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Published online 11 January 2008
Published in Soil Sci Soc Am J 72:73-82 (2008)
DOI: 10.2136/sssaj2007.0025
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

The Effect of Long-Term Annual Application of Biosolids on Soil Properties, Phosphorus, and Metals

J. L. Schrodera,*, H. Zhanga, D. Zhoub, N. Bastac, W. R. Rauna, M. E. Paytond and A. Zazulake

a Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078
b Institute of Soil Science, Nanjing, China
c School of Environment and Natural Resources, The Ohio State Univ., Columbus, OH 43210
d Dep. of Statistics, Oklahoma State Univ., Stillwater, OK 74078
e STV Incorporated, 80 Ferry Blvd., Stratford, CT 06615

* Corresponding author (jschrod{at}okstate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The USEPA Part 503 rule did not directly specify the amount of sludge-borne P that can be applied but allowed application rates based on the recommended N requirement of a crop. Monitoring of long-term application of biosolids is important to protect both soil and water quality. Biosolids and ammonium nitrate were annually applied for 13 yr to winter wheat at six plant available N (PAN) rates: 0, 45, 90, 180, 269, 536 kg N ha–1 yr–1. Biosolids application did not increase soil pH but increased levels of total C, total N, NH4–N, NO3–N, and electrical conductivity (EC). Soil concentrations of Cd, Cu, Pb, Mo, and Zn were greater in biosolid-treated plots but were <24% of their respective cumulative pollutant loading rates. Application of biosolids increased plant micronutrient B, Cu, Fe, and Zn concentrations. The addition of biosolids to the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll) increased Mehlich 3 plant available P (M3P) and water soluble P (WSP) and these P levels were well correlated with biosolids application rate. Application of biosolids at rates of ≥2 times the recommended agronomic rate of 90 kg PAN ha–1 resulted in M3P concentrations that exceeded an environmental threshold of 200 mg kg–1 established by the USDA-NRCS for the land application of manures in Oklahoma non-nutrient limited watersheds. Significant relationships were found between the ammonium oxalate P saturation index (PSIox) and M3P as well as between PSIox and WSP. It appears the repeated long-term application of biosolids above the N agronomic rate should be avoided and application should be based on other criteria such as an agronomic P threshold, an environmental P threshold, or a P site index.

Abbreviations: Alox, ammonium oxalate extractable Al • CRM, certified reference materials • EC, electrical conductivity • Feox, ammonium oxalate extractable Fe • M3P, Mehlich3 P • opm, oscillations per minute • PAN, plant available N • Pox, ammonium oxalate extractable P • WSP, water soluble P


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In 1988 ocean disposal of treated biosolids was banned, leading to an increase in land application of biosolids as an alternative to landfilling or incineration (NRC, 2002). Approximately 60% of the 5.6 x 106 dry tons of biosolids disposed of annually in the United States are land applied (NRC, 2002).

The USEPA (1995b) defines biosolids as "the primarily organic solid product yielded by municipal wastewater treatment processes that can be beneficially recycled." Biosolids have been utilized successfully as soil amendments on forests, rangelands, and reclamation sites as well as agricultural lands and golf courses (Basta, 2000; NRC, 2002). Land application of biosolids is regulated under 40 CFR Part 503, which was promulgated by the USEPA in 1993 (USEPA, 1993). The regulations, commonly referred to as the Part 503 rule, established management practices for the land application of biosolids. This included maximum concentrations and loading rates for chemical pollutants and treatments for controlling and reducing pathogens, which were intended to be protective of human health and the environment (USEPA, 1993; NRC, 2002).

The Part 503 rule does not directly regulate the amount of P in biosolids that can be land applied but adopts a similar approach to that traditionally used with animal manures (USEPA, 1993; Shober and Sims, 2003). Rather the Part 503 rule allows biosolids to be applied based on the recommended N rate for the crop to be grown. However, land application of animal manures to meet crop N needs leads to an accumulation of P in soil (Sharpley et al., 1999) because the N/P ratio of animal manures is less than N/P ratio of about 8:1 taken up by most crops and pastures (USDA, 2001). Land application of biosolids based on the recommended N rate has the potential for many of the same problems associated with manure application because the N/P ratio of biosolids is typically narrower than the N/P ratio needs of the crops (O'Connor et al., 2005).

