Relative Availability of Phosphorus in By-Products and By-Product Amended Soils
Interest in the relative availability of P in by-products and in by-product amended soils has increased for two reasons. The first is the goal of reducing P availability to reduce its potential environmental impact, and the second is accounting for relative differences in by-products that might be useful in determining relative risk of P loss for assessment tools such as the P index.
Wastewater treatment processes can influence the availability of P in biosolids. Of particular interest is the addition of metal salts, specifically Fe and Al, and lime used in some wastewater treatment processes. A decrease in the P solubility of biosolids resulting from certain chemical additions could reduce the risk of P loss to surface water.
Maguire et al. (2001) tested the effect of lime and metal salt addition and digestion on the extractability of P in eight biosolids and one poultry litter product in a 51-d incubation study using two soils. Six of the eight biosolids were treated with metal salts (usually Fe), two received addition of lime and metal salts, and two were not treated with either. Five of the biosolids underwent anaerobic digestion. Each of the biosolids and the poultry litter were incorporated at a rate of 8 Mg ha–1. The addition of biosolids consistently increased the water-soluble phosphorus (WSP) concentrations compared with the unamended soil. A rapid decrease in WSP was observed after the first week of incubation, followed by a more gradual decrease until week three, and minimal change thereafter. Biosolids not treated with a metal salt showed the greatest increases and sustained the greatest concentrations of WSP, Fe oxide strip–extractable P, and Mehlich-1 P throughout the incubation. Increased soil pH associated with the application of biosolids treated with lime may have resulted in the greater P extracted from those soils. Results suggested the possibility of predicting changes in soil P following additions of biosolids using the biosolids properties. Both the Mehlich-3 soil test and the molar ratio of P to (Al + Fe) measured with acid-peroxide digestion or oxalate extraction resulted in "reasonably accurate" predictions of soil P changes after biosolids application.
O'Connor et al. (2004) assessed phytoavailability of P in 12 biosolids relative to triple superphosphate (TSP) fertilizer in a greenhouse study using two P-deficient, sandy soils. Relative phytoavailability ranged from very low (not detectable) to greater than 100%. The materials were placed into groups of high (>75% of TSP), moderate (25–75% of TSP), or low ( TSP) phytoavailability. The high group contained two biosolids produced by a biological P removal process and without any metal salt additions. The moderate group had biosolids produced by conventional treatment processes, including composting. Within the low group were materials with high total Fe and Al concentrations and that were developed into products with low moisture content.
Reducing the solubility of P in manure before land application with chemical amendments (e.g., alum) can reduce P loss in runoff. Moore et al. (2000) (Table 4) and Shreve et al. (1995) reported increased dissolved reactive and total P concentrations in runoff from pastures amended with untreated poultry litter compared with alum-amended litter applications. Dou et al. (2003) tested coal combustion by-products and alum as amendments for reducing soluble P in dairy, swine, and broiler litter. Alum additions reduced readily soluble P (inorganic P) by 99, 86, and 80% in the dairy, broiler, and swine samples, respectively, as compared with the control. Fluidized bed combustion fly ash and flue gas desulfurization by-product decreased readily soluble P by 50 to 80% at the highest application rates. Sequential extraction results from both the alum and by-product treatments suggest that manure P was shifted from the water-soluble fraction into more stable fractions. These shifts reduce the potential for P transport offsite as soluble P in runoff, but have minimal influence on the long-term P availability to the crop.
Robinson and Sharpley (1996) compared the short-term extractability, sorption, and fractionation of P in soils treated with poultry litter leachate or KH2PO4. The short-term extractability of P was lower in the soils treated with litter leachate than with KH2PO4, as measured by extraction with Fe-oxide strips. Soils treated with KH2PO4 had an average of 50% more Fe-oxide strip P than soils treated with litter leachate. Following removal of solution P by water extraction, leachate-treated soils had greater amounts of NaHCO3–P than KH2PO4–treated soils. Phosphorus sorption was greater from the litter leachate than from the KH2PO4 solution for all soils. Averaged across all soils, P sorption maximum was 548 and 304 mg kg–1 for the leachate-treated and KH2PO4–treated soils, respectively. Results suggest that poultry litter may maintain more residual P in soil than KH2PO4 due to lower solution P and strip P, but greater NaHCO3–P.
