff or percolating water initially free of, or containing little, dissolved P, desorption of P occurs. Under specific desorption conditions (time of contact, degree of turbulence of the suspension, temperature, etc.), the concentration of dissolved reactive phosphorus (DRP) in solution or runoff/drainage, c, is given by

[2]
where W is the water to soil ratio, Wc is the amount of desorbed P, Y is the equation of the desorption curve, and c0 is the initial concentration of DRP in the soil solution.

Equation [2] has one important implication that becomes apparent when one considers two soils with the same c0 value that desorb P according to Freundlich equations with identical b but different A values (the latter constant reflects the magnitude of the soil PSC). One can thus write

[3]

[4]
where, for soil i, Wci is the amount of desorbed P, Ai is the amount of P previously sorbed at unit solution P concentration, and ki is the fraction of sorbed P that is desorbed.

Figure 2 shows a plot of the amount of desorbed P against W, which was obtained by solving Eq. [3] and [4] for two soils with the following constants: c0 = 2 mg L-1; A1 = 500 mg kg-1; A2 = 100 mg kg-1; and b = 0.5. Even though the A1 to A2 ratio is 5, the plots show that the ratio between the amounts of desorbed P is only about 1.5 at W = 10; 2 at W = 100; and 3 at W = 1000. Thus, the influence of the soil PSC on the amount of desorbed P decreases with decreasing W. This is also the case when P desorption is described by a straight line [Y = k(c0 - c1)] or by the Temkin isotherm [Y = k log(c0/c1)].



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  		Fig. 2. Logarithmic plot of the amount of desorbed P as a function of the solution to soil ratio (W) for two hypothetical soils exhibiting Freundlich desorption curves with the stated kA values and b = 0.5. The initial dissolved reactive phosphorus (DRP) concentration in the soil solution was c0 = 2 mg L-1 in both cases.

 
Whatever the W value, the amount of desorbed P is relatively insensitive to the shape of the Freundlich curve (determined by b) for soils with identical values of kA and c0. For instance, for two soils exhibiting desorption curves with k1A1 = k2A2 = 250 mg kg-1, c0 = 0.3 mg L-1, b1 = 0.6, and b2 = 0.3, the respective amounts of desorbed P are 2.9 and 2.8 mg kg-1 at W = 10, and 105 and 110 mg kg-1 at W = 10000.

Figure 3 shows the hypothetical plot of the amount of desorbed P against the initial solution P concentration (c0) for soils possessing the same P desorption curve (A = 500 mg kg-1, k = 0.5, and b = 0.5) but differing in c0, all at selected values of W. The slope of the plot decreases with increasing W (it is ~1 at W = 10 and ~0.5 at W = 10000), that is, the amount of desorbed P tends to be proportional to c0 at low W values and to c0.50 at high W values. Therefore, the differences imposed by the initial solution P concentration on the amount of desorbed P tend to decrease with increasing W.



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  		Fig. 3. Logarithmic plot of the amount of desorbed P, at selected values of W, as a function of the initial dissolved reactive phosphorus (DRP) concentration in the soil solution for a population of hypothetical soils exhibiting identical P desorption curves (A = 500 mg kg-1, k = 0.5, and b = 0.5) but differing in solution DRP.

 

   SOILS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 SOILS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The P desorption data for 12 soils from European agricultural areas having a long history of fertilization with P were examined. About 50 kg were collected by the end of the winter of 1995 from the Ap horizons of single soil pits, air-dried, sieved through a 2-mm screen, and stored at room temperature in air-tight plastic sacks, from which subsamples were taken for analysis. The properties of the soils and the methods used to characterize them were reported by Delgado (1996), Barberis et al. (1996), Ruiz et al. (1997) and Delgado and Torrent (1997)(1999, 2000). General soil data for the soils are given in Table 1, and data on P forms and P sorption–desorption properties in Table 2. In particular, two commonly used STPs were considered: the Olsen P (Olsen et al., 1954), which is a good index for the plant-available P, and the oxalate-extractable P (van der Zee and van Riemsdijk, 1988), which provides a measure of the P associated with P-reactive Al and Fe components in many soils.


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  		Table 1. Selected soil properties.{dagger}

 

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  		Table 2. Extractable forms of phosphorus.

