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Journal of Environmental Quality 30:1023-1033 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Surface Water Quality

Relationships between Biosolids Treatment Process and Soil Phosphorus Availability

R.O. Maguirea, J.T. Simsa, S.K. Dentelb, F.J. Coalec and J.T. Mahb

a Dep. Plant and Soil Science, Univ. of Delaware, Newark, DE 19717-1303
b Dep. Civil and Environmental Engineering, Univ. of Delaware, Newark, DE 19717-1303
c Dep. Natural Resource Science and Landscape Architecture, Univ. of Maryland, College Park, MD 20742-5821

Corresponding author (rmaguire{at}udel.edu)

Received for publication May 31, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Laws mandating phosphorus (P)-based nutrient management plans have been passed in several U.S. Mid-Atlantic states. Biosolids (sewage sludge) are frequently applied to agricultural land and in this study we evaluated how biosolids treatment processes and biosolids P tests were related to P behavior in biosolids-amended soils. Eight biosolids generated by different treatment processes, with respect to digestion and iron (Fe), aluminum (Al), and lime addition, and a poultry litter (PL), were incubated with an Elkton silt loam (fine-silty, mixed, active, mesic Typic Endoaquult) and a Suffolk sandy loam (fine-loamy, siliceous, semiactive, thermic Typic Hapludult) for 51 d. The amended soils were analyzed at 1 and 51 d for water-soluble phosphorus (WSP), iron-oxide strip–extractable phosphorus (FeO-P), Mehlich-1 P and pH. The biosolids and PL were analyzed for P, Fe, and Al by USEPA 3050 acid–peroxide digestion and acid ammonium oxalate, Mehlich-1, and Mehlich-3 extractions. Biosolids and PL amendments increased extractable P in the Suffolk sandy loam to a greater extent than in the Elkton silt loam throughout the 51 d of the incubation. The trend of extractable WSP, FeO-P, and Mehlich-1 P generally followed the pattern: [soils amended with biosolids produced without the use of Fe or Al] > [PL and biosolids produced using Fe or Al and lime] > [biosolids produced using only Fe and Al salts]. Mehlich-3 P and the molar ratio of P to [Al + Fe] by either the USEPA 3050 digestion or oxalate extraction of the biosolids were good predictors of changes in soil-extractable P following biosolids but not PL amendment. Therefore, the testing of biosolids for P availability, rather than total P, is a more appropriate tool for predicting extractable P from the biosolids-amended soils used in this study.

Abbreviations: 1 d, 51 d, Day 1 and Day 51 of the incubation study • FeO-P, iron-oxide strip–extractable phosphorus • PL, poultry litter • Pox, Feox, Alox, oxalate-extractable phosphorus, iron, and aluminum • USEPA 3050 P, Fe, Al, and Ca, total sorbed phosphorus, iron, aluminum, and calcium by the USEPA 3050 acid–peroxide digestion method • WSP, water-soluble phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS losses from agricultural crop land have been implicated as one of the major factors responsible for accelerated eutrophication of surface waters at many locations in the USA, such as the Great Lakes, Chesapeake and Delaware Bays, Lake Okeechobee, and the Everglades (Daniel et al., 1998). While problems with eutrophication are long-standing, recent concerns about the potential human health impacts of Pfiesteria spp. in the Mid-Atlantic region of the USA have accelerated efforts to reduce nonpoint-source pollution by agricultural nutrients (Burkholder and Glasgow, 1997). In particular, there is now a focus on the buildup of P to "excessive" concentrations in many soils of the Chesapeake Bay watershed, due to more P being added in animal manures, biosolids, and fertilizers than is removed in crop harvest. These "excessive" soil P concentrations can be measured by environmental soil P tests, such as WSP and FeO-P, that have been related to P loss from agricultural land or by agronomic soil tests such as Mehlich-1 and Mehlich-3 that estimate available P for crop growth (Pote et al., 1996; Sharpley et al., 1995; Sims and Heckendorn, 1991).