The implementation of the Part 503 rule is assigned to the individual states and their varying regulations and guidelines. Shober and Sims (2003) performed a national survey to evaluate which of the U.S. states have restrictions or are considering restrictions on biosolids application based on concerns about the environmental effects of P. They reported that 24 of the 54 U.S. states and territories have regulations, guidelines, or legislation that can be used to limit land application of biosolids based on P, 13 of which have an environmental threshold for P. Additionally, Shober and Sims (2003) found that 59% of the states require monitoring of both soils and biosolids for P, 2% require monitoring of soil P only, 11% require testing of biosolids P only, and 28% do not require monitoring of biosolids or soils. Repeated applications of biosolids to the same property may result in cost savings because new lands do not have to be located and permitted. From a management standpoint, one must consider the cumulative effects on soils and plants (Cogger et al., 2001). Monitoring of long-term application of biosolids is important to protect both soil and water quality. While a multitude of papers have been published on biosolids application, few researchers have thoroughly documented the effect of annual long-term application on a wide array of soil properties. Rather these studies have focused on one particular item such as P or trace metals.

The objectives of this study were to evaluate the effect of the long-term repeated application of biosolids on (i) soil properties, (ii) metal levels in soils, and (iii) soil P concentrations. The annual application of biosolids and the duration makes this study unique.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site and Treatments
A winter wheat (Triticum aestivum L.) field experiment was established on a Norge loam soil (USDA-NRCS, 2001b) at the Agronomy Research Station, Stillwater, OK in the fall of 1993. The experimental site has mean annual temperature of 16°C and a mean annual precipitation of 86 cm. Treatments were a complete factorial within a randomized complete block experimental design with three replicates composed of two N sources. The two N sources were anaerobically digested biosolids and ammonium nitrate (34–0-0) applied at six rates (0, 45, 90, 180, 269, and 539 kg PAN ha–1 yr–1). The recommended N agronomic rate for wheat assuming a yield goal of 40 bushel acre–1, which is equivalent to 2727 kg ha–1, is approximately 90 kg PAN ha–1 (Zhang and Raun, 2006). Thus, the application of biosolids ranged from 50 to 600% of the recommended agronomic rate. Plot size was 5 x 10 m. Winter wheat was annually planted from 1993–2005. The nitrogen sources were annually applied preplant in the fall and disk incorporated to a depth of 15 cm. Biosolids were applied on a total N rate and on a dry weight basis to supply PAN. Application rates of biosolids varied yearly with both N and moisture content and are shown in Table 1 .


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  		Table 1. Biosolids application rates on a dry-weight basis (Mg ha–1) for the different treatment rates from 1993–2005.

 
Soil and Biosolids Analyses
Soil samples were collected from all plots in August 2005. Ten cores at a depth of 0 to 15 cm were obtained per plot and combined to make a composite sample.

Mehlich 3 extractable P was determined by shaking duplicate 2.0-g samples of soil and 20 mL of M3 solution (0.2 M CH3COOH, 0.25 M NH4NO3, 0.015 M NH4F, 0.013 M HNO3, and 0.001 M EDTA) in 50-mL centrifuge tubes for 10 min on an end-to-end shaker (150 oscillations per minute [opm]) (Mehlich, 1984). The samples were then centrifuged at 5211x g for 10 min, filtered (Fisherbrand P4 filter paper) and analyzed colorimetrically for P (Murphy and Riley, 1962). Water-soluble P was determined similarly to M3P (i.e., same soil/solution ratio), except deionized distilled water was used as the extractant (Fuhrman et al., 2005).

