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DIVISION S-2-SOIL CHEMISTRY

Phosphorus Sorption, Desorption, and Buffering Capacity in a Biosolids-Amended Mollisol

^a Agronomy Dep., Iowa State Univ., Ames, IA 50011-1010 USA

mlthomps{at}iastate.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
To investigate the impact of biosolids amendments to soil on the sorption, desorption, and buffering capacity of P, laboratory experiments were conducted on soil samples collected from a field study on a Mollisol amended with three levels of biosolids. The potential for sorption of additional P and the binding intensity of P were evaluated by applying the two-surface Langmuir model to sorption isotherms. Over the range of equilibrium P concentrations in this study, the ability of the soil to sorb added P decreased due to biosolids amendment. Addition of biosolids to the soil also decreased indices of the P-binding intensity at both the high- and low-affinity sites. The P equilibrium buffering capacity (PEBC) significantly decreased and the equilibrium P concentration (EPC) significantly increased after biosolids amendment. P desorption from soil samples with and without biosolids amendment was investigated for different equilibration periods and at various liquid/solid ratios. The amount of P that could be desorbed from the soil significantly increased after biosolids amendment. The effects of biosolids amendments on indices of soil P sorption–desorption phenomena (binding energy, PEBC, and EPC) imply a large increase in the P concentration of the soil solution. The increase of soluble forms of P in soil solution of this soil, which was heavily amended with biosolids, could enhance the loss of P in runoff and P movement below the root zone.

Abbreviations: EPC, equilibrium P concentration • PAN, potentially available N • PEBC, P equilibrium buffering capacity • Q/I, quantity/intensity • SOC, soluble organic C

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
BIOSOLIDS REFERS TO PRIMARY WASTEWATER SOLIDS that are tested, treated, and processed and that meet federal and state regulations for beneficial use by land application or other methods (WERF, 1997). Land application of biosolids can improve the physical properties of soils and provide nutrients necessary for plant growth (Metzger and Yaron, 1987; Pierzynski, 1994). Biosolids-application rates are normally determined by calculating the amount of N needed by the crop grown and comparing that value with an estimate of potentially available N (PAN) in the biosolids. But when biosolids-application rates are based on PAN only, P-application rates are generally much greater than crop needs.

Continued application of P in amounts greater than crop needs results in an accumulation of P in soil surface horizons (Sui et al., 1999). Erodible soils with high P levels may be nonpoint sources of P, especially in agricultural watersheds (Abrams and Jarrell, 1995). Because of the high P-fixation capacity of most soils, vertical movement of P through the soil profile is usually considered to be unimportant. But increases in soluble P in soil drainage waters and subsurface horizons have been reported in response to P amendments and accumulation in soil surface horizons (Heckrath et al., 1995; Smith et al., 1995; Eghball et al., 1996; James et al., 1996). Whether high-P biosolids amendment will result in increased mobility of P is an important issue for managing biosolids and soils where biosolids are applied.

When a material containing P is applied to a soil that initially has low levels of P, the soluble forms of P become increasingly less soluble with time (Holford et al., 1997). From the agronomic point of view, this is a concern because P that is strongly retained by the soil is less available for plant uptake. But from the environmental point of view, strong retention of P by soil may prevent losses of soluble P in runoff as well as movement to ground water. Thus, sorption and desorption reactions of P and the P-buffering capacities of soils may play an important role in both the agronomic and environmental aspects of P management.

Sorption of P by soil minerals and organic compounds initially proceeds by a rapid, exothermic ligand-exchange reaction with functional groups at mineral surfaces (Frossard et al., 1995). Following the rapid reaction, slower reactions such as liquid-state diffusion into micropores, solid-state diffusion, or discrete precipitation may occur. Unfortunately, it is extremely difficult to distinguish between precipitation of insoluble phosphate salts and sorption of phosphate. In this paper, we use the term sorption to cover all types of retention mechanisms.