There is concern that these high-P soils represent an increased risk for nonpoint-source pollution of surface waters (Sharpley et al., 1996; Sims et al., 1998; Sims et al., 2000). Where water quality standards are not being met, states must develop total maximum daily loads, which are a "quantitative analysis of the assimilative capacity of a water body," with the load split between point and nonpoint sources (Parry, 1998). Laws mandating nutrient management plans have recently been passed in Delaware, Maryland, Pennsylvania, and Virginia. In Maryland, the Water Quality Improvement Act of 1998 mandates P-based nutrient management of manures and biosolids (municipal sewage sludges), causing concern for municipalities that rely upon land application as a means to beneficially recycle biosolids (Simpson, 1998). Under the Delaware Nutrient Management Act of 1999, the "application of P to ‘high’ P soils cannot exceed a 3-yr crop removal rate" (Sims, 1999). In practice this means that "high" P soils will only receive P applications once every 3 yr. Although biosolids are only applied to a small proportion of agricultural land, current N-based management plans have been shown to lead, with time, to soil P accumulation above the levels needed for optimum crop growth (Maguire et al., 2000a; Peterson et al., 1994).

The effect of wastewater treatment processes (particularly the addition of metal salts and/or lime) on the availability of P in biosolids, relative to fertilizers and animal manures, is an area of particular interest today. If biosolids P is less plant available and less mobile than manure and fertilizer P, then biosolids P represents less risk of P loss to surface waters. There is evidence that the metal content of biosolids decreases the P availability. Corey (1992) found that as the [aluminum (Al) + iron (Fe)] to P ratio increased, there was a trend for decreased availability of P. Maguire et al. (2000a) found that farm soils with a history of biosolids applications had significantly higher concentrations of oxalate-extractable P and Fe, as well as a trend of higher oxalate-extractable Al, relative to soils that had not received biosolids. Maguire et al. (2000b) characterized inorganic P fractions in biosolids-amended soils and found that Fe-P and Al-P were the predominant forms in soils of the Mid-Atlantic USA and that there was a trend for higher Al-P and significantly greater Fe-P where biosolids had been applied compared with unamended soils. Maguire et al. (2000a)(b) concluded that the increase in soil components responsible for P retention (predominantly Fe and Al) may mitigate the increased P release into runoff or leaching waters that would be expected from the increase in soil P associated with biosolids applications. Wen et al. (1997) found that biosolids were not as good a source of P for crops as composted livestock manure and concluded that this was due to the addition of Fe and Al during biosolids production. Soon and Bates (1982) reported that Fe and Al in biosolids applications over 7 yr increased soil Fe and Al oxide content, which in turn increased P retention.

Concerns about P loss from agriculture and past research emphasize the need to understand how biosolids treatment processes affect P availability immediately following biosolids application, as well as to evaluate biosolids tests and to determine which soil tests are the most appropriate for biosolids-amended soils. Accordingly, the objectives of this work were to characterize the influence of biosolids treatment processes (particularly digestion and addition of metals and lime) on the potential mobility and availability of biosolids P in two soil types with time. If nutrient management shifts to a P basis, it is necessary to understand how biosolids P behaves in soils relative to other P sources. Therefore, we included a poultry litter sample for comparison. An evaluation of biosolids tests for predicting extractable P in biosolids-amended soils was also conducted.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Collection and Characterization
Two Ultisols were obtained for use in this study (an Elkton silt loam and a Suffolk sandy loam). The soils had similar soil test P values, but different textures and organic matter contents (Table 1). The soils were collected by sampling the Ap horizon (0–20 cm) of setback areas (zone of no biosolids application) on two Maryland farms where biosolids had been applied for crop production purposes. After sampling, the soils were air-dried and ground to pass through a 2-mm sieve, prior to analysis and subsequent use in the incubation study.


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  		Table 1. Selected properties of the two soils used in the incubation study.

 
Soil pH, buffer pH (Adams–Evans buffer), organic matter (OM = loss on ignition -0.3%), and cation exchange capacity (CEC) were measured by standard methods of the University of Delaware Soil Testing Laboratory (Sims and Heckendorn, 1991). Soil moisture content at field capacity was determined as described by Tan (1996). Each soil was analyzed for P as follows:

(i) Water-soluble P (WSP): 1:10 soil to deionized water, 1-h reaction time, filtration through a 0.45-µm Millipore membrane.