Acidified ammonium oxalate extractable P, Al, and Fe (Pox, Alox, Feox) were determined by shaking 1.5-g samples of soil with 30 mL of 0.5 M (COONH4)2•H2O at pH 3.0 in 50-mL centrifuge tubes (Schoumans, 2000). The samples were shaken at 150 opm in the dark for 2 h and centrifuged for 10 min at 5211 x g. The supernatant was decanted, filtered (Fisherbrand P4 filter paper) and analyzed for P, Al, and Fe using inductively coupled plasma–atomic emission spectroscopy (ICP–AES; Spectro CirOs, Fitchburg, MA). The PSIox was computed using the P, Al, and Fe contents (mmol kg–1) according to Eq. [1] (Schoumans, 2000):

Formula 1[1]
Soil P saturation has been used in the evaluation of the environmental fate of P. Phosphorus saturation is defined as the amount of P sorbed divided by the P sorption capacity of the soil. The concept of P saturation is meaningful as it estimates the degree to which P sorption sites have been filled and indicates the potential desorbability of soil P (Beauchemin and Simard, 1999). Phosphorus saturation has been highly correlated with P desorption such that P desorption increases with higher degrees of P saturation (Sibbesen and Sharpley, 1997). Phosphorus saturation is viewed as an environmental indicator of soil P because it has been found to be a good indicator of P availability to runoff and leachate (Kleinman and Sharpley, 2002).

Soil pH was determined in a 1:1 soil/deionized water suspension (Thomas, 1996) and electrical conductivity was measured using the saturated paste method as described by Rhoades (1996). Total C and total N were determined by dry combustion using a LECO CN 2000 (LECO Corp., St. Joseph, MI) (Nelson and Sommers, 1996). Ammonium N (NH4–N) and nitrate N (NO3–N) were extracted with 1.0 M KCl (shake time of 30 min) and analyzed using a Lachat Quickchem 8000 automated flow-injection analyzer (Zellweger Analytics, Milwaukee, WI) (Mulvaney, 1996). Calcium and Mg were extracted using the M3P procedure described above and analyzed by inductively coupled plasma atomic emission spectroscopy (ICP–AES). Soil samples were extracted for Fe, Zn, Cu, and B using a mixture of diethylenetriaminepentaacetate (DTPA) and sorbitol followed by analysis with ICP–AES (Gavlak et al., 2003). Sulfate-sulfur was extracted with Ca(H2PO4)2 and analyzed by ICP–AES. Standard reference samples from North American Proficiency Testing Program (NAPTP) were analyzed for quality assurance and quality control of soil pH, M3P, electrical conductivity, total C, total N, NH4–N, NO3–N, Ca, Mg, Fe, Zn, B, Cu, and SO4–S. Reference samples were evaluated every 10 samples with control limits set by NAPT.

Heavy metal (i.e., As, Cd, Cr, Cu, Pb, Mo, Ni, and Zn) were determined by acid wet digestion according to U.S. Environmental Protection Agency Method 3050B (USEPA, 1996) followed by analysis by ICP–AES. Blanks, spikes (known concentrations), and certified biosolids-treated reference soil (CRM005–050, RTC Corp., Laramie, WY, USA) were digested and analyzed for quality assurance and quality control in the determination of metal content in soil. Spike concentrations were added at approximately the same concentration of the metal measured in the soil. The certified reference soil was analyzed every 10 samples. Digested blanks contained metals below detection limit. Mean recoveries of metal in certified reference soil (CRM005–050, RTC Corp., Laramie, WY, USA) ranged from 91 to 97% with relative standard deviations ranging from 2.3 to 5.0% . Spike recoveries for metals in soil ranged from 95 to 100%.

Biosolids obtained from the city of Stillwater, OK waste water treatment plant were analyzed similarly to the soil for the same heavy metals as well as total P. Blanks, spikes and a certified biosolid reference material (CRM031–040, RTC Corp., Laramie, WY, USA) were digested and analyzed for quality assurance and quality control in the determination of metal content in soil. Certified reference biosolid was evaluated every 10 samples. Digested blanks contained metals below detection limit. Mean recoveries of metal in certified biosolid (CRM031–040, RTC Corp., Laramie, WY, USA) ranged from 90 to 103% with relative standard deviations ranging from 1.0 to 4.3%. Spike recoveries for metals in biosolids ranged from 95 to 100%.

