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Soil Science Society of America Journal 63:1174-1180 (1999)
© 1999 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Fractionation of Phosphorus in a Mollisol Amended with Biosolids

Yaobing Suia, Michael L. Thompsona and Chao Shanga

a Dep. of Chemistry and Biochemistry, South Dakota State University, Brookings, SD 57007 USA

mlthomps{at}iastate.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Information about soil P fractions is useful to predict the bioavailability of P in soil as well as to predict the likelihood of its transport. In this study, we used a sequential fractionation procedure to investigate the forms of P in a Mollisol amended at the soil surface with biosolids (i.e., anaerobically digested sewage sludge). Soil samples from three depths (0–5, 5–20, and 20–35 cm) were collected from a Cumulic Vertic Endoaquoll in a field experiment with three biosolid application rates, two vegetation treatments [hybrid poplar–cottonwood trees (Populus x euramericana — clone NC-5326) and switchgrass (Panicum virgatum L.)], and four replications per treatment. The Hedley fractionation scheme (dividing soil P into six empirical fractions [water-soluble, NaHCO3-soluble inorganic and organic P; NaOH-soluble inorganic and organic P; HCl-soluble P, and residual P)] was employed. After 6 yr of continuous application of biosolids to poplar plots, the absolute concentrations of all P fractions at the 0- to 5-cm depth increased significantly (P < 0.05). Some P fractions at the 5- to 20-cm depth increased significantly, whereas at the 20- to 35-cm depth, none of the fractions was affected by biosolids amendment. At the 0- to 5-cm depth of both poplar tree and switchgrass plots, the relative concentrations of some of the P fractions (e.g., HCl–P, NaOH–OP, and residual P) decreased rather than increased. Because NaHCO3–IP and H2O–P increased in the biosolids-amended soil at rates disproportionate to their concentrations in the biosolids, we conclude that HCl–P applied with biosolids was transformed to more labile forms.

Abbreviations: IP, inorganic P • OP, organic P


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
TO INVESTIGATE the effects of biosolids amendments (anaerobically digested sewage sludge) on forms of soil P, we would prefer to identify and quantify individual P compounds in biosolids and in soil, but because the chemistry of soil P is so complex, it is almost impossible to identify individual P compounds. Instead, classes of soil P compounds are often defined functionally by the extractants that remove them from soil material in a sequential fractionation scheme. Sequential fractionation procedures are based on the assumption that chemical extractants selectively dissolve discrete groups of P compounds, and such operationally defined soil P fractions are subject to broad interpretations. Nevertheless, the information obtained from P fractionation schemes has been useful for interpretations of soil development (Walker and Syers, 1976; Smeck, 1972, 1985; Cross and Schlesinger, 1995; Nair et al., 1995) as well as plant availability of P (Tiessen and Moir, 1993; Cox et al., 1997).

Many sequential fractionation strategies have been developed to quantify different forms of P in soils. The method developed by Chang and Jackson (1957), with later modifications such as those of Petersen and Corey (1966) and Williams et al. (1967), has been widely used for investigations of the forms and transformations of soil P, although some problems in interpretation have been cited for this method (Williams and Walker, 1969). Another widely used sequential P-fractionation approach was developed by Hedley et al. (1982). This procedure aims at quantifying plant-available (H2O- or NaHCO3-extractable P), Ca-associated (HCl-extractable), Fe-oxide- and Al-oxide-associated inorganic P (NaOH-extractable), as well as labile and stable organic P.

Losses of P in surface runoff and by subsurface transport via drainage tiles normally occur in water-soluble and sediment-bound forms (Ryden et al., 1973; Sharpley et al., 1995). The extent to which these forms are bioavailable determines the degree to which they can stimulate eutrophication in surface water bodies. Biologically available P has been defined by Sonzogni et al. (1982) as "the amount of inorganic P a P-deficient algal population can utilize over a period of 48 h or longer." Dorich et al. (1985) and Sharpley et al. (1991) reported that the fraction of P in soil and sediment extracted with 0.1 M NaOH under the conditions of 1000:1 or 500:1 solution/soil ratio and 16 h of shaking was correlated well with algal uptake of P. Sims (1993) has presented a detailed discussion of the importance of P in the soil environment. More information about the forms of P in biosolids-amended soils is needed to assess the environmental consequences of the very high levels of P that can occur in such soils.

