Identification and Quantification of Organic and Inorganic forms of Phosphorus in Animal Manures Generated by Dietary Modification.
Gurpal Toor, Univ of Arkansas, Biological and Agricultural Engineering, 203 Engineering Hall, Fayetteville, AR 72701, Derek Peak, Univ of Saskatchewan, Dept of Soil Science, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada, Barbara Cade-Menun, Stanford Univ, Dept Geological & Envl. Sci., Stanford, CA 94305-2115, J. Thomas Sims, Univ of Delaware, Dept of Plant and Soil Sciences, Newark, DE 19717-1303, and Brian Haggard, USDA-ARS/Bio & Agric. Eng. Dept., 203 Engineering Hall, Fayetteville, AR 72701.
The tremendous interest in the fate and transformations of Phosphorus (P) applied to soils in animal manures today stems from long-standing agricultural concerns about the most efficient means to beneficially recycle the P in manures and increased regulation on use of manures as soil amendments to prevent nonpoint source pollution of surface and shallow ground waters. The typical approach used to characterize P in animal manures is to measure total P, however, recently there has been growing interest in measuring soluble P in manures as direct relationship between soluble P in manures and surface runoff and leaching has been established. At the same time, researchers are now using advanced spectroscopic techniques to understand P species in animal manures. In this talk, we shall discuss results of two studies where we used chemical sequential fractionation, Nuclear Magnetic Resonance (NMR), and X-ray Absorption Near Edge Structure (XANES) spectroscopy as methods to understand different P forms in dairy and poultry manures. In the first study, we employed chemical sequential fractionation and XANES spectroscopy to speciate P in manures produced by broiler chickens and turkeys fed either normal diets, or diets with reduced amounts of mineral P and/or phytase. Results showed that dicalcium phosphate was the predominant mineral in broiler litters (65-79%) followed by aqueous phosphate, and phytic acid, however, no hydroxylapatite was observed. Greater than 77% of P in normal turkey manure was present as dicalcium phosphate, while turkey manure from modified diets had equal proportions of dicalcium phosphate and hydroxylapatite. Water extractable P (1:200 litter to water extraction ratio) was 56 to 77% in broiler litter, whereas, aqueous phosphate determined with XANES was less than 18% indicating that water extraction likely dissolved mineral forms of P (e.g., dicalcium phosphate). Reducing mineral P and adding phytase in diets reduced manure total P contents by up to 50% and resulted in decreasing phytic acid and increasing dicalcium phosphate in broiler litters, and in formation of hydroxylapatite in turkey manures. Importantly, the inclusion of phytase coupled with reductions in mineral P in poultry diets did not affect the aqueous phosphate contents in broiler litters. In the second study, we established quantitative relationships between P forms in diets, feces, and manures collected from six Northeastern and Mid-Atlantic States commercial dairy farms that had low- to high- P in diets (0.36 to 0.53%) by using NMR spectroscopy. NMR analysis indicated that orthophosphate was lower in dairy diets than feces. In contrast, the phytic acid was higher in dairy diets than feces. The net increase in orthophosphate in the dairy feces (5-31%) was accompanied by a decrease in phytic acid (9-32%), which is attributed to the hydrolyzation of phytic acid to orthophosphate by microbial phytase in the dairy cows rumen. Phytic acid was further degraded to orthophosphate in dairy manures thereby increasing orthophosphate by 4 to 20%. Contents of orthophosphate diesters (phospholipids, deoxyribonucleic acid) were twice as high as in dairy feces (6-11%) as diets (2-5%) suggesting the excretion of microbial cells in feces. However, dairy manures had lower orthophosphate diesters than feces. The continuous degradation of organic forms of P (orthophosphate monoesters and diesters) to inorganic P (orthophosphate) from dairy feces to manures has the potential to enrich manures with inorganic P and increase risk of P losses to waters. We also performed sequential chemical fractionation and XANES investigation on these dairy diets, feces, and manures to understand inorganic P forms as NMR provided detailed information about organic P species in these organic materials but no information about inorganic P minerals, which, in turn, would influence the inorganic P leaching (see other presentation on P leaching in soils amended with these manures). The results of chemical fractionation and XANES analysis are currently being processed. We believe that use of these advanced spectroscopic techniques along with a fractionation scheme that could be used by land managers would go in a long way to better understand organic and inorganic P forms in the present day animal manures so that correct manure management strategies should be formulated and implemented in intensive animal production systems not only to decrease the risk of P losses via surface runoff and leaching to waters but also to effectively utilize animal manures for sustainable agricultural production.