Sorption-isotherm techniques have been widely used to compare the sorption of P by different soils. The Langmuir equation can be used to calculate parameters that are indices of the capacity for and the intensity of P sorption by the soil. The Langmuir model must be used cautiously to describe P retention by soils, however. For example, Harter (1984) has shown that sorption maxima estimated by the Langmuir equation could be in error by more than 50% if the entire isotherm is not used in calculating sorption parameters. Because the simple Langmuir model does not adequately describe P sorption by soil when surfaces have more than one type of elementary site (i.e., sites with different sorption energies), a two-surface Langmuir equation has been used in some P sorption studies (Holford et al., 1974). A complete derivation of the two-surface Langmuir equation has been given by Sposito (1982).

An alternative model to describe soil P equilibria is the quantity/intensity (Q/I) model (Peaslee and Phillips, 1981). The advantage of using Q/I relationships is that they allow the prediction of both P retention and release in soils (Kpomblekou-A and Tabatabai, 1997). The P-buffering capacity of a soil is its ability to resist a change in the P concentration of the solution phase. Phosphorus-buffering capacities of soils can be related to both plant nutrition and environmental pollution. The Q/I model can be applied to either adsorption or desorption experiments, but because of hysteresis, values for the P-buffering capacity obtained from adsorption curves are generally different from those obtained from desorption curves (Peaslee and Phillips, 1981; Okajima et al., 1983). Hartikainen (1991) and Kpomblekou-A and Tabatabai (1997) used low concentrations of P in solution to yield a single, straight Q/I line that crossed sorption and desorption ranges to overcome the discrepancies created by sorption–desorption hysteresis.

Barrow (1979) showed that P desorption from soils was influenced by desorption time and the liquid/solid ratio. He also found that the effects of these factors on P desorption from soils incubated with phosphate for different periods were not the same. Similar data are, however, scanty with regard to the effects of desorption time and liquid/solid ratio on P desorption from soils amended with biosolids. By investigating the effects of desorption time and liquid/solid ratio on P desorption from biosolids-amended soil materials, we might better predict the relative rate and extent of P desorption under varied environmental conditions, such as flooding, different rainfall intensities, and different rainfall durations.

The objectives of this study were (i) to determine the effects of biosolids amendments on the P sorption potential and P-buffering capacity in a Mollisol, and (ii) to investigate the impact of heavy biosolids amendment on P desorption under varying extraction periods and liquid/solid ratios.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soils and Biosolids
The soil used in this study is located on the Skunk River flood plain near Ames, IA; it is classified as a fine, smectitic, mesic Cumulic Vertic Endoaquoll (Zook soil series). The study was designed as a randomized complete-block experiment with four blocks and three biosolids treatments. The three biosolids treatments were a control (no biosolids applied), a low biosolids-application rate, and a high biosolids-application rate. Poplar trees (Populus x euramericana, clone NC-5326) were planted in each experimental plot (15 x 60 m) before biosolids were applied.

Biosolids produced by the Ames Water Pollution Control Facility (Ames, IA) were sprayed as a suspension (5% solids) on the soil surface with large application trucks. Because the vegetation was permanent, no tillage was used to incorporate the biosolids into the soil. The average amount of biosolids (dry wt.) applied annually to poplar tree plots from 1991 to 1996 was 6.4 Mg ha^-1 for the low biosolids-application rate and 11.5 Mg ha^-1 for the high biosolids-application rate. The biosolids-application rates were chosen with the target of supplying 170 kg ha^-1 of plant-available N at the low application rate. Dry matter in the biosolids ranged from 3.2 to 5.5 g L^-1, and pH ranged from 7.2 to 7.6. The range of total P in the biosolids dry matter was 17 to 32 g kg^-1. These analyses of the biosolids were supplied to us by the Environmental Protection Agency–certified laboratory at the Ames Water Pollution Control Facility. Selected characteristics of soil samples collected from the research plots are given in Table 1 . The particle-size distribution of the surface horizon of the soil was 250 g kg^-1 sand, 350 g kg^-1 silt, and 400 g kg^-1 clay (Sui et al., 1999).