Statistical Analyses
Analysis of variance was performed with PROC GLM in PC SAS Version 9.2 (SAS Institute, 2001). Certain planned comparisons were evaluated with either an LSMEANS statement (when the contrast involved a comparison of two means) or a CONTRAST statement. Of primary interest was the comparison of biosolids to ammonium nitrate at a fixed rate of fertilizer. The CONTRAST statement was used to fit a linear and quadratic contrast for the rate of fertilizer for each type of fertilizer (biosolids and ammonium nitrate). Simple linear correlations were performed for M3P and WSP against P added, for DTPA extractable Cu, Fe, and Zn against biosolids added, for PSIox against biosolids added, and for PSIox against M3P and WSP using PROC CORR (SAS Institute, 2001).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Biosolids Characteristics
The applied biosolids had a wide range of chemical and physical properties. Overall, the biosolids were neutral (median pH = 6.5), low in total C (median total C = 204 g kg–1), contained large amounts of P (median P = 35.4 g kg–1 with P/N ratios ranging from 1.01 to 3.42) and displayed low C/N ratios (median C/N = 10.0) (Table 2 ). Total N and C to N ratios were similar to those reported by the USEPA (1995b) and by O'Connor et al. (2004). O'Connor et al. (2004) examined 12 biosolids they believed representative of the residuals produced nationally and reported that total N ranged from 15 to 70 g kg–1 while the C/N ratio ranged from 5.3 to 13. However, the median P content of the studied biosolids was considerably higher than the national median of 23 g kg–1 reported by USEPA (1995b) but within the range of 20 to 40 g kg–1 found by O'Connor et al. (2004). The biosolids contained large amounts of NH4–N as compared with NO3–N, which is consistent with the anaerobic digestion process. Similarly, Sommers (1976) found that a significant amount of N in biosolids existed as NH4–N.


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  		Table 2. Physical and chemical properties of the applied biosolids.

 
The biosolids applied from 1993 to 2005 contained low concentrations of metals as compared with limits set by USEPA (1993) (Table 3 ). Metal concentrations in our biosolids were considerably less than the national average with the exception of Pb (USEPA, 1990). The biosolids used in our study could not possibly qualify as exceptional quality (EQ) biosolids (USEPA, 1993) because of Mo and Pb. The Mo concentrations of applied biosolids exceeded EQ guidelines for all years while Pb concentrations were greater from 1993 to 1997 (Table 3). Additionally, Pb concentrations of biosolids were greater than the USEPA Part 503 ceiling pollutant concentrations during 1993 and 1995.


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  		Table 3. Metal and phosphorus content of biosolids applied from 1993 to 2005.

 
Effect of Biosolids and Ammonium Nitrate on Soil Characteristics
Application of biosolids did not significantly change (p > 0.05) soil pH as compared with the untreated control (Table 4 ). Our results are similar to those of several other researchers who have reported the long-term application of biosolids did not significantly effect soil pH (Maguire et al., 2000a; Cogger et al., 2001; Gaskin et al., 2003; Shober et al., 2003). Application of ammonium nitrate significantly decreased (p < 0.001) soil pH as compared with the untreated control, especially in the higher N rates. Our results are consistent with several other researchers who found that long-term repeated applications of ammonium nitrate reduced soil pH (Eghball, 1999, 2002; Moore and Edwards, 2005; Mullen et al., 2005).


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  		Table 4. Physical and chemical properties of the Norge soil (fine-silty, mixed, active, thermic Udic paleustoll) sampled in 2005 following 13 yr of biosolids and ammonium nitrate fertilizer applications.

 
Excessive soil salinity may reduce the growth and yield of many crops and is commonly determined by measuring the electrical conductivity (EC) of a solution extracted from a water-saturated soil paste. The larger application rates of biosolids (i.e., rates of 269 and 539 kg PAN ha–1) increased EC (p < 0.001) as compared with the untreated control, whereas application of ammonium nitrate did not significantly elevate EC (Table 4). Even the greatest application rate of biosolids produced an EC that was approximately an order of magnitude less than the EC of 7.4 dS m–1 reported by Soltanpour and Follett (1995) to decrease wheat yield by 10%.