 
Data from experiments where P was desorbed to 0.002 M CaCl2 were used. The Ca concentration and ionic strength of this solution are within the ranges expected for the solution of agricultural soils (Helmke, 2000) and that observed for the saturation extract of the 12 soils studied (data not shown); on the other hand, this Ca concentration does not differ excessively from the average Ca concentration of continental ground and surface waters (Stumm and Morgan, 1970). Desorption experiments were conducted at solution to soil ratios ranging from 10 to 10000 under near-static and turbulent conditions. In the former, suspensions were allowed to stand for different times up to 340 d and shaken only before each sampling. In the latter, suspensions were stirred vigorously (20 rad s-1) on a rotating rod–blade assembly for 2 h. Samples of the suspensions were taken at different times, filtered through a 0.22-µm membrane, and analyzed for DRP in the filtrate. The concentration of DRP in the 1:1 soil to water extract (P1:1) was determined in the supernatant obtained by centrifugation (acceleration, 1.1 x 105 m s-2) of the suspension previously shaken on a reciprocating shaker oscillating at 2.5 Hz at 298 K for 16 h. The saturation extract was prepared according to Rhoades (1996). Dissolved reactive P was analyzed by the molybdenum-blue method (Murphy and Riley, 1962). All determinations were done in duplicate.

Regression analyses were performed with Costat (CoHort Sofware, Monterey, CA). The significance of the differences between the slopes of the regression lines was tested by analysis of variance, which was based on comparing the sum of separate regressions with the joint regression (Quenouille, 1966).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 SOILS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Properties of the Soils
The soils studied ranged widely in physical and chemical properties (Tables 1 and 2). Evidence for application of fertilizer P in excess of crop requirements was provided by Olsen P values, which were all higher than the critical levels for most field crops (10–15 mg kg-1) (Kamprath and Watson, 1980). The values of Olsen P, as well as those of mixed resin-extractable P and isotopically exchangeable P (IEP), were rather divergent, and so was plant P uptake in a pot experiment with eight successive crops. Differences in P contents and forms among soils were reflected in the P1:1, which ranged from 0.09 to 10.4 mg L-1. There was no significant (P < 0.05) correlation between P1:1 and the other P-related properties listed in Tables 1 and 2.

The P1:1 differed little from, and was highly correlated with, the DRP concentration in the saturation extract (P1:1 = 0.10 + 0.93 x DRP in the saturation extract; R2 = 0.99). For this reason, and because the 1:1 extract is more readily prepared than the saturation extract, P1:1 can be used as a proxy for the DRP concentration in the saturation extract.

Phosphorus Desorbed to 0.002 M CaCl2
The loss of P from agricultural soils in subsurface flow or surface runoff can occur under diverse conditions. Generally, one can assume the ratio between soil mass and water (dilute electrolyte) mass in contact with it, time of contact, and degree of mixing of soil and water to be the most important factors influencing soil P loss. It is assumed here that the effect of these factors (related to the different soil loss scenarios) can be elucidated from simple laboratory experiments. For instance, the loss of P in runoff can be somewhat mimicked by experiments involving mixing soil under turbulent conditions with a dilute electrolyte (e.g., 0.002 M CaCl2) at W values of 50 to 2500, which approximately represent typical extreme sediment concentrations in runoff. The loss of P through macropore flow is probably related to the amount of P desorbed in experiments carried out at low W values (<100) and times of contact ranging from a few hours to a few days. Also, release of P from sediments on the bottom of a lake to a relatively still mass of water above it can probably be predicted using large W values (>1000) and times of months to years. In order to develop an approach effective in all these situations, data from desorption experiments involving W, time and the rheological conditions (near-static or turbulent) of the suspension as the main variables were examined.

Figure 4 shows the logarithmic plots of the cumulative amount of P desorbed under near-static conditions as a function of P1:1 for 1, 6, and 340 d, at W = 30, 100, 300, 800, 2500, and 10000. For each time–W combination, the logarithms of the amount of desorbed P and P1:1 were linearly correlated, with R2 > 0.82 (P < 0.001), except for desorption at 340 d at W = 100, where R2 = 0.69 (P < 0.01) (Table 3). Thus, the amount of desorbed P tended to be proportional to P1:1 raised to a power equal to the slope of the regression line in this plot. At each desorption time, the slope decreased significantly (P < 0.05) with increasing W. For identical W values, differences in slope at different times were not significant. On average, the slope was ~0.75 at W = 100, ~0.65 at W = 800, ~0.50 at W = 2500, and ~0.40 at W = 10000.



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  		Fig. 4. Logarithmic plot of the amount of P desorbed under near-static conditions at different times and W values as a function of the dissolved reactive phosphorus (DRP) concentration in the solution of 12 European soils. The slopes corresponding to the regression lines for each time and W value are shown as insets.

 

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  		Table 3. Values of R2 for the regressions of the logarithm of the amount of desorbed P against the logarithm of different P saturation indices.

 
Figure 5 shows similar plots obtained in experiments under turbulent conditions, using two different times (5 min and 2 h) and four W values (100, 800, 2500, and 10000). Correlation between variables was always significant (P < 0.05), with R2 values ranging from 0.40 to 0.92 (Table 3). As in Fig. 4, the slope of the regression lines decreased with increasing W, with an average value of ~0.65 at W = 100, ~0.50 at W = 800, ~0.48 at W = 2500, and ~0.23 at W = 10000. The slope and R2 values increased significantly if the soil with the highest P1:1 (D3, 10.4 mg L-1) was excluded (Table 3). This soil exhibited an anomalous behavior under turbulent conditions, with unexplained erratic changes in the concentration of DRP in solution. With D3 excluded, the slope was ~0.78 at W = 100, ~0.64 at W = 800, ~0.65 at W = 2500, and ~0.32 at W = 10000.