Siddique and Robinson (2003) investigated P sorption and extractability in soils receiving poultry litter, poultry manure, cattle slurry, and urban sewage sludge. All of the organic sources of P and KH2PO4 decreased the P sorption maximum and binding energy and increased CaCl2–P and resin P in all the soils. The changes increased in the order: poultry manure sewage sludge 2PO4suggested that soil solution P concentrations in the poultry manure, litter, and sewage sludge treatments were controlled by large amounts of available Ca. Organic acids released in the cattle slurry treatments were hypothesized to increase solubility compared with the KH2PO4–treated soils by blocking P sorption sites, thus reducing the P sorption capacity. Findings from Robinson and Sharpley (1996) and Siddique and Robinson (2003) suggest that P from inorganic sources that contained organic by-products does not undergo identical reactions in soils and that these differences should be taken into account when addressing agronomic or environmental issues from P. Few states or regulatory agencies address the potential differences, though a few states are moving to incorporate phosphorus availability coefficients (PACs) for various organic sources of P in P index values (Shober and Sims, 2003).
Phosphorus in Runoff and Erosion
Application of animal manures and biosolids based on the N requirement of the crop typically results in overapplication and accumulation of P in the soil. High concentrations of soil P increase the potential of P loss and surface water quality degradation. Phosphorus loss in surface runoff is mainly controlled by transport (i.e., runoff and erosion) and source (e.g., manure, biosolids, or fertilizer applications, soil test P level) factors (Pote et al., 1996).
Eghball and Gilley (1999) evaluated the effects of N or P-based applications of beef cattle manure or compost on runoff concentrations of dissolved P, FeO sink P, particulate P, and total P from no-till and disked fields using rainfall simulations. Runoff concentrations of dissolved and FeO sink P were significantly greater from the no-till than the disked treatment. Tillage did not affect total P concentration in the runoff. Nitrogen-based manure and compost applications resulted in runoff dissolved P concentrations significantly greater than 1 mg L–1; however, dissolved P concentrations for the P-based treatments were not significantly different than 1 mg L–1 during both simulated rainfall events. This indicates that N-based manure or compost application to no-till systems will result in P enrichment of surface water bodies. The researchers concluded that P-based manure and compost application appears to achieve beneficial agronomic and environmental goals with respect to P.
Waste source and treatment processes may explain differences in P availability and their potential loss in surface runoff (Frossard et al., 1996). Withers et al. (2001) compared P transfer in surface runoff from plots amended with inorganic fertilizer, cattle manure, and sewage sludge. The specific amendments were TSP, liquid cattle manure (LCS), liquid anaerobically digested sewage sludge (LDS), and dewatered cake sludge (DSC). Laboratory analysis of the waste materials found substantially more P extracted from the cattle manure compared with the sludges using water and NaHCO3. The liquid amendments (LCS and LDS) had higher HCl-extractable P than the DSC sample. High P solubility for both the NaHCO3– and HCl-extractable fractions in the cattle manure indicated that most P was inorganic. Based on laboratory analyses, the relative differences in the potential P release to water can be ranked in the order TSP > LCS > LDS > DSC. Application of either inorganic P fertilizer or liquid cattle manure transferred more P to surface water than application of liquid or dewatered biosolids. This is probably due to lower P solubility in water and/or NaHCO3 associated with the biosolids amendments.
Many studies have measured edge of field P losses or concentrations in runoff; however, there is a strong need for interpretation of such data relative to potential environmental impact. Such an interpretation would probably be watershed specific, but should be more instructive than merely making relative comparisons or comparing dissolved P concentrations in runoff with an arbitrary criterion (e.g., 1 mg P L–1).
