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TECHNICAL REPORT
Waste Management

Phosphorus Leaching from Biosolids-Amended Sandy Soils

^a Agric. and Biol. Eng. Dep., Pennsylvania State Univ., University Park, PA 16802
^b Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611

* Corresponding author (hae1{at}psu.edu)

Received for publication May 14, 2001.

	ABSTRACT

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

 
Increasing emphasis on phosphorus (P)-based nutrient management underscores the need to understand P behavior in soils amended with biosolids and manures. Laboratory and greenhouse column studies characterized P forms and leachability of eight biosolids products, chicken manure (CM), and commercial fertilizer (triple superphosphate, TSP). Bahiagrass (Paspalum notatum Flugge) was grown for 4 mo on two acid, P-deficient Florida sands, representing both moderate (Candler series: hyperthermic, uncoated Typic Quartzipsamments) and very low (Immokalee series: sandy, siliceous, hyperthermic Arenic Alaquods) P-sorbing capacities. Amendments were applied at 56 and 224 kg P_T ha^-1, simulating P-based and N-based nutrient loadings, respectively. Column leachate P was dominantly inorganic and lower for biosolids P sources than TSP. For Candler soil, only TSP at the high P rate exhibited P leaching statistically greater ( = 0.05) than control (soil-only) columns. For the high P rate and low P-sorbing Immokalee soil, TSP and CM leached 21 and 3.0% of applied P, respectively. Leachate P for six biosolids was <1.0% of applied P and not statistically different from controls. Largo biosolids, generated from a biological P removal process, exhibited significantly greater leachate P in both cake and pelletized forms (11 and 2.5% of applied P, respectively) than other biosolids. Biosolids P leaching was correlated to the phosphorus saturation index (PSI = [P_ox]/[Al_ox + Fe_ox]) based on oxalate extraction of the pre-applied biosolids. For biosolids with PSI approximately 1.1, no appreciable leaching occurred. Only Largo cake (PSI = 1.4) and pellets (PSI = 1.3) exhibited P leaching losses statistically greater than controls. The biosolids PSI appears useful for identifying biosolids with potential to enrich drainage P when applied to low P-sorbing soils.

Abbreviations: BPR, biological phosphorus removal • CM, chicken manure • PSI, phosphorus saturation index • TSP, triple superphosphate

	INTRODUCTION

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

 
BENEFICIAL REUSE OF BIOSOLIDS and manures based on crop nitrogen (N) requirements usually supplies phosphorus (P) in excess of crop needs. Excess soil P is not harmful to plants when biosolids are used for N fertility (Peterson et al., 1994). Off-site migration to aquatic systems is, however, a major concern because P is the limiting nutrient in most freshwaters (Sharpley and Beegle, 1999). Phosphorus moves from agricultural fields in a dissolved form or attached to soil particles. When soil P levels are not excessive, up to 90% of the P transported from cropland is bound to soil particles (Sharpley and Beegle, 1999). Thus, erosion control measures prevent significant off-site P movement. Yet, some agricultural fields have soil test P levels in the high and very high categories, and off-site transport of soluble P can be important. Soluble P is directly available to algae (Sonzogni et al., 1982) and thus particularly relevant to water quality degradation.

The growing concern over excess P in soils has led to regulatory changes that could dramatically affect land-based recycling of biosolids. For example, the Water Quality Improvement Act in Maryland mandates P-based nutrient management for manures and biosolids by 2005 (Simpson, 1998). Recent Florida legislation requires P-based biosolids application rates in watersheds associated with P-sensitive water bodies (Sec. 373.4595 FL Statutes). Compared with N-based nutrient management, a P-based approach dictates substantially lower waste application rates. This means larger land area requirements and higher costs to transport wastes outside sensitive watersheds. Increasingly stringent effluent discharge limits exacerbate the problem because more efficient P removal from wastewaters elevates biosolids P. Municipalities are concerned that mandated P-based nutrient management may force them to abandon biosolids recycling programs in favor of landfill disposal.