Our objectives in this study were (i) to quantitatively document the effects of biosolids amendments on soil P fractions in a Mollisol and (ii) to qualitatively investigate transformations among soil P fractions after biosolids application.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Research Site, Biosolids, and Soil Characteristics
Located on the Skunk River flood plain ({approx}2 km from the river), the soil in the study area is a fine, smectitic, mesic Cumulic Vertic Endoaquoll. Slope at the site is 0%, and groundwater depth is normally {approx}3.5 m. Before biosolids amendments began, the surface horizon of the soil was sampled at eight locations in the research plots. In the upper 30 cm at those sites, the particle-size distribution (determined by the pipette method of Gee and Bauder [1986]) was 216 ± 42 g kg-1 sand, 372 ± 26 g kg-1 silt, and 412 ± 48 g kg-1 clay.

The study was designed as a randomized complete-block experiment with four blocks and three biosolids treatments. Two types of vegetation, hybrid poplar–cottonwood trees (Populus x euramericana — clone NC-5326) and switchgrass, were planted adjacent to each other in strips in each block in 1990. The three biosolids treatments included a control (no biosolids applied), a low biosolids application rate, and a high biosolids application rate (see rate details below).

Biosolids produced by the Ames Water Pollution Control Facility (Ames, IA) were sprayed as a suspension ({approx}5% solids) onto 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 amounts of biosolids annually applied to switchgrass plots from 1991 to 1993 were 8.4 Mg (dry matter) ha-1 for the low biosolids application rate and 14.6 Mg ha-1 for the high biosolids application rate. After 1993, biosolids application to switchgrass was stopped because the switchgrass had limited tolerance to the highest application rate and to the heavy wheel traffic that occurred during biosolids application.

The average amounts of biosolids annually applied to poplar tree plots from 1991 to 1996 were 6.4 Mg ha-1 for the low biosolids application rate and 11.5 Mg ha-1 for the high biosolids application rate. Dry matter in the biosolids ranged from 3.2 to 5.5%, and pH ranged from 7.2 to 7.6. The range of total P in the dry matter was 1.7 to 3.2%, and the calculated amounts of P applied in the biosolids are given in Table 1 . These analyses of the biosolids were supplied to us by the Environmental Protection Agency–certified laboratory at the Ames Water Pollution Control Facility.


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  		Table 1 Annual and cumulative amounts of P applied to poplar and switchgrass plots

 
We conducted two sampling campaigns in the fall of 1996. First, soil samples were collected at depths of 0 to 5 and 5 to 25 cm within each experimental plot (15 by 60 m). Four subsamples from the same depth were collected on a grid pattern in each plot and mixed to form a composite sample from that plot. The samples were air dried and ground to pass a 2-mm sieve, and organic C and total N were determined by dry combustion with a CHN analyzer (Leco Corp., 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 suspension. For the P fractionation study reported here, soil samples were also collected from the research plots at depths of 0 to 5, 5 to 20, and 20 to 35 cm. A procedure similar to that described above was used to collect and composite soil samples from each plot.

Sequential Fractionation of Soil Phosphorus
A modification of the methods of Hedley et al. (1982) and Tiessen and Moir (1993) was selected in this study to extract empirically defined pools of P. A diagrammatic representation of this scheme is given in Fig 1 . A 0.5-g air-dried, <2-mm soil sample was placed in a 50-mL centrifuge tube and was sequentially extracted with 30 mL each of deionized water, 0.5 M NaHCO3 (pH = 8.2), 0.1 M NaOH, and 1 M HCl. Each extraction ran for 16 h of end-to-end shaking. After each extraction, the tubes were centrifuged at 26860 g for 15 min at 5°C. Then the supernatant was passed through a 0.22-µm filter. After the final extraction, residual P was determined in the soil material left in the centrifuge tubes by extraction with H2SO4–H2O2 (Tiessen and Moir, 1993). The procedure was performed in duplicate on each composite sample from each replication. Phosphorus in a sample of biosolids collected at the treatment facility was fractionated with the same procedure for comparison (Table 2) .