(ii) Iron-oxide strip–extractable P (FeO-P): 1:40 soil to 0.01 M CaCl2 + iron-oxide coated filter paper strip, 16-h reaction time, dissolving of Fe and P from the filter paper strip for 1 h in 1 M H2SO4 (Chardon et al., 1996).

(iii) Mehlich-1 P: 1:4 soil to 0.05 M HCl + 0.0125 M H2SO4, 5-min reaction time, filtration through Whatman #2 filter paper.

(iv) Mehlich-3 P: 1:10 soil to 0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.13 M HNO3 + 0.001 M EDTA.

(v) Oxalate P (Pox): 1:40 soil to 0.2 M acid ammonium oxalate (pH 3), 2-h reaction time (in the dark) (McKeague and Day, 1966).

The oxalate extract was analyzed for P, Al, and Fe and the USEPA 3050 extract for P, Al, Fe, and Ca by inductively coupled plasma–atomic emission spectroscopy (ICP–AES); all other extracts were analyzed for P colorimetrically by the molybdate blue method of Murphy and Riley (1962).

Biosolids and Poultry Litter Collection and Characterization
Eight biosolids produced by a range of wastewater treatment processes, with respect to digestion and addition of lime and metal salts [e.g., FeCl3, Al2(SO4)3], were collected from municipal wastewater treatment plants in the Mid-Atlantic region of the USA (Table 2). The biosolids were dried at 60°C and ground to pass an 0.8-mm screen in a stainless steel Wiley mill. All biosolids were analyzed for (i) total P by nitric acid–perchloric acid digestion (Olsen and Sommers, 1982); (ii) USEPA 3050 P, Al, Fe, and Ca by the USEPA 3050 acid–peroxide digestion method (a measure of the metals adsorbed by biosolids constituents that does not include metals associated with silicates, but often extracts 75 to 90% of total metals measured by more complete digestion methods; USEPA, 1986); (iii) oxalate-extractable P, Al, and Fe, as described above; (iv) water-soluble P, Mehlich 1-P, and Mehlich 3-P, as described above; and (v) total carbon (C) and nitrogen (N) by combustion in a LECO (St. Joseph, MI) CNS analyzer. A poultry litter (PL; mixture of poultry manure and sawdust) sample obtained from a cooperating farmer was analyzed in the same manner as the biosolids.


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  		Table 2. Selected properties of the biosolids and poultry litter (PL) used in the incubation study.

 
Incubation Study
Each of the eight biosolids and the PL were incorporated, in triplicate, with the two soils at a rate equivalent to 8 Mg/ha (assuming 2242 Mg topsoil/ha, which equates to 52 g of soil and 0.1855 g biosolids/PL on a dry weight basis). This represents an application rate similar to those typically used when biosolids are applied in accordance with N-based crop management practices. The amended soils and control soils (no biosolids or PL added) were incubated at 70% of field capacity in polyethylene containers for 51 d at 25°C. Subsamples were removed for analysis at four time intervals (1, 6, 22, and 51 d). Two holes were cut in the tops of the incubation containers to allow gaseous exchange and prevent anaerobic conditions during the incubation. Soil moisture content was maintained at the desired value by adding deionized water at weekly intervals. At each sampling interval the incubating soils were analyzed immediately (before drying) for WSP and FeO-P. At the beginning (1 d) and end (51 d) of the incubation soils were also removed and air-dried for analysis of Mehlich-1 P, oxalate-extractable P, Al, and Fe and pH (only one pH measurement was possible per treatment, due to insufficient incubated soil; the soil from all three reps had to be combined for pH measurement, and therefore no statistics are given for incubation pH). Where the results are expressed as changes in WSP, FeO-P, and Mehlich-1 P (relative to the control) per 100 kg USEPA 3050 P/ha added in the biosolids or PL, the values were calculated as:

where the P rate is the rate of USEPA 3050 P applied in the biosolids or PL in kg/ha.

