Effect of Manure Management on Carbon Evolution and Water Extractable Phosphorus

Soils with excessive phosphorus (P) levels due to manure application are an environmental concern because water extractable P (WEP) in runoff from these soils can contribute to increased amounts P in surface water, which