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  		Fig. 1 Sequential P fractionation scheme, modified from that of Hedley et al. (1982)

 

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  		Table 2 Fractionation of P in a sample of biosolids from the Ames Water Pollution Control Facility.{dagger}

 
Our procedure differed from the standard Hedley fractionation procedure in that we used H2O as the first extractant instead of equilibrating the soil sample with an anion-exchange resin. We found in a preliminary study that an anion-exchange resin was not required to extract detectable levels of labile P from the soil samples of our study. This fraction is intended to represent the most labile P in the soil, and we believe that P in the H2O extract would be more closely related to bioavailable P in surface runoff from a soil than would resin-exchanged P.

Analytical Methods
Total P in the filtrates of the NaHCO3 and NaOH extracts was determined by digesting aliquots of those filtrates in an autoclave at 103.5 kPa and 121°C (60 min for the NaHCO3 extract and 90 min for NaOH extract) with acidified ammonium persulfate (Method 4500-P B5) (Greenberg et al., 1992). Orthophosphate P in the initial filtrates and in the digests of each soil extract was determined colorimetrically by the method of Murphy and Riley (1962). Absorbance was determined at a wavelength of 712 nm. The difference between total P and inorganic P in the extracts represents organic P.

The data were statistically analyzed using the General Linear Model Procedure of SAS Institute (1989). Differences in each P fraction among the three biosolids 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
 
After 6 yr of biosolid amendments, organic C, total N, and total P had increased significantly, and pH had decreased significantly in the upper 5 cm of the amended plots (Table 3) . The differences between the biosolids-amended and unamended soil were most pronounced in the poplar plots, because these plots received biosolids fully 3 yr longer than did the switchgrass plots. The addition of biosolids increased organic C, total N, and total P at the depth of 5 to 25 cm in the poplar plots as well, but there was no statistical difference between the plots receiving the two application rates. Application of biosolids had no effect on the pH of the poplar plots at 5 to 25 cm. Similarly, at the 5- to 25-cm depth in the switchgrass plots, there was no statistically discernible effect of 3 yr of application of biosolids on any of the measured parameters.


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  		Table 3 Mean values of organic C, total N, total P, and pH of soils in plots treated with various levels of biosolids.{dagger}

 
Total P in the upper 5 cm of the soil of our study increased to levels as great as 2833 mg kg-1 in the poplar plots and 1913 mg kg-1 in the switchgrass plots that received the high rate of biosolids. The low rate of biosolids corresponded approximately with the "agronomic rate" of biosolids application, defined by the content of plant-available N in the biosolids. But even at this application rate, total P in the upper 5 cm of the soil increased to 1810 mg kg-1 in the poplar plots and to 1086 mg kg-1 in the switchgrass plots.

Absolute Concentrations of Phosphorus Fractions
Our main interest in this study was to document how the large amounts of P added to the soil were distributed into the various fractions of soil P at three depths. At the 0- to 5-cm depth in both poplar tree and switchgrass plots, the absolute concentrations of all P fractions were significantly influenced by biosolids amendments, at least at the high application rate (Tables 4 and 5) . In contrast, at the deepest sampling increment (20–35 cm) of both vegetation treatments, none of the P fractions was substantially affected by biosolids application. At the intermediate soil depth sampled (5–20 cm), the influence of biosolids amendments was variable. For example, at the 5- to 20-cm depth of the poplar tree plots, the absolute concentrations of all inorganic P fractions increased after biosolids applications, although some of the increases were not statistically significant. In the switchgrass plots, the absolute concentrations of P fractions at the 5- to 20-cm depth increased slightly in the biosolids-amended plots, but none of the increases was significant, according to the standard analysis of variance procedure.


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  		Table 4 Absolute concentrations of soil P fractions in popular plots

 

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  		Table 5 Absolute concentrations of soil P fractions in switchgrass plots§

 
Organic P fractions (extracted by NaHCO3 and NaOH) at the 5- to 20-cm depth tended to increase with the level of biosolids amendment, but the increases were not statistically significant (Tables 4 and 5). Several studies (Hannapel et al., 1964; Frossard et al., 1989; Eghball et al., 1996; Chardon et al., 1997) have reported that the translocation of organic P fractions can contribute to the transport of P in soil. But in our study, the concentration of organic forms of P in the biosolids was small, and we cannot unequivocally conclude that downward movement of organic P fractions occurred. It is likely that some of the increases in organic P forms in our study were indirectly related to biosolids additions through the stimulation of plant growth, root development, and other biological activity in the soil that comes with large additions of C and N.