Statistical Analysis
All statistical analyses of these data were performed using the PROC GLM procedure (SAS Institute, 1989). The 0.05 probability value was used to determine significant difference. Differences between means were evaluated using least significant difference (LSD).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Properties
The two soils used in this study are representative of the major soil types in the Mid-Atlantic USA where biosolids are applied. The Elkton silt loam was finer-textured and more acidic, and had a higher organic matter content and cation exchange capacity than the coarse-textured, low organic matter, low cation exchange capacity Suffolk sandy loam (Table 1). The Elkton, and similar soil types, are usually found in the Piedmont region, while the Suffolk and other sandy loams and loamy sands predominate in the coastal plain. The Elkton soil also had much higher concentrations of amorphous (oxalate-extractable) Al and Fe than the Suffolk soil (Table 1).

The soils were in the "low" (Suffolk) and "medium" (Elkton) soil P fertility categories according to current soil test P criteria of the University of Delaware Soil Testing Laboratory (based on the Mehlich-1 soil test for P: low = 0 to 12 mg/kg; medium = 13 to 24 mg/kg; optimum = 25 to 50 mg/kg; excessive = >50 mg/kg). Oxalate P, Mehlich-3 P, and FeO-P were also higher in the Elkton soil, but WSP was slightly lower than in the Suffolk sandy loam (Table 1).

Biosolids and Poultry Litter Properties
The biosolids used in this study represent material from the major types of wastewater treatment plant processes in the Mid-Atlantic region (Table 2). Six of the eight biosolids (Biosolids 3–8) were treated with metal salts (primarily Fe), two were treated with lime and metal salts (Biosolids 3 and 4), and two were not treated with metal salts or lime (Biosolids 1 and 2). Five of the biosolids were digested anaerobically (2, 5, 6, 7, and 8); the other three (1, 3, and 4) were undigested materials.

All biosolids had relatively high N concentrations (average = 42 g/kg) and low C to N ratios (<11:1), suggesting that they would be good sources of plant-available N (Table 2). Total P concentrations in biosolids ranged from 10.5 to 30.0 g/kg (average = 21.3 g/kg), while USEPA 3050 P ranged from 6.8 to 24.5 g/kg (average = 16.5 g/kg) and Pox from 4.9 to 32.9 g/kg (average = 21.2 g/kg). Note that average total P was almost identical to average Pox, while USEPA 3050 P was ~80% of total P and Pox. However, these three measurements of P in the biosolids were all significantly correlated with each other (total P: USEPA 3050 P, r = 0.94; total P: Pox, r = 0.90; USEPA 3050 P: Pox, r = 0.93 [all significant at the 0.001 probability level]). The amounts of USEPA 3050 P added to the soils in the N-based biosolids application rate used in this study (8 Mg/ha) ranged from 54 to 196 kg P/ha and averaged 133 kg P/ha (Table 3).


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  		Table 3. Amounts of total sorbed (USEPA 3050) and oxalate-extractable P, Al, Fe, and Ca added to soils in the incubation study in biosolids or poultry litter (PL){dagger}.

 
Of some interest with regard to the availability and speciation of P in biosolids is the fact that total P (Table 2) and Pox usually exceeded the USEPA 3050 P (Table 3), which is regarded as "total sorbed P" (average value for biosolids Pox = 21.2 g/kg; average equivalent P added = 169 kg/ha). This suggests that most of the P in biosolids is in a form that can be rapidly and easily extracted by oxalate, a chemical extracting solution known to remove P from amorphous oxides of Fe and Al in soil (van der Zee and van Riemsdijk, 1988). Compared with total P and USEPA 3050 P, oxalate was less effective at extraction of P when biosolids had been limed and had high concentrations of Ca (Biosolids 3 and 4), possibly due to precipitation of Ca oxalate in the extracting solution.