Sound management of land-based biosolids recycling demands a full understanding of P transformations in the soil–plant environment, but leaching of biosolids P has received limited investigation (Kyle and McClintock, 1995; Sui et al., 1999). Unlike nitrogen, leaching of P has not traditionally been viewed as a major ground water problem. In many soils, abundant P-sorbing oxide components in surface horizons and subsoils keep leachate P levels well below eutrophication thresholds. Peterson et al. (1994), summarizing a 12-year study where liquid digested sludge was applied to a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudoll) in Wisconsin, concluded: "We do not need to worry about P leaching to ground water because leaching is practically zero." While true for many locations, downward movement of P from organic wastes is potentially significant in areas with shallow ground water and coarse-textured soils of low P-sorbing capacity (Eghball et al., 1996; Harris et al., 1996; Lu and O'Connor, 2001). Deep leaching of P through sandy soils of the Atlantic Coastal Plain is particularly a concern in states with areas of intensive animal agriculture (Sims et al., 1998; Novak et al., 2000). Such conditions present unique challenges for P management. Because surface and ground water systems are hydrologically linked, leached P often moves to surface water via lateral subsurface flow (He et al., 1999).

The major objective of this study was to compare P leaching from biosolids, chicken manure (CM), and triple superphosphate (TSP) sources applied to sandy soils representative of Florida and other eastern Coastal Plain states. Greenhouse column studies allowed P leaching to be examined in a crop growth environment. Eight biosolids materials from a variety of treatment schemes were studied since wastewater treatment and sludge processing methods markedly influence biosolids P mobility (Kyle and McClintock, 1995; Richards et al., 1997). The P forms and solubilities in the materials were extensively characterized so leaching data could be interpreted on the basis of P-source chemical properties.

	MATERIALS AND METHODS

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

 
Phosphorus-Source Materials
Phosphorus sources included eight biosolids materials, chicken manure (CM), and triple superphosphate (TSP). Details on the biosolids sources are given in Table 1. The selected biosolids represent a variety of processes expected to influence P behavior, for example, chemical addition for P removal, biological phosphorus removal (BPR), pelletization, composting, and post-alkaline treatment. Pairs of biosolids materials from the same wastewater source were selected to evaluate the effect of various processes used to meet Class A standards of the Part 503 regulations (USEPA, 1994). Layer CM was obtained from the Orlando, FL region. The TSP, from a local fertilizer distributor, was labeled as being 46% citric acid–soluble P₂O₅ (20% P).

Greenhouse Experiment
We prepared 126 columns for the greenhouse investigation. Each column consisted of a 15-cm-diameter x 45-cm-long PVC tubing fitted with netting to prevent soil loss and a PVC cup to collect drainage. Columns contained 15 cm of the A horizon of either the Candler or Immokalee soils over 28 cm of a sand (E horizon of the Myakka series: sandy, siliceous, hyperthermic Aeric Alaquods) of negligible P-retention capacity (Table 2). The Candler soil has moderate P-sorbing capacity and the Immokalee soil has very low P-sorbing capacity as indicated by the Fe_ox + Al_ox (Table 2).

	RESULTS AND DISCUSSION

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

 
Phosphorus-Source Properties
Table 3 provides detailed characteristics of the P sources. Except for the N-Viro (Toledo, OH) product, all biosolids had total N levels (average N_T = 50 g kg^-1) and P levels (average P_T = 27.3 g kg^-1) typical of biosolids produced nationally. The relatively low N and P contents of the N-Viro product reflect ammonia volatilization and dilution of the biosolids cake with alkaline materials (e.g., cement kiln dust). Inorganic P forms dominated all P sources (approximately 80–90%, Table 3). Even the Tarpon Springs and Largo materials, generated by BPR processes, contained predominantly (75–85%) inorganic P. Total concentrations of major elements (Al_T, Fe_T, Ca_T, and Mg_T) were also representative of biosolids produced nationally, and reflected individual wastewater and sludge treatment processing. Thus, Fe_T or Al_T concentrations were generally <10 g kg^-1, unless chemicals were added to the waste stream, for example, Fe (Baltimore) or Al (Tarpon Springs) for P removal and Philadelphia where Fe-containing water treatment residuals are discharged to the sanitary sewer. High Ca_T content of the Tarpon Springs N-Viro product reflects post-alkaline treatment, and identifies the material as a possible liming agent (approximately 30% CaCO₃ equivalent).