Poplar and switchgrass plots differed considerably in the amounts of labile P (H2O-soluble and NaHCO3-soluble) that occurred at the 0- to 5-cm depths. This difference was especially evident in the control plots where labile P was more concentrated at the surface of the tree plots than the switchgrass plots. Switchgrass roots were abundant very close to the soil surface, and as a result, bioavailable P at that depth was probably taken up more effectively than in the poplar plots. In the plots amended with biosolids, translocation of labile P may have occurred more readily in the poplar plots because intercepting roots were less abundant near the surface than they were in the switchgrass plots.

Regression Analyses
We used a linear regression analysis to define the relationship between the absolute concentration of a soil P fraction and the amount of P applied with the biosolids. As shown in Table 6 , at the 0- to 5-cm depth in both poplar and switchgrass plots, concentrations of several P fractions were linearly related to the cumulative amount of P added with biosolid amendments (P < 0.05). For the poplar plots, the linear relationships between the fractions of H2O–P and NaHCO3–IP and the amount of P applied with biosolids were very significant (P < 0.001).


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  		Table 6 Linear regression expressions that describe the change in concentration of each P fraction at 0- to 5-cm depth (Y) as a function of cumulative P added in biosolids (X)*

 
In the linear models of Table 6, the slope of each equation reflects the increase in the concentration of a soil P fraction with each increment of P applied with biosolids. So, the greater the slope, the greater was the rate of increase for a particular fraction of P. At the 0- to 5-cm depth in the poplar plots, the order of the rates of increase in concentrations of P fractions was: NaOH–IP > NaHCO3–IP {approx} HCl–P > NaOH–OP > H2O–P > residual P > NaHCO3–OP. At the 0- to 5-cm depth in switchgrass plots, the order of the rates was identical, except that the H2O-soluble fraction did not increase as rapidly as the residual fraction of P. These rates of increase reflect the dominance of inorganic P in the biosolids (Table 2). Yet, the relatively large rate of increase in NaHCO3–IP is not consistent with its abundance in the biosolids and suggests that its concentration may have increased as a result of direct transformation from other P forms.

At the 5- to 20-cm depth in the poplar tree plots, the order of the rate increases in concentrations of P fractions was: (Table 7) . At the 5- to 20-cm depth in switchgrass plots, the order of increase in absolute concentrations of P was (Table 7). The rates of increases of all P fractions were less at the 5- to 20-cm depth than at the 0- to 5-cm depth. For example, in poplar plots the rates of increase in H2O–P and NaHCO3–IP at the 0- to 5-cm depth were nine- and sixfold, respectively, those at the 5- to 20-cm depth.


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  		Table 7 Linear regression expressions that describe the change in concentration of each P fraction at 5- to 20-cm depth (Y) as a function of cumulative P added in biosolids (X)*§

 
Therefore, the effect of biosolids amendments on soil P fractions at the 5- to 20-cm depth can be interpreted differently, depending on which statistic is used. The standard analysis of variance for the 5- to 20-cm depths indicated that the absolute concentrations of all P fractions in switchgrass plots and of some fractions in poplar tree plots were not significantly increased by biosolids application (P > 0.05) (Tables 4 and 5). On the other hand, regression analysis indicated that, although the concentration increases were small, biosolids amendments did increase most of the P fractions (P < 0.05) at a linear rate at the 5- to 20-cm depth in both poplar and switchgrass plots (Table 7).

Transformations of Phosphorus Applied with Biosolids
Because the portion of applied biosolids captured by the litter layer and not mixed with the underlying soil in the plots is unknown, our data do not allow us to quantify transformations of P fractions applied with biosolids. But changes in the relative concentrations of soil P fractions at the 0- to 5- and 5- to 20-cm depths in both poplar and switchgrass plots suggest that transformation of P did occur concomitant with biosolids amendments (Tables 8 and 9) . The relative concentration of a soil P fraction is the proportion of that fraction in the total amount of P. The data in Tables 8 and 9 are directly calculated from those in Tables 4 and 5, respectively. If there had been no transformation of biosolids-applied P from one fraction to another, then we might expect that a large amount of a given P fraction in the biosolids would increase the proportion of that fraction of P in the amended soil. For example, the relative concentrations of NaOH–IP and HCl–P were high in the biosolids (Table 2); thus, the proportion of NaOH–IP and HCl–P at the 0- to 5-cm depth of the biosolids-amended plots might be expected to increase in comparison with the control plots.