The greatest application rate of 539 kg PAN ha–1 biosolids increased total soil C (two-fold increase) as compared with the untreated control but ammonium nitrate did not increase total soil C (Table 4). Similarly, some researchers have reported that long-term application of biosolids did not increase total soil C (Gaskin et al., 2003; Shober et al., 2003). However, other researchers have found the long-term application of biosolids increased total soil C (Tester, 1990; Maguire et al., 2000a). The differences in the studies may be due to different cropping systems, biosolids type, and/or climate. Total N was increased by the higher application rates of 269 and 539 kg PAN ha–1 biosolids but not by the lesser rates, whereas the application of ammonium nitrate did not increase (p > 0.05) total N. Our results are similar to those of Sui et al. (1999) who found that long-term (>6 yr) application of biosolids at rates similar to our application rates of 269 and 539 kg PAN ha–1 only slightly increased total N. Long-term application of biosolids increased (p < 0.05) soil NH4–N by a factor of two while the greatest application rate of 539 kg N ha–1 ammonium nitrate increased (p < 0.001) soil NH4–N by a factor of four (Table 4). Conversely, the maximum rate of biosolids increased soil NO3–N by a factor of six while at the same rate ammonium nitrate only increased NO3–N by a factor of three. Currently Oklahoma recommends 40 mg kg–1 PAN for a yield goal of 40 bushels acre–1, which is equivalent to 2727 kg ha–1 (Zhang and Raun, 2006). Therefore, the maximum rate of biosolids has resulted in sufficient residual soil N for at least one growing season while the application rates of 269 and 539 kg PAN ha–1 of ammonium nitrate increased soil N above the recommended rate. However, because the rooting depth of wheat is >15 cm, deeper sampling would be required to adequately assess profile available N.

Effect of Biosolids and Ammonium Nitrate on Secondary Nutrients and Micronutrients
Application of biosolids did not increase (p > 0.05) concentrations of Ca, Mg, or SO4–S (Table 5 ). The larger application rates of biosolids (i.e., N rates of 269 and 539 kg PAN ha–1) increased (p < 0.01) levels of B while application rates of ≥ 180 kg PAN ha–1 significantly increased (p < 0.001) concentrations of Cu, Fe, and Zn. Our results are similar to those of other researchers who have reported that biosolids applications increased DTPA extractable Cu and Zn (Barbarick et al., 1998; Cogger et al., 2001; Barbarick and Ippolito, 2003; Evanylo et al., 2006). A significant relationship existed between DTPA extractable Cu and biosolids applied (p < 0.001, r2 = 0.87) (Fig. 1A ). Additionally, significant relationships (p < 0.001) existed between DTPA extractable Zn and biosolids applied for (r2 = 0.92) (Fig. 1B) and between DTPA extractable Fe and biosolids applied (r2 = 0.80) (Fig. 1C). Similarly, Evanylo et al. (2006) reported highly significant relationships (r2 = 0.99) for the relationships between DTPA extractable Cu and Zn and biosolids applications rate. Application of ammonium nitrate did not increase micronutrients with the exception of Fe, which was significantly increased at the greater treatment rates (Table 5).


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  		Table 5. Secondary nutrients and micronutrients content for the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll) sampled in 2005 following 13 yr of biosolids and ammonium nitrate fertilizer applications.

 

Figure 1
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  		Fig. 1. The relationships between (A) DTPA extractable Cu and total biosolids applied, (B) DTPA extractable Zn and total biosolids applied, and DTPA extractable Fe and total biosolids applied for the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll). ***p < 0.001.