Inorganic P was dominantly in Fe- and Al-associated forms (NaOH extract), except for the N-Viro products where Ca and Mg forms (HCl extract) dominated (Table 3, Part B). The sum of sequentially extracted P forms is usually taken to represent total inorganic P. Given the dominance of biosolids P_T by inorganic forms, we expected the good correlation (r² = 0.94) found between the sum of sequential extracts and P_T.

The Mehlich-I soil test was also applied to biosolids. Previous work (O'Connor and Sarkar, 2000) suggested that Mehlich P in biosolids was at least qualitatively related to bioavailable and leachable P, although not as indicative as KCl-P. Mehlich P was poorly correlated (r² = 0.13) to P_T for the biosolids examined. Extractable-P values were very low for N-Viro materials (Table 3, Part B) because extract pH values were alkaline rather than acid. Citric acid–soluble P appeared correlated with total P except for the N-Viro material, where the material's alkalinity confounded extractability interpretation. Previous work (O'Connor and Sarkar, 2000) suggested that citric acid–soluble P was not indicative of bioavailable or leachable P. The P_ox was well correlated (r² = 0.76) with P_T, indirectly confirming the sequential extraction dominance of Fe- and Al-P forms in most materials. Oxalate extracts amorphous Fe and Al and, thus, is expected to release P associated with amorphous Fe and Al solids.

We also calculated the P saturation index (PSI = [P_ox]/[Al_ox + Fe_ox]) from the molar P_ox, Fe_ox, and Al_ox (Table 3, Part B). The PSI is a measure of the degree to which biosolids P is potentially bound with Fe and Al. Thus, PSI values <1 suggest excess Fe and Al for binding of P (little available P), whereas values >1 suggest available P beyond that associated with Fe and Al precipitates. Obviously, P-binding soil components will affect P availability once biosolids are land applied, but the PSI value of biosolids may prove useful as an index of biosolids-P lability. Since only Fe_ox and Al_ox are considered, the index is not as useful for Ca-dominated materials (N-Viro products, TSP, CM). Nevertheless, the PSI qualitatively identifies the Largo materials as possibly high labile P sources and Baltimore (high Fe) materials as poor labile P sources. There is a qualitative (but not infallible) correlation of PSI values with KCl-extractable P values that suggests the need for further investigation. The PSI is inversely related to the molar ratio of (Fe_T + Al_T) to P_T value recommended by Pastene (1981) to characterize biosolids-P availability. Pastene (1981) suggested that ratios <1 were characteristic of biosolids capable of supplying large quantities of soluble P, whereas ratios >1 were indicative of sources with poor P supply.

All biosolids examined here are low in trace elements based on information (not presented) supplied by the producers, except the Largo material. It contains relatively high Mo (approximately 60 mg Mo kg^-1). The ceiling concentration (USEPA, 1994) for Mo, however, is 75 mg Mo kg^-1, so even the Largo material can be land applied in compliance with USEPA guidelines.

Chicken manure was similar in total N, C to N ratio, and P_T concentration to the biosolids materials (Table 3, Part A), and was also dominated by inorganic forms of P (Table 3, Part B). The CM had high KCl-P (4.89 g kg^-1), exceeded only by the Largo cake among the biosolids. Notably, the CM is much lower in Fe_T and Al_T than most biosolids, but Ca_T was high, characteristic of layer chicken wastes (Bhattacharya and Taylor, 1975).

Our analysis confirmed the P_T (Table 3, Part A) and citric acid–soluble P (Table 3, Part B) levels of the TSP fertilizer reported by the manufacturer (Table 3). Much of the P was KCl extractable as expected for a commercial fertilizer, and Ca was the dominant cation (137 g kg^-1), but impurities in the rock phosphate treated with H₃PO₄ to produce the fertilizer contributed to significant amounts of Fe, Al, and Mg (Table 3, Part A).