The sorption isotherm of P was modeled with the simple Langmuir and the two-surface Langmuir equations. The simple Langmuir equation has the form

(1)

where Q is the P sorbed by soil (mg kg^-1),C is P concentration in the equilibrium solution (mg L^-1), parameter b is the predicted sorption maximum, and k represents a constant related to the energy of sorption. The two-surface Langmuir equation is defined as

(2)

where Q is the amount of P sorbed, C is the equilibrium P concentration, b₁ and b₂ are the high- and low-affinity maxima of P sorption, and k₁ and k₂ are related to high- and low-affinity binding energies of P sorption. Two straight lines can be obtained from a plot of Q/C vs. Q if isotherm data fit the equation well, and sorption parameters can be determined from the plot (Sposito, 1982). In our study, we determined sorbed P as the amount of P removed from solution by the soil sample after 24 h. On the basis of these data, we used the two-surface Langmuir equation to calculate an index of the maximum amount of added P that might be retained by a soil, and we refer to it as the soil's P sorption potential.

The set of initial dissolved P concentrations used in the sorption experiments reflected the high concentrations of water-soluble P that occurred in both the soils and the biosolids employed in this study. As we have reported elsewhere, in the uppermost 5 cm of the soil amended with biosolids, labile P (i.e., water-soluble plus bicarbonate-soluble inorganic P) occurred at concentrations of 430 mg P kg^-1 soil at the low biosolids-application rate and 644 mg P kg^-1 soil at the high biosolids-application rate (Sui and Thompson, 1999). In addition, about 12% of P added in the biosolids occurred in labile fractions, resulting in a mean annual application to the soil surface of 18 (low rate) to 33 (high rate) kg labile P ha^-1 soil yr^-1 (1991–1996) (Sui et al., 1999). Other workers have estimated the maximum P sorption potential of a soil sample by measuring the amount of P sorbed from a single addition of 1500 mg P kg^-1 soil sample (e.g., Bache and Williams, 1971; Eghball et al., 1996). The last point in our sorption series represented an addition of 1612 mg P kg^-1 soil.

Desorption of Phosphorus from Soils
Soil samples used for the P desorption study were collected in the fall of 1996 at a depth of 0 to 5 cm from the four unamended plots and from the four plots amended with the high rate of biosolids application. These samples were collected on a grid basis from each plot, air-dried and ground to pass a 2-mm sieve, and then mixed to form a composite sample for each replication. Finally, the composited plot samples were thoroughly mixed to produce two samples representative of the unamended and biosolids-amended soil. In the desorption study, nine desorption times (0.5, 1, 3, 6, 12, 24, 48, 72, and 96 h) and five liquid/solid ratios (10, 30, 60, 120, and 240) were chosen (compare Barrow, 1979). A 0.5-g subsample was put into either a 50-mL centrifuge tube with screw cap or a 250-mL plastic bottle with a screw cap and was extracted with 5, 15, 30, 60, or 120 mL of 0.5 mM CaCl₂ (i.e., liquid/solid ratios of 10, 30, 60, 120, or 240). Chloroform was not added to the suspensions in the desorption experiment. Each set of tubes or bottles with the same liquid/solid ratio was shaken end-to-end for each of the nine desorption periods. After the equilibration period, the tubes were immediately centrifuged at 26860 x g for 15 min; the suspensions in the plastic bottles were transferred into 50-mL centrifuge tubes and then centrifuged. The supernatants were passed through a 0.4-µm membrane filter, and the filtrates were analyzed for P as indicated below. The procedure was performed in duplicate on each composited sample from each of the two biosolids treatments.

Analytical Methods
Total organic C and total N in the soil were determined by dry combustion with a CHN analyzer (Leco, St. Joseph, MI). Total P was measured after digestion with H₂SO₄–H₂O₂–HF (Bowman, 1988). Soil pH was measured in a 1:1 soil/water or soil/CaCl₂ suspension. Inorganic P in the filtrates of each soil extract was determined colorimetrically by using the method of Murphy and Riley (1962). Absorbance was determined at a wavelength of 712 nm.

