Effects of Manure Management on Phosphorus Biotransformations and Losses During Animal Production

Abstract

Regional intensification of animal production has drastically changed the natural landscape and the ecological balance on millions of hectares of agricultural lands. Animal manure, which is a substantial source of environmental phosphorus (P), has accumulated and now often exceeds the nutrient utilization capacity of the plant production sector within watersheds with a high density of livestock feeding operations. How manure is handled, collected, stored, and disposed of determines the dominant types of transformations and bioavailability of the various P forms excreted in the manure (given the animal species and its diet). Livestock and poultry utilize feed P inefficiently, and unavoidable P excretion occurs even at recommended feeding levels. P excretion increases over and above that level, in proportion to the rise in dietary P intake. Although the use of dietary phosphohydrolases improves feed digestibility and recovery of feed organic P forms, the practice also increases the proportion of water-extractable P excreted in feces. The latter phenomenon may appear advantageous during the reuse of manure as a biofertilizer in crop production, but high levels of water-soluble P also present undue risk of dispersal and contamination of the environment. Linkages between production system characteristics and animal manures were examined to gain an improved understanding of management-induced transformations of manure P forms on the farm. Co-transformations of manure C and N influence the turnover of P and accumulation of water-extractable P in stored manures and amended soils. Mineralization of organic P forms occurs in manure in spite of elevated initial water-extractable P levels. A wide C:P molar ratio enhances manure organic P mineralization, given the need of manure-borne organisms to assimilate C substrates and obtain metabolic energy. P losses from animal production systems occur primarily via P transport in rainwater or snowmelt, and in water percolating through soil. The identification of likely loss pathways on grazinglands, animal production facilities, manure storage structures, and treated fields contributes to the development of comprehensive mitigation strategies to ultimately reduce the environmental footprint of animal agriculture.