Soil Properties
Selected properties of the two A-horizon materials (Immokalee and Candler) and the E2-horizon material (Myakka series) are given in Table 2. All materials are acidic sands, low in organic matter. The Candler soil contains more Fe_ox and Al_ox than the Immokalee soil, which is reflected in a smaller PSI and greater tendency to sorb P (relative P adsorption 15.3 vs. 5.3). The Candler soil contains more P_T, P_ox, and Mehlich P than the Immokalee, but both soils are classified as "very low" (<10 mg P kg^-1) in available P by Florida soil test interpretation guidelines (Hanlon et al., 1990). The Myakka sand is extremely low in Fe_ox and Al_ox, and has essentially no P-sorbing capacity. Any P leached from the treated surface soils in the greenhouse columns would, thus, be expected to pass freely through the base E2 sand to drainage. Organic P dominates P_T in both A-horizon materials. Inorganic P tends to be associated with Fe and Al forms extracted by NaOH, especially in the Candler soil. The lack of plant-available P is reflected in the low Mehlich P and KCl-P values.

Greenhouse Leaching Study
Both organic and inorganic P collected in drainage was quantified for the Candler and Immokalee columns (Table 4). The Candler soil possesses approximately three times the P-sorbing capacity of the Immokalee soil (Table 2) and this was reflected in the leaching behavior. Only the high-P rate of TSP lost significantly more P than the other treatments or controls. The percentage of applied P leached varied with P rate from 1.7 to 21.7% in the TSP treatments, and the CM at the higher P rate lost 0.9% of applied P. No biosolids treatments lost >0.45% of applied P and no treatments were statistically different from the soil-only controls. Apparently, differences in biosolids-P leachability (discussed below) were effectively masked by even the modest sorptive capacity of the Candler soil. Most U.S. soils probably contain sufficient P-retention capacity (Fe and Al oxides) to attenuate labile biosolids P in the zone of incorporation. Thus, studies have reported that leaching of biosolids P is minor or negligible (Peterson et al., 1994; Sui et al., 1999). For most locations then, restricting biosolids application rates to the P needs of the crops would normally be unnecessary to minimize leaching concerns.

View larger version (17K): [in this window] [in a new window]   Fig. 1. Cumulative P leached (percent of total applied) from Immokalee soil amended with various P sources applied at 224 kg P ha-1.

View larger version (11K): [in this window] [in a new window]   Fig. 2. Phosphorus leached (percent of total applied) from Immokalee soil amended with biosolids and chicken manure as a function of KCl-P of the materials (P rate = 224 kg ha-1).

Although the proportion of organic P in the Largo biosolids is similar to the other biosolids, the amount of labile P, as measured by KCl extraction, is appreciably greater. The Largo facility is a conventional activated sludge process modified to carry out biological phosphorus removal (BPR), a process where the microbes accumulate P in excess of metabolic requirements. According to Stratful et al. (1999), BPR sludges contain more plant-available P, and thus are excellent fertilizers. Our findings tend to confirm this notion since the two facilities employing BPR (Largo and Tarpon Springs) contained appreciably greater KCl-P than the other materials (Table 3, Part B).

The KCl-P alone is not, however, a reliable guide to leaching behavior. For example, the Largo pellets and Tarpon Springs cake had similar KCl-P values, 3.51 and 1.99 g kg^-1, respectively. Yet, the Tarpon Springs material exhibited virtually no P leaching compared with 2.5% loss of applied P for Largo pellets. It has long been recognized (Häni et al., 1981) that the use of inorganic chemicals in wastewater and sludge treatment can markedly influence biosolids-P availability. Corey (1992) suggested that the bioavailability of biosolids P should be inversely correlated with the (Fe_T + Al_T) to P_T ratio. The Largo pellets had Fe_T + Al_T of about 1.3%, whereas the Tarpon Springs cake contained greater Fe_T + Al_T (approximately 4%), primarily due to alum addition for P removal at Tarpon Springs. Thus, despite similar P_T (Table 3, Part A), P solubility and leachability for Largo pellets can be expected to be greater than for Tarpon Springs cake.