The data were statistically analyzed using the General Linear Model Procedure of SAS Institute (1989). Differences among treatments were examined by a standard analysis of variance procedure, with means separation by Tukey's procedure.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Analysis of Phosphorus Sorption Isotherms
Phosphorus Sorption Isotherms
Figure 1 shows the complete isotherms for P sorption by the control and biosolids-amended soil samples. Each point on the figure represents a mean of composited samples from four replicate plots. Over the large range of initial P concentrations employed, biosolids-amended samples sorbed less P from the solution than did the unamended samples. When the initial solution P concentration was <1.0 mM (equivalent to adding 620 mg of P kg^-1 of soil), P sorption was the least in the samples collected from the plots amended with the high application rate of biosolids. That trend changed when the initial solution P concentration was >1.8 mM (equivalent to adding 1116 mg of P kg^-1 of soil), at which point P sorption was the least in the samples collected from the plots amended with the low application rate of biosolids.

View larger version (16K): [in this window] [in a new window]   Fig. 1 Phosphorus sorption isotherms for a Mollisol without and with two rates of biosolids amendment. Each point on the figure represents a mean of four replicate plots

View larger version (11K): [in this window] [in a new window]   Fig. 2 Phosphorus sorption isotherms plotted according to the simple Langmuir equation. Each point on the figure represents a mean of four replicate plots

View larger version (11K): [in this window] [in a new window]   Fig. 3 Phosphorus sorption isotherm plotted according to the two-surface Langmuir equation (soil without biosolids amendment)

View this table: [in this window] [in a new window]   Table 2 P sorption and buffering parameters of soil samples (0–10 cm).

View larger version (14K): [in this window] [in a new window]   Fig. 5 Phosphorus Q/I relationships for a Mollisol without and with two rates of biosolids amendment

Exchange of Phosphorus and Soluble Organic Carbon
We found evidence that orthophosphate added in the sorption experiments displaced exchangeable organic anions. Others have also studied the competition of orthophosphate-P with simple organic anions (such as acetate, citrate, oxalate, malate, and amino acids) at mineral surfaces (Nagarajah et al., 1970; Kafkafi et al., 1988; Geelhoed et al., 1998). At the research site of our study, Han and Thompson (1999) showed that soluble organic C was increased by biosolids amendments to the soil. To investigate the impact of P sorption on soluble organic carbon (SOC) in soil samples of the present study, we first determined the amount of SOC that could be extracted with no P added. We found that the amounts of SOC removed from the samples when the initial solution P concentration was 0, were 69, 151, and 191 mg of organic C kg^-1 of soil in samples from the control, low-biosolids, and high-biosolids treatments, respectively. By subtracting these SOC values from the amounts of SOC extracted in solutions containing different concentrations of P, we obtained the net SOC that was exchanged by the added P. The amount of P sorbed correlated well with the net amount of organic C desorbed from all the soil samples (Fig. 4) . Therefore, exchange of P for organic anions appeared to be a significant chemical reaction during P sorption by the soil samples.

View larger version (14K): [in this window] [in a new window]   Fig. 4 The relationship between P sorbed from solution and soil organic C desorbed from the soil samples

Other phenomena in the biosolids-amended soils could also have influenced the maximum P sorption potential of the soil, as calculated from the two-surface Langmuir equation. For example, Traina et al. (1986) found that simple organic acids can extract Al from an acidic, smectitic soil, and so create new P sorption sites in the form of Al–organic acid complexes. The addition of biosolids at the research site not only directly introduced organic compounds to the soil but probably also stimulated microbial activities in the surface horizon, producing additional organic acids (Han and Thompson, 1999). Clay minerals in the soil of the present study are dominated by smectite (unpublished data), and pH values in the biosolids-amended soil were 5.4 to 5.5 (Table 1). Clearly, from the sorption isotherms alone, it is not possible to distinguish among all the potential, complex reactions that might affect sorption of P in the biosolids-amended soils.