Some biosolids contained appreciable quantities of Fe, Al, and Ca, which for the most part reflected the wastewater treatment processes used. Biosolids produced using Fe treatment had higher concentrations of USEPA 3050 Fe (15.7 to 40.1 g/kg) than those without Fe treatment (3.1 to 10.6 g/kg). For the two biosolids to which lime was added, Ca concentrations in the biosolids were markedly elevated (>90 g/kg Ca) compared with unlimed biosolids (8.1 to 28.8 g/kg Ca). USEPA 3050 Al concentrations ranged from 2.7 to 14.4 g/kg, except for Biosolids 5, where a mixture of Fe and Al had been added during wastewater treatment, and thus the Al concentration was much higher (51.3 g/kg). As with P, oxalate tended to be a more effective chemical extractant for Al and Fe (average of 13.6 and 25.7 g/kg, respectively) from biosolids compared with the USEPA 3050 method (average of 12.3 and 20.2 g/kg, respectively). This suggests that, as with P, much of the Al and Fe in biosolids is in amorphous forms. In contrast with the biosolids, the PL used in this study had similar, but slightly lower, USEPA 3050 P (13.3 g/kg) and Ca (15.0 g/kg) concentrations and much lower Al and Fe concentrations than most biosolids. Poultry litters often contain appreciable Ca and P because of the use of Ca phosphates and limestone in poultry feed.

In general, we observed that amending soils with biosolids based on plant N requirements adds considerably more P (~135 kg P/ha) than is removed in crop harvest (~15 to 40 kg P/ha). This supports past research that suggests that applying biosolids to meet crop N requirements can lead to overapplication of P (Kelling et al., 1977; Peterson et al., 1994). However, depending on the wastewater treatment plant process used, biosolids can also add appreciable amounts of Fe, Al, and Ca, relative to the amount of P applied (Table 3), which, as noted by Maguire et al. (2000a), may mitigate the potential for P losses in runoff or leaching. Adding PL also supplies considerably more P (106 kg/ha) than is removed in crop harvest, which agrees with past research that shows elevated levels of soil P associated with intensive animal agriculture (Sims et al., 2000; van der Molen et al., 1998).

Biosolids Effects on Water-Soluble Phosphorus and Iron-Oxide Strip–Extractable Phosphorus in Amended Soils
We observed a consistent pattern for the effect of biosolids and PL (except for PL in the Elkton silt loam; Table 4) on WSP concentrations in both soils, as illustrated by results for four representative biosolids and the control in Fig. 1 (data for the other biosolids and PL were similar and are not presented). Adding biosolids initially increased the concentration of WSP relative to the unamended soil. This was followed by a rapid decrease in WSP by the end of the first week of the incubation, a continued (but smaller) decrease in WSP by 3 wk, and little change thereafter. The increases in WSP observed were always greater throughout the incubation in the sandy, low organic matter Suffolk soil (Fig. 1b) than in the fine-textured, higher organic matter Elkton soil (Fig. 1a), although they narrowed with time in some cases.


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  		Table 4. Changes in water-soluble P, Fe-strip P (FeO-P), and Mehlich-1 P during the incubation study. Results are expressed as changes in each extract of soil P, relative to the control, per 100 kg USEPA 3050 P/ha added in the biosolids or poultry litter (PL).

 


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  		Fig. 1. The influence of biosolids type on water-soluble phosphorus concentrations in the (a) Elkton and (b) Suffolk soils during the 51-d incubation study. Data are for Biosolids 1, 2, 4, and 6 (B1, B2, B4, B6). Note differences in scale for the y axis.

 
The patterns we observed with FeO-P concentrations during the incubation were similar, but had some important differences relative to the WSP trends. As with WSP, adding biosolids or PL initially increased FeO-P, relative to the unamended soils, followed by a decline during the first 3 wk (Fig. 2). The FeO-P concentrations were also consistently higher than WSP (from 2- to 10-fold), as would be expected since FeO-P measures both WSP and P that easily desorbs from soils onto an Fe oxide strip (a "P sink") during a 24-h equilibration (Menon et al., 1997). However, the differences in FeO-P between soil types were not as distinct. Unlike WSP, after the initial decrease in FeO-P, we observed a tendency for FeO-P concentrations to increase gradually with time. This increase in FeO-P occurred in both the unamended soils and biosolids-amended soils and in some cases FeO-P reached concentrations at or slightly above initial FeO-P values (Fig. 2a,b).