Phosphorus that accumulates in the soil can be susceptible to loss via leaching, particularly in coarse-textured soils where large amounts of animal manure or biosolids have been applied. Leaching of soil P can lead to ground water contamination in areas with shallow water tables or contribute to the degradation of surface water through subsurface flow, particularly when tile drainage is used. Phosphorus can leach in sandy soils because these soils quickly become P saturated due their low contents of P-binding constituents (e.g., clays and Fe- and Al-oxides). The saturation of the P sorption capacity in the surface soil could lead to further saturation of the deeper soil horizons (Sims et al., 1998).
Novak et al. (2000) evaluated the effects of long-term (10 yr), intensive swine manure application on soil P accumulation and movement in a Coastal Plain soil. Three soil cores were taken within the spray field in 1991 (4 yr of manure application) and 1997 (10 yr of manure application). In 1991, the mean surface soil (0–15 cm) value of Mehlich-3 P was 376 mg P kg–1 soil, substantially greater than the maximum soil test value for crop production (100 mg P kg–1). High values (168 mg P kg–1) were also observed down to the 30- to 45-cm depth, suggesting that 4 yr of manure applications was sufficient to cause P accumulation in the topsoil and some leaching. In 1997, soil Mehlich-3 P concentrations were significantly greater for the spray field compared with the control area down to 90 to 135 cm (Fig. 4) . In a similar study using cattle feedlot manure, Eghball et al. (1996) found greater STP concentrations to 1.8 m in soils amended with manure compared with inorganic fertilizer. Minimal amounts of fertilizer P had moved deeper in the soil than 1.1 m. At similar P loading rates, P movement was deeper in the manure treatment than the fertilizer treatment. The researchers found no correlation between the P adsorption maximum or P index and soil P movement.
Phosphorus leaching was low in Coastal Plain soils amended with biosolids whose high concentrations of Al and Fe increased P sorption and reduce P solubility (Elliot et al., 2002). Similar results were found by Siddique et al. (2000) in a study on soils whose surface horizon texture was loam. Significantly greater amounts of P leached from inorganic fertilizer–amended soil than sludge-amended soil. The difference in P mobility was explained by the lower P solubility in the sludge compared with the fertilizer.
New Tools for Understanding and Minimizing the Environmental Impacts of Phosphorus
Phosphorus Loss Assessment Tools
In 1999, the USDA and the USEPA issued a national strategy outlining P-based nutrient management approaches for the land application of animal manure in response to increasing concern over nonpoint-source P pollution of surface waters associated with the agronomic use of animal wastes. The three approaches base P management on either (i) STP and crop response, (ii) an environmental soil P threshold limit, or (iii) a P index that identifies fields with the greatest risk for P loss. Relative advantages and disadvantages of each approach were discussed previously. The P index identifies areas where the risk of P transport is probably greater than other areas by accounting for source and transport factors that control P loss in runoff. Several investigators have demonstrated that a P index can accurately rank sites based on their potential for P loss in runoff (Birr and Mulla, 2001; Gburek et al., 2000; Sharpley, 1995).
An evaluation of the three P-based nutrient management strategies was conducted on 10 Pennsylvania farms using site management and transport factors from the Pennsylvania P index (Weld et al., 2002). The revised universal soil loss equation (RUSLE) and soil surveys were used to calculate soil loss for each site. A surface runoff class was assigned based on the soil permeability class and slope. Also included in transport factor category was contributing distance, generally defined as the distance between a given field and the edge of a body of water. Source factors included soil test P (Mehlich-3 P), fertilizer rate, fertilizer application method, manure rate, manure application method, and P availability coefficient. A P index value was calculated by multiplying the summed transport, source, and management factors. Results from this study found that the P index was less restrictive, more flexible, and allowed for more management options compared with the other two strategies. In addition, the P index strategy had the lowest first year total cost estimate across all 10 farms.
Modifications of Animal Diets to Reduce Phosphorus Losses in Runoff
Livestock diet modifications can change the digestibility, forms, and amounts of P in feed, and can subsequently alter the quantity and forms of P excreted in manure. Dairy, swine, and poultry are the animal species for which these modifications are most appropriate. Three strategies can be used: (i) reductions in P supplementation to the diet without reducing animal performance, (ii) supplementation of phytase enzyme to increase the digestibility of phytic acid P in the diet, and (iii) use of high-available-phosphorus (HAP) corn in the diet. These changes can reduce P losses in runoff following manure land applications.