Quantity/Intensity Analysis
To determine equilibrium P concentrations and P-buffering capacities, we plotted quantity/intensity (Q/I) diagrams by using data for the three lowest initial P concentrations in the sorption isotherms. These data included both P sorption and desorption reactions (Peaslee and Phillips, 1981; Hartikainen, 1991; Kpomblekou-A and Tabatabai, 1997). In Fig. 5 , the intercept of the line with the x-axis represents the equilibrium P concentration (EPC), and the slope represents the P equilibrium buffering capacity (PEBC). The complete set of Q/I parameters for the soil samples from the three different biosolids treatments is given in Table 2. The EPC of samples from the control treatment was very close to 0, whereas the EPCs of biosolids-treated samples substantially increased (1–2 mg L^-1) (P < 0.05). The increase in soluble P suggests that P loss in runoff and by leaching would occur more readily in a biosolids-amended soil than in unamended soil. Conversely, biosolids amendments substantially decreased the PEBC of the soil (P < 0.05), that is, lowering the ability of the soil to modulate the effect of further P additions on solution concentrations of P.

Desorption of Phosphorus
The soil that we studied is located on a flood plain and is periodically saturated with water. Therefore, we investigated the conditions under which P could be desorbed from soil in the control plots and from soil in plots amended with the high application rate of biosolids. The effects of the liquid/solid ratio and equilibration period on desorbed P are illustrated in Fig. 6 .

View larger version (27K): [in this window] [in a new window]   Fig. 6 The effects of liquid/solid ratio and equilibration period on P desorbed from (a) soil samples from the control plots and (b) samples from the high biosolids-application rate plots

In the unamended soil samples, the extraction period itself had little impact on the concentration of desorbed P at liquid/solid ratios of 10, 30, and 120 (Fig. 6). At liquid/solid ratios of 60 and 240, the solution concentration of desorbed P did increase after 48 or 72 h. In contrast, for the biosolids-amended samples at liquid/solid ratios 60, the concentration of P in solution increased substantially during the first 24 h before equilibrium was reached (as much as 40% at the liquid/solid ratio of 240). These desorption experiments suggest that under flooding conditions or during long periods of sustained rainfall, considerably more P could be desorbed from the surface of soil heavily amended with biosolids than from one without biosolids amendment.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Our laboratory studies suggest that biosolids amendments were likely to increase P concentration in the soil solution of the amended soil. Over the range of equilibrium P concentrations in this study, the ability of the soil to sorb added P decreased due to biosolids amendment. Adding biosolids to the soil also decreased indices of the P-binding intensity at both the high- and low-affinity sites. The EPC of the soil was significantly increased by biosolids amendment, implying that readily soluble P in the biosolids-amended soil surface horizon could be large. The PEBC of the soil was decreased by biosolids amendment, implying that the ability of the soil to remove P from solution decreased after biosolids amendment. Finally, biosolids amendment influenced the rate at which P could be desorbed from the soil. The biosolids-amended samples released soluble P more quickly and in greater quantities than did the unamended soil samples.

	ACKNOWLEDGMENTS

 
The study was part of the Ames Agroforestry Project, a cooperative research project of Iowa State University, the City of Ames (IA) Water Pollution Control Facility (WPCF), and the Iowa Department of Natural Resources. We thank L. Schultz for assistance in laboratory analyses and A.M. Blackmer and A.P. Mallarino for helpful comments on the manuscript. We gratefully acknowledge the Iowa Agriculture and Home Economics Experiment Station, the City of Ames, and the Leopold Center for Sustainable Agriculture for funding the research. Journal Paper No. J-18216 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, Project No. 3359, was supported by Hatch Act and State of Iowa funds.

Received for publication December 17, 1998.

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