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  		Fig. 2. The influence of biosolids type on iron-oxide strip–extractable phosphorus (FeO-P) concentrations in the (a) Elkton and (b) Suffolk soils during the 51-d incubation study. Data are for Biosolids 1, 2, 4, and 6 (B1, B2, B4, B6).

 
When comparing the changes in soil P concentrations for the different biosolids types during the incubation study, it is important to remember that the biosolids application rates used resulted in the addition of variable rates of total P, Al, Fe, and Ca (Table 3). To account for the differences in P added, we normalized the differences in soil P extracts on the basis of total P added. Thus we calculated the changes in WSP, FeO-P, and soil test P (Mehlich-1 P), relative to the unamended soil, initially and at the end of the incubation study, and expressed them per 100 kg USEPA 3050 P added in each biosolids (Table 4). For example, adding Biosolids 1 to the Elkton soil added 145 kg P/ha and increased WSP, relative to the control soil, by 1.12 mg P/kg. This change was equivalent to an increase of 0.77 mg WSP/kg per 100 kg P/ha added. From this perspective (the change in soil P associated with equal rates of added P) it is clear that the biosolids not treated with a metal salt (Biosolids 1 and 2) consistently had the greatest initial increases and maintained the highest concentrations of WSP, FeO-P, and Mehlich-1 P in the incubation study. Also, these increases were greater in the Suffolk soil than in the Elkton soil, possibly due to the Elkton soil having a greater P sorption capacity, as would be expected from its greater content of Alox and Feox, compared with the Suffolk soil (Table 1). Biosolids treated only with metal salts (Biosolids 5, 6, 7, 8) consistently led to the smallest increases in all three forms of soil P. Those treated with Fe and lime (Biosolids 3 and 4) led to intermediate changes in soil P. The effects of adding PL on WSP, FeO-P, and Mehlich-1 P tended to be intermediate between the biosolids treated with metal salts only and those with no metal salt treatment, especially after 51 d.

The greater P extracted from soils amended with biosolids that had been treated with lime, compared with those that had no lime treatment, may be associated with increased soil pH, as the pH in soils amended with Biosolids 3 and 4 was generally greater than that of the other amended soils (Table 5). The unlimed biosolids and poultry litter generally increased the soil pH relative to the unamended control, but not to the same degree as the limed biosolids.


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  		Table 5. The pH in the unamended and amended Elkton and the Suffolk soils on Day 1 and Day 51 of the incubation study{dagger}.

 
Predicting Changes in Soil Phosphorus from Biosolids Properties
One of the objectives of this study was to determine whether changes in soil P following biosolids amendment could be predicted from the properties of the biosolids. To do this, we correlated the changes in various soil P measurements shown in Table 4 with (i) several direct measures of the P concentration in the biosolids (USEPA 3050 P, Pox, Mehlich-3 P, and Mehlich-1 P) and (ii) the molar ratio of P to [Al + Fe] in the biosolids, because past work has shown that P extractability from biosolids is affected not only by the biosolids P concentrations, but by metals that can adsorb or precipitate P (McCoy et al., 1986; Corey, 1992). In other words, a low P availability could be correlated with either a low P level or a high [Al + Fe] concentration, and the P to [Al + Fe] ratio is indicative of both factors. The PL was left out of these correlations, as our focus was on the effect of biosolids treatment on P availability. Our results suggest that both the Mehlich-3 soil test and the molar ratio of P to [Al + Fe], measured with either the USEPA 3050 acid–peroxide digestion method or by oxalate extraction, could predict, with reasonable accuracy, changes in soil P following the addition of biosolids to these two soils (Table 6). We also noted that the predictive accuracy of these biosolids tests tended to decrease with time (from 1 to 51 d of the incubation), as shown for the linear relationships between Mehlich-3 P and the molar ratio of USEPA 3050 P to [Al + Fe] and WSP (Fig. 3), FeO-P (Fig. 4), and Mehlich-1 P (Fig. 5). While these results are promising, we feel there is a need to evaluate these tests with a wider range of soils and biosolids before recommending a specific biosolids test for the availability of P in biosolids-amended soil. There is also a need to expand these results outside the laboratory in field trials that use "wet" (undried or ground) biosolids. Encouragingly, the similarity in trends we obtained when using the data from 1 and 51 d of the incubation study (Fig. 3 to 5) suggest that a short-term incubation study, where only the final values for soil P are measured and compared with biosolids properties, could allow for rapid evaluation of these or other "quick tests" for P availability in biosolids.