 
Effect of Biosolids and Ammonium Nitrate on Heavy Metal Concentrations
Some metal concentrations in soil were increased by application of biosolids as compared with the reference plot that did not receive biosolids: Cd (p < 0.05), Cu (p < 0.001), Pb (p < 0.001), Mo (p < 0.001), and Zn (p < 0.001) (Table 6 ). Cadmium in biosolids-treated soils ranged from 2.5- to 3.3-fold greater than the mean for the reference plot. Similarly, biosolids treated soils were elevated in Cu (2.5- to 4.2-fold), Pb (1.4- 2.5-fold), Mo (2.9- 8.8-fold), and Zn (1.4- 3.5-fold) as compared with the control plot. Cadmium, Cu, Pb, and Zn increased linearly with application rate while Mo displayed both a quadratic and linear relationship with application rate. Concentrations of As, Cr, and Ni did not significantly increase (p > 0.05) with the application of biosolids. Our results differ somewhat from those of Mullen et al. (2005) who sampled these same plots in 2000 and reported that soil Cu and Pb doubled with the highest application rate and that soil Mo was increased by a factor of four at the highest application rate. In their study, they also found application of biosolids did not increase As, Cr, or Ni in soil. The difference in our study and the one by Mullen et al. (2005) is probably due to the digestion procedure used. Mullen et al. (2005) used a HNO3, HClO4 procedure while our study used USEPA Method 3050B, which uses HNO3 followed by H2O2, and is a much stronger digestion. Despite repeated annual applications of biosolids for 13 yr and increased metal concentrations in soil, none of the metals were greater than reported concentrations in normal or agricultural soils of the United States (Adriano, 2001) (Table 6). As it was expected, the application of ammonium nitrate did not significantly increase any of the heavy metals.


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  		Table 6. Concentrations of heavy metals in the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll) sampled in 2005 following thirteen years of biosolids and ammonium nitrate fertilizer applications.

 
A significant increase in the concentration of a pollutant does not necessarily result in increased risk to plants, animals, or humans. Risk assessments were used to determine cumulative pollutant loading rates (CPLRs) as a component of the Part 503 regulations. These CPLRS represent the maximum amount of a pollutant that can be added to a soil without causing adverse effects on plants, animals, and humans (USEPA, 1993). One way of addressing the issue of potential risk to human health and the environment is the comparison of measured cumulative loading rates of pollutants with their respective CPLRs (Shober et al., 2003). For the measurement of cumulative loading rates, we assumed a soil mass of 2 x 106 kg ha–1, which is equivalent to a bulk density of 1.33 g cm–3 (USEPA, 1995b). We also used the CPLR for Mo published by O'Connor et al. (2001) (Table 7 ) because to our knowledge the original published CPLR of 18 kg ha–1 for Mo is under re-examination (USEPA, 1995a). The measured percentage of CPLRS for biosolids applied in our study ranged from 1.3% for Cd to 23.3% for Pb. The small percentages of CPLRs indicates that the land application of biosolids had not significantly increased the risk to human health.


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  		Table 7. Measured mean pollutant loading rates per hectare for a 15-cm depth of biosolids incorporation in the greatest rate of biosolids treatment (539 kg N ha–1) and the percentage of the cumulative pollutant loading rates.

 
Effect of Biosolids on Extractable Phosphorus and Other Parameters
The annual application of biosolids to soil resulted in a wide range (9.83–401 mg kg–1) of soil M3P and WSP (2.42–18.2 mg kg–1) (Table 8 ). Both M3P and WSP significantly increased (p < 0.001) with the application of biosolids. Currently, Oklahoma considers a M3P concentration of 33 mg kg–1 as 100% sufficient for wheat production (Zhang and Raun, 2006). All biosolids application rates resulted in M3P concentrations greater than that required for wheat production. Forty bushel of wheat per acre is considered a good yield in Oklahoma. The recommended N agronomic rate for wheat in Oklahoma assuming a yield goal of 40 bushel acre–1 is 90 kg PAN ha–1, which is equivalent to 2727 kg ha–1 (Zhang and Raun, 2006). The recommended N agronomic rate of 90 kg PAN ha–1 of biosolids did not result in soil P concentrations greater than the environmental threshold of 200 mg kg–1 established by the USDA-NRCS Code 590 (USDA-NRCS, 2001a) for the land application of manures in Oklahoma non-nutrient limited watersheds, while rates ≥ 180 kg PAN ha–1 of biosolids produced soil P levels greater than the environmental threshold. Our results are similar to those of several other researchers who have reported that long-term application of biosolids increased soil P (Cogger et al., 2001; Maguire et al., 2000a, 2000b; Penn and Sims, 2002; Mantovia et al., 2005).