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  		Table 8 Relative concentration of P fractions in poplar tree plots

 

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  		Table 9 Relative concentration of P fractions in switchgrass plots

 
Indeed, the relative concentration of NaOH–IP did increase, but the proportion of HCl–P at the 0- to 5-cm depth of soil amended with biosolids in both poplar tree and switchgrass plots decreased (Tables 8 and 9). At the same time, the relative concentration of NaHCO3–IP at the 0- to 5-cm depth of plots amended at the high rate of biosolids application increased from 7 to 14% in poplar plots and from 14 to 17% in switchgrass plots. Similarly, the relative concentration of H2O–P at the 0- to 5-cm depth of plots amended at the high rate of biosolids application increased from 3 to 6% and 1 to 4% in poplar and switchgrass plots, respectively. In the poplar plots, the relative concentration of H2O–P also increased at the 5- to 20-cm depth.

We hypothesize that HCl–P in biosolids was dissolved and transformed into other P fractions, especially NaHCO3–IP and H2O–P. Soil pH may have influenced the transformation of biosolids-applied HCl–P into other P fractions. Although the pH of the applied biosolids was relatively high (7.2–7.6), after 6 yr of biosolids amendments at the high rate, soil pH at the 0- to 5-cm depth had decreased from 6.2 to 5.0 under poplar and to 5.8 under switchgrass. These declines in pH values of the biosolids-amended soils most likely occurred when NH+4–N and organic N applied in the biosolids were oxidized to NO-3. We suggest that the relatively low soil pH favored conversion of the biosolids-applied HCl–P into more labile forms that could be more easily translocated to the 5- to 20-cm depth.

Another interpretation is possible: The large addition of nutrients in the biosolids could have stimulated biological activity in the soil sufficiently such that the increase in labile forms of P originated not directly from dissolution of HCl–P but from mineralization of organic forms of P. We reject this alternative, reasoning from the model of McGill and Cole (1981) that significant release of P from organic matter in the soil occurs only when the supply of inorganic P is limited. Such was certainly not the case in the soils of this study.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
The effect of biosolids amendments on soil P fractions was greater at the soil surface where biosolids were applied than it was deeper in the soil. After 6 yr of continuous biosolids applications to poplar plots and after 3 yr of biosolids applications to switchgrass plots, the absolute concentrations of all P fractions at the 0- to 5-cm depth increased. Concentrations of inorganic P fractions (NaOH–IP, NaHCO3–IP, HCl–P) increased the most as a result of biosolids amendments. At the 5- to 20-cm depth in poplar tree plots, H2O–P, NaHCO3–IP, NaOH–IP, and HCl–P were significantly increased by biosolids amendments. On the other hand, at the 20- to 35-cm depth of both poplar tree and switchgrass plots, none of the soil P fractions was influenced by biosolids amendments during the period of our study.

At the 0- to 5-cm depth of both poplar tree and switchgrass plots, the relative concentrations of all P fractions except NaHCO3–OP changed after the biosolids amendments. The relative concentrations of NaOH–OP, HCl–P, and residual P decreased, whereas those of H2O–P, NaHCO3–IP, and NaOH–P increased after the biosolids amendments. At the 5- to 20-cm depth of the poplar plots, the relative concentrations of all P fractions except NaHCO3–OP changed after the biosolids amendment, and the trends were similar to those at the 0- to 5-cm depth. We hypothesize that HCl–P applied with biosolids was transformed to more labile forms (NaHCO3–IP and H2O–P) as a result of the relatively low pH of the biosolids-amended soil.SAS Institute 1989


   ACKNOWLEDGMENTS
 
The study was conducted at 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 F.A. Khan for assistance in the field and laboratory, and we thank J.-C. Fardeau and R.J. Killorn for comments on this manuscript. We acknowledge the Iowa Agriculture and Home Economics Experiment Station, the City of Ames, the Leopold Center for Sustainable Agriculture, the U.S. Department of Energy, and the U.S. Environmental Protection Agency for funding that supported this research. Journal Paper no. J-17989 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Project no. 3359, supported by Hatch Act and State of Iowa funds.

Received for publication August 26, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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Y. Sui and M. L. Thompson
Phosphorus Sorption, Desorption, and Buffering Capacity in a Biosolids-Amended Mollisol
Soil Sci. Soc. Am. J., January 1, 2000; 64(1): 164 - 169.
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