Swine manure has some of the greatest P contents compared with other livestock (Barnett, 1994). Typical swine diets include corn and/or soybean meal, whose P is largely in the form of myo-inositol hexakisphosphate (phytate). Swine are unable to use phytate P due to insufficient amounts of the phytase enzyme in their upper digestive tract. For this reason, supplements of inorganic P are often used in swine diets, thus further increasing the amount of P excreted in the manure.
Microbial phytase and HAP are two products that can eliminate the need for inorganic P supplement in swine diets. Microbial phytase catalyzes the hydrolysis of phytate in the upper digestive tract and makes orthophosphate available for use. Several studies have shown that microbial phytase is effective in improving P adsorption and reducing P excretion. High-available-P corn contains substantially less phytate than normal corn, but does not differ in total P content. This product is not commercially available at present but may be a promising substitute for normal corn in swine diets. High-available-P corn diets for swine and young poultry have resulted in improved P utilization and decreased P excretion compared with diets using conventional corn (Spencer et al., 2000). Phosphorus digestibility was further increased when phytase was added to HAP corn diets (Sands et al., 2001).
Baxter et al. (2003) evaluated the effects of swine diets formulated with normal and HAP corn, with and without phytase additions, on the relative amounts of various P fractions in the manure. The researchers also studied the changes in the relative contents of these P fractions when manure was stored as slurry. Fecal total P concentration was reduced 17, 19, and 40% in diets formulated with phytase, HAP corn, and phytase plus HAP corn, respectively (Table 5). Phytase supplements had no effect on water-soluble dissolved molybdate-reactive phosphorus (DRP) when added to the normal corn diet, but significantly reduced DRP by 27% when in the HAP corn diet (Table 5). Fecal phytate P was significantly reduced in all the treatments compared with the control diet, with the HAP plus phytase diet having the lowest phytate P concentration (Table 5). Reductions were observed across all diets in the relative pools of DRP, phytate P, dissolved organic P, and acid-soluble organic P over a storage time of 150 d. Significant effects of diet on the relative amounts of different P pools, averaged across storage time, indicated that diet may affect the availability and types of P in slurry that contains different-aged material.
Synchrotron Radiation–Based Spectroscopic Approaches to Study Phosphorus
Identification of P species in high-P soils is important for studying P mobility and possible transport to ground and surface water. The processes responsible for the sorption of P in soils are primarily chemisorption and precipitation of secondary P-containing solid phases. Little advancement has occurred in the study of these phenomena because of limitations in the available techniques. The notable exception to this is the use of synchrotron-based techniques, which is a recent application for identification of P species in soils. Beauchemin et al. (2003)
used X-ray absorption near-edge structure spectroscopy (XANES) and sequential chemical fractionation to characterize the dominant solid-phase P species in a range of soils. Previous work by Hesterberg et al. (1999)
found XANES spectroscopy to be an acceptable method for characterizing P species in soil. Advantages of this method include the elimination of sample pretreatment and element specificity (Beauchemin et al., 2003
). Results from chemical fractionation found the largest P fraction in moderately labile (NaOH-P) or nonlabile pools (HCl-P and H2
–P). In a soil with a history of manure application, P was mainly associated with Al or Fe oxides in the surface horizon. This was also true in another, nonmanured acidic soil. The B horizon of both samples was dominated by HCl-P, indicating the presence of Ca-bound P minerals. Results from XANES spectroscopy indicated that all samples, especially acidic soil samples, had phosphate adsorbed on Fe- or Al-oxide minerals (Fig. 5
) . These data from the XANES spectral fitting significantly correlated with proportions of NaOH-extractable inorganic P. Both the alkaline and acidic soil samples contained Ca-phosphates according to results from chemical fractionation and XANES spectroscopy. The authors conclude that XANES spectroscopy can directly identify some P species in soil.
Peak et al. (2002) used XANES to study P forms in alum-amended poultry litter. Alum amendment converts P from weakly adsorbed and Ca phosphate forms to forms associated with amorphous Al(OH)3.