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  		Table 8. Total amount of P applied, mean extractable soil P, mean ammonium oxalate P saturation index (PSIox), and mean ammonium oxalate-extractable P (Pox), Al (Alox), and Fe (Feox) for each treatment of long-term biosolids experiment.

 
Soil P saturation has been used in the evaluation of the environmental fate of P. Phosphorus saturation is defined as the amount of P sorbed divided by the P sorption capacity of the soil. The concept of P saturation is meaningful as it estimates the degree to which P sorption sites have been filled and indicates the potential desorbability of soil P (Beauchemin and Simard, 1999). Phosphorus saturation has been highly correlated with P desorption such that P desorption increases with higher degrees of P saturation (Sibbesen and Sharpley, 1997). Phosphorus saturation is viewed as an environmental indicator of soil P because it has been found to be a good indicator of P availability to runoff and leachate (Kleinman and Sharpley, 2002). Similarly to M3P and WSP, PSIox was significantly increased (p < 0.001) by all application rates of biosolids and ranged from 3.01% in the untreated control to 44.1% at the greatest treatment rate (Table 8). Phosphorus extracted by acid ammonium oxalate method ranged from 55 to 1350 mg kg–1 in the biosolids-amended plots. Concentrations of Alox ranged from 885 to 1628 mg kg–1 while Feox ranged from 1103 to 2157 mg kg–1. All of the treatments significantly increased Pox (p < 0.001) as compared to the untreated control. However, only application rates ≥ 180 kg PAN ha–1 significantly increased (p < 0.001) Feox and Alox as compared with the untreated control (Table 8). Additionally, application rates of ≥180 kg PAN ha–1 significantly increased [Alox + Feox]. Thus, the greater rates of biosolids application increased P sorption as well as P, thus offsetting some of the increase in PSIox. Similary, Maguire et al. (2000b) examined soils collected from eight farms with a history of biosolids application. They found that Pox and Feox were higher on all eight biosolids-amended soils as compared to setback areas (no application of biosolids), whereas Alox was higher in six of the eight soils.

Relationships between Long-Term Biosolids Addition and Extractable Soil Phosphorus
A significant relationship (r2 > 0.87, p < 0.001) existed between M3P and biosolids-borne P with a slope of 0.03 (Fig. 2A ). The slope of 0.03 indicates that approximately 33 kg ha–1 of biosolids-borne P would be required to increase M3P by 1.0 mg kg–1 under normal wheat production practices. Our results differ from those of Zhang et al. (2006), who evaluated the long-term application of triple super phosphate (TSP, 0–46–0), and reported a slope of 0.07 for the M3P-P added relationship. This means that more than double the amount of P must be added in the form of biosolids to increase M3P by 1.0 mg kg–1 as compared with the application of a commercial fertilizer. This is probably because of increased adsorption of P due to elevated concentrations of Alox and Feox in the biosolids plots (Table 8). Similarly a significant relationship (r2 = 0.76, p < 0.001) existed between WSP and biosolids-borne P with a slope of 0.001 (Fig. 2B). This slope indicates that approximately 1000 kg ha–1 of biosolids P would be required to increase WSP by 1.0 mg kg–1 under normal wheat production practices.


Figure 2
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  		Fig. 2. The relationships between (A) Mehlich 3 P and P added and (B) water soluble P and P added from biosolids for the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll). ***p < 0.001.