The crucial role of Al and Fe in P mobility has recently been more precisely defined by measuring the reactive portion of Al, Fe, and P using various extraction techniques and P saturation indices (Chardon et al., 2000). The oxalate-based PSI has been used as a risk indicator of soluble P losses from soils. Conceptually, PSI is the molar ratio of readily soluble P to the amorphous Al and Fe components capable of strong fixation of P. Heretofore, the PSI has been applied to soils and waste-amended soils (e.g., Chardon et al., 2000; Maguire et al., 2000).

We explored the utility of the PSI as an indicator of biosolids-P leachability. In the Immokalee soil of low P-sorbing capacity, we expected the applied biosolids to dominate P chemistry. We plotted the amount of biosolids P leached from the amended Immokalee soil as a function of the biosolids PSI (trendline and square data points in Fig. 3) . The PSI is based on the molar ratio of P_ox to Al_ox + Fe_ox, and appreciable P leaching logically occurs only when the amount of reactive P is stoichiometrically greater than the amorphous oxides (primarily responsible for P retention in acid soils). For the materials examined here, no appreciable P leaching occurred from soils amended with biosolids of PSI < approximately 1.1. Only Immokalee soil amended with Largo cake (PSI = 1.4) and pellets (PSI = 1.3) exhibited P leaching losses greater than the control soils.

View larger version (13K): [in this window] [in a new window]   Fig. 3. Phosphorus leached (percent of total applied) from Immokalee soil (P rate = 224 kg P ha-1) as a function of the biosolids phosphorus saturation index (PSI) (trendline and square data points). No P source exhibited significant leaching for the Candler soil, and only Largo materials are shown ("x" data points). The Myakka soil data is from a previous study.

	SUMMARY AND CONCLUSIONS

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

 
Soils common to most of the USA contain sufficient P-sorptive capacity (Al and Fe oxides) to prevent significant P leaching and to mask inherent differences in the P solubilities of biosolids materials. This was the case for the Candler soil in this study. In contrast, the Immokalee and similar Coastal Plain soils have little ability to immobilize soluble P contained in land-applied fertilizer materials. However, the leachable fraction of P_T as measured by KCl or water extraction (Table 2; Sui et al., 1999; Withers et al., 2001) is very small (<5%) for many biosolids. Thus, leaching of P appears to be quite small for most biosolids, even when applied to meet crop N requirements on sandy soils with limited P-sorbing capacity. Biosolids produced in BNR facilities where Al or Fe salts are not added for P removal seem to be exceptional in this regard.

Water- and neutral salt–extractable P determinations are potentially useful for identifying biosolids and other P sources where P leaching may be significant. However, the soil PSI concept, employed for identifying critical source areas for P surface runoff (Chardon et al., 2000), may be superior for gauging P leaching potential in low P-sorbing soils. We found that the PSI based on oxalate extraction of the pre-applied biosolids material was a useful indicator of P leachability. While not commonly run in soil testing, oxalate extraction would be a rapid test that readily fits into standard laboratory operations. Control of P leaching from biosolids-amended soils should be based on ensuring sufficient reactive Al + Fe to immobilize labile P in the biosolids. Adequate Al and Fe exist in many biosolids because of chemical additions in wastewater and sludge treatment processes or because water treatment residuals are discharged to sanitary sewers. For biosolids and manures with meager Al and Fe concentrations, chemical additions (Moore et al., 1999) or co-application with water treatment residuals (O'Connor and Elliott, 2000) can dramatically increase P fixation and, in turn, eliminate concern over P leaching in biosolids-amended soils.

	ACKNOWLEDGMENTS

 
This research was supported in part by a project grant (99-PUM-2T) from the Water Environment Research Foundation.

	REFERENCES

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

 

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