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  		Fig. 5. Logarithmic plot of the amount of P desorbed under turbulent conditions at different times and W values as a function of the dissolved reactive phosphorus (DRP) concentration in the solution of 12 European soils. The slopes corresponding to the regression lines for each time and W value are shown as insets.

 
Our results are basically consistent with the relationship illustrated in Fig. 3, that is, the amount of desorbed P is a function of the concentration of DRP in the soil solution raised to a power that decreases with increasing W, from ~0.9 at W = 30 to ~0.4 at W = 10000. Because the soils studied were heterogeneous in P sorption–desorption properties, one should expect the amount of desorbed P to be dictated not only by the DRP in the soil solution (according to Fig. 3), but also by the desorption capacity of the soil, at least at high W values (as suggested by Fig. 2). However, the variance in the amount of desorbed P accounted for did not increase significantly when various P sorption and desorption properties of the soil (e.g., the P sorption maximum or the P buffering capacity measured in experiments of P uptake by plants) were added to P1:1 in the regression analysis. The fact that DRP concentration in the soil solution seems to influence the amount of P desorbed to a dilute electrolyte more than other P sorption and desorption–related soil properties is certainly intriguing. The fraction of sorbed P that is desorbable may decrease with increasing P sorption capacity of the soil because higher P sorption capacities may be related to higher proportions of soil components with a high affinity for phosphate.

Good correlation between water-soluble P (Sissingh, 1971) and P desorbed to water in successive extractions was found previously by Hooda et al. (2000) for a relatively heterogeneous soil group; these authors only used a narrow water to soil ratio, not very different from that of the water-soluble P method. By contrast, Pote et al. (1999) found the correlation between concentration of DRP in runoff and water-soluble P to differ among soil series. These results do not contradict our conclusions because these authors found that, for any given level of water-extractable P, soils that produced the least volume of runoff (i.e., with the lowest W values) also had the lowest loads of DRP in the runoff. So, if a plot of the type shown in Fig. 4 were used, the points corresponding to the soils associated with low W values would lie lower that those corresponding to the soils associated with high W values, consistent with our hypotheses.

Correlation between the amount of desorbed P and the Pox/(Alox + Feox) ratio was significant (P < 0.05) in only 9 of the 26 desorption experiments illustrated in Fig. 4 and 5 (Table 3). This is probably the result of acid oxalate dissolving P forms that are unlikely to be released in significant amounts to a dilute electrolyte because of their low solubility or slow dissolution kinetics; such forms must include detrital apatite (Uusitalo and Tuhkanen, 2000) and much of the poorly soluble Ca or Ca-rich pedogenic phosphates in calcareous and limed acid soils (Delgado and Torrent, 2000). When the calcareous soils (E1, E2, and I3) were removed from the regression, the correlations between desorbed P and Pox/(Alox + Feox) became significant (P < 0.05) in all cases except for the experiment under turbulent conditions at W = 10000 at t = 5 min (Table 3). In summary, the Pox/(Alox + Feox) index seems useful for acidic soils where the P sorption–desorption properties are largely determined by the poorly crystalline Al and Fe compounds, as was the case with the soils studied by Hooda et al. (2000).

The amount of desorbed P was correlated to Olsen P in all but one of the desorption experiments illustrated in Fig. 4 and 5 (Table 3). Olsen P accounted for less variance in desorbed P than did P1:1 in all cases but two. Thus, even though Olsen P is useful to predict plant P uptake over a wide range of soils, it is clearly inferior to P1:1 in predicting P environmental risks.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 SOILS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study provides evidence that the concentration of DRP in the 1:1 soil to water extract (P1:1) is a useful index for predicting the potential release of P from soil to water in different P desorption scenarios and across a wide range of soils. The amount of P desorbed to a dilute electrolyte under different desorption conditions was correlated with P1:1 raised to a power that decreases with increasing water to soil ratio. The predictive value P1:1 was higher than that of Olsen P. A DSSP index based on oxalate extraction of P, Al, and Fe had no significant predictive value unless the calcareous soils were not considered. Even in this case, this index had a lower predictive value than P1:1.

Future use of the proposed index, like that of STP- or DSSP-based indices, will require careful calibration for each desorption scenario. However, the proposed index seems relatively insensitive to soil type, which makes it potentially useful in areas with high soil diversity. One added advantage of this index is that its determination is quite simple and requires little laboratory time.


   ACKNOWLEDGMENTS
 
This work was funded by the European Union (Contracts AIR CT92-0303 and IC15 CT98-0103).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 SOILS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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