The Influence of Manure Phytic Acid on Phosphorus Solubility in Calcareous Soils

Manure characteristics can influence the potential for P transfer in runoff following land application of manures. This research assessed the influence of manure characteristics on P solubility in calcareous soils using manures from poultry (Gallus Domisticus) fed a variety of grain-based diets with the manures containing a range of total P (5.6–16.4 g P kg), water-extractable P (WEP, 0.9–4.7 g P kg), phytic acid P (0.1–7.6), total N/P ratios (2.6–5.1), and total C/P ratios (19.5–75.7). In addition, mono-ammonium phosphate fertilizer and reagent grade inositol hexaphosphate (phytic acid [PA]), were included, as well as a control treatment with no P additions. Treatments were incorporated into two soils (Portneuf [Coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcids] and Millville [Coarse-silty, carbonatic, mesic Typic Haploxerolls]) at three rates (10, 20, and 40 mg P kg) and incubated for a total of 18 wk with subsamples taken at 2, 5, 9, and 18 wk. Soil samples were analyzed for inorganic and organic NaHCO3 (Olsen) extractable P and select soils were analyzed at 0 and 12 wk by P nuclear magnetic resonance spectroscopy (NMR) for soil P characterization. The percentage of WEP and PA (of total P) in the manures were linearly related (r 2 5 0.94). Increases in Olsen P over time were positively related to the percentage of monoester P in the treatments. At 2 wk, there was a strong negative correlation between the amount of PA added in the treatments and increases in Olsen P. However, by 18 wk, Olsen P was more closely related to the amount of C orN addedwith the treatments. Changes in PA content of manures due to dietary modification may influence P sorption on calcareous soils in the short-term while other characteristics such as C/P ratio may exert a stronger influence over changes in soil test P over longer time periods. ANIMAL PRODUCTION in the USA is valued at more than $100 billion and has consolidated significantly over the past 20 yr, with a larger number of animals being produced on an increasingly smaller land base (Kellogg et al., 2000). Such consolidation of animal production can generate regional and farm-scale nutrient surpluses that can potentially contribute to nonpoint source nutrient pollution ofwater bodies, because nutrient imports in feed and mineral fertilizer can exceed nutrient exports (Sharpley et al., 1994; Sims et al., 1998). Overapplication or mismanagement of manure can increase the risk of degrading surface and ground water quality with excess nutrients (Sharpley, 1996; Sims et al., 1998, 2000). Phosphorus is a particular problem, because it can accumulate in soil and reach concentrations greater than those needed for optimum crop production. This is due in part to unfavorable N/P ratios in manures relative to the uptake of these nutrients by most crops, which results in overapplication of P when manures are applied to meet the N requirement of the crop (Mikkelsen, 2000). As a result, long-term manure application to agricultural land often leads to soil P accumulation which has the potential to accelerate P transfer in runoff to water bodies. This process can contribute to eutrophication in freshwater ecosystems, and numerous examples of water quality impairment associated with P pollution from animal operations now exist (Boesch et al., 2001; Burkholder and Glasgow, 1997; U.S. Geological Survey, 1999). The environmental fate of P in animal manures is partly determined by its chemical composition. However, few studies have fully characterized manure P and determined the effect of various P compounds on P reactivity in soils. It has been shown that the soil sorption capacity differs for various organic P compounds, with inositol hexaphosphate (also called phytate-P, PA) being very tightly bound by soils while other organic P compounds such as nucleotides, DNA, and glucose phosphates are more mobile in soils (Celi and Barberis, 2005). Manure characteristics, other than organic P content, may also affect the manure P retention in soils. For example, the addition of manure can lead to complexation of Fe and Al by organic ligands, which decreases precipitation of P with these metals. These ligands can also compete for P sorption sites, which increase the concentration of soluble P (Iyamuremye et al., 1996). As new feed technologies aimed at P reduction in manures are adopted, the P composition of manures may be altered, which in turn can potentially influence its reactivity in soils. For example, the addition of phytase enzymes to improve P availability in feeds can result in a decrease in total P in manure by up to 40% (Sims et al., 1999). This reduction in total P may also be accompanied by a change in the forms of P in the manure. For example, research has shown that these manures contain greater amounts of inorganic P than manure generated from conventional feeds (Sims et al., 1999; Maguire et al., 2003, 2004). The use of genetically mutated corn (containing low concentrations of PA) for feed can also reduce the total P excreted by monogastric animals (Spencer et al., 1998) and may change the P composition in the manure. As feed modifications result in manures with less PA, a situation may be created with A.B. Leytem, USDA-ARS, 3793N 3600E, Kimberly, ID 83341-5076; D.R. Smith, USDA-ARS, National Soil Erosion Research Laboratory, West Lafayette, IN; T.J. Applegate, Purdue University, Dep. of Animal Science, West Lafayette, IN; and P.A. Thacker, University of Saskatchewan, Dep. of Animal and Poultry Science, Saskatoon, SK, Canada. Received 5 Jan. 2006. *Corresponding author ([email protected].