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  		Table 6. Correlation coefficients (r) between biosolids properties and changes in water-soluble P, Fe-strip P (FeO-P), and Mehlich-1 P during the incubation study, relative to the control, per 100 kg USEPA 3050 P ha-1 added in the biosolids.

 


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  		Fig. 3. Relationship between (a) Mehlich-3 phosphorus in biosolids and (b) the molar ratio of USEPA 3050 phosphorus to [Al + Fe] in biosolids and the changes in water-soluble phosphorus (WSP), normalized per 100 kg P/ha added, in the Elkton and Suffolk soils during the incubation study.

 


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  		Fig. 4. Relationship between (a) Mehlich-3 phosphorus in biosolids and (b) the molar ratio of USEPA 3050 phosphorus to [Al + Fe] in biosolids and the changes in iron-oxide strip–extractable phosphorus (FeO-P), normalized per 100 kg P/ha added, in the Elkton and Suffolk soils during the incubation study.

 


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  		Fig. 5. Relationship between (a) Mehlich-3 phosphorus in biosolids and (b) the molar ratio of USEPA 3050 phosphorus to [Al + Fe] in biosolids and the changes in Mehlich-1 phosphorus, normalized per 100 kg P/ha added, in the Elkton silt loam and Suffolk sandy loam soils during the incubation study.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
There is increasing pressure in the Mid-Atlantic region of the USA for agricultural nutrient management plans to be based on P recommendations. Our results and past research have shown that applying biosolids or PL according to a nutrient management plan based on N will lead to the application of P in excess of crop requirements and hence a buildup of P in soils to levels above those required for crop production. However, P in biosolids may not be as mobile or available as P from other sources, such as PL, depending on the biosolids treatment process.

Biosolids type (e.g., wastewater treatment process used) affected the concentrations of soil P throughout the incubation study. When normalized to equal amounts of USEPA 3050-P added, the general trend was for higher concentrations of WSP, FeO-P, and Mehlich-1 P in soils amended with biosolids produced without the use of metal salts, followed by biosolids treated with metal salts and lime, and then biosolids produced only using metal salts. The PL tended to cause increases in WSP and FeO-P, which were intermediate between the biosolids treated with metal salts only and those that received no metal additions. As WSP and FeO-P are thought of as environmental soil P tests, and if nutrient management plans dictate switching from an N basis to a P basis to protect surface water quality, our results suggest that the P in biosolids treated with metal salts during production should not be treated the same as P in biosolids treated with no metal salts or P in PL. Further research comparing biosolids with a variety of animal manures is required to verify this point. The increase in P in the biosolids- or PL-amended soils was greater initially than after 51 d and also greater in the Suffolk sandy loam than the Elkton silt loam, indicating the importance of temporal and soil properties as well as biosolids properties in determining P availability and mobility in amended soils.

The USEPA 3050 P, oxalate-extractable P, and Mehlich-1 P in the biosolids were poor indicators of changes in WSP, FeO-P, and Mehlich-1 P in biosolids-amended soils. However, Mehlich-3 P and the molar ratio of P to [Al + Fe], by either the USEPA 3050 acid–peroxide digestion or oxalate extraction of the biosolids, were good indicators of changes in soil P following biosolids amendment. Therefore, testing of biosolids for availability of P, rather than total P, can be a useful tool for predicting extractable P from biosolids amended soils.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
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