 
Relationships between Extractable Soil Phosphorus and Phosphorus Saturation Index
Several researchers have shown that M3P is significantly related to WSP for both soils amended with either manure or an inorganic P source (McDowell and Sharpley, 2001; Burt et al., 2002; Sims et al., 2002; Fuhrman et al., 2005). Mehlich-3 P was significantly related to WSP (p < 0.001, r2 = 0.89) for the biosolids treatments (Fig. 3 ). Additionally, PSIox was significantly related to P added from biosolids (p < 0.001, r2 = 086) (Fig. 4 ). Previous work has shown PSIox–extractable P relationships to be highly significant for soils amended with manure and/or an inorganic P source (Sims et al., 2002; Tarkalson and Mikkelsen, 2004; Davis et al., 2005; Zhang et al., 2006). Significant relationships were observed between PSIox and M3P (p < 0.001, r2 = 0.98) (Fig. 5A ) and between PSIox and WSP (p < 0.001, r2 = 0.90) (Fig. 5B).


Figure 3
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  		Fig. 3. The relationship between water soluble P and Mehlich3 P for the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll). ***p < 0.001.

 

Figure 4
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  		Fig. 4. The relationship between the phosphorus saturation index calculated with acid ammonium oxalate extraction data (PSIox) and P added from biosolids for the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll). ***p < 0.001.

 

Figure 5
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  		Fig. 5. The relationships between (a) Mehlich 3 P and P saturation index calculated with acid ammonium oxalate extraction data (PSIox) for the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll) and (B) water soluble P and PSIox for the Norge soil. ***p < 0.001.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In our study, we over-applied biosolids to some treatments to increase the number of agronomic years. The rates applied ranged from 50 to 600% of the recommended agronomic rate.

The effect of application of biosolids on soil characteristics was variable. Soil pH was not changed by the application of biosolids but the application of ammonium nitrate significantly decreased soil pH. Soil EC was increased at the two greatest rates of biosolids application but was not high enough to affect wheat yields. Application of biosolids had little effect on total soil C except for the greatest application rate, which doubled soil C. Soil total N, NH4–N, and NO3–N increased in biosolids application rates of ≥180 kg PAN ha–1.

Concentrations of soil Ca, Mg, and SO4–S were not affected by the application of biosolids whereas DTPA-extractable Cu, Fe, and Zn increased when biosolids with ≥180 kg PAN ha–1 were applied. Boron increased slightly with the greatest biosolids application rate. Application of ammonium nitrate did not increase micronutrients with the exception of Fe, which was increased by a factor of 2.5 in the greatest application rate. Significant relationships were observed between DTPA extractable Cu, Zn, and Fe and biosolids applied.

The biosolids applied during the experiment met the USEPA exceptional quality criteria for heavy metals with the exception of Mo and Pb. Soil Cd, Cu, Pb, Mo, and Zn increased with the application of biosolids while As, Cr, and Ni did not. Long-term annual applications of biosolids did not increase soil heavy metal concentrations above normal background concentrations.

Due to long-term biosolids application, a wide range of soil P levels was observed in our study. The addition of biosolids to the Norge soil increased M3P and WSP and these P levels were well correlated with biosolids application rates. Our study indicates that approximately double the amount of P must be added in biosolids as compared with a commercial fertilizer to increase M3P by one unit. This is probably due to strong binding of P by Alox and Feox. This supports the concept advanced by other researchers that P guidelines for biosolids probably need to take into account the strong adsorption of P by Al and Fe oxides present in biosolids. Even the smallest long-term biosolids application rate resulted in M3P concentrations greater than that required for wheat production (33 mg kg–1). Application of biosolids at rates of greater than two times the recommended agronomic rate of 90 kg PAN ha–1 resulted in M3P concentrations that exceed an environmental threshold of 200 mg kg–1 established by the USDA-NRCS Code 590 for the land application of manures in Oklahoma non-nutrient limited watersheds. Significant relationships were found between M3P and WSP. Additionally, the PSIox–M3P and PSIox–WSP relationships were found to be highly significant.

Our research clearly shows that the long-term application of biosolids using an N-based approach increases soil P above crop production needs. Currently Oklahoma requires that both biosolids and receiving soils be tested before biosolids application. However, Oklahoma has not established a threshold value for soil P in regard to biosolids application. It appears that the application of biosolids above N agronomic rate should be regulated based on other criteria such as an agronomic P threshold, an environmental P threshold, or a P site index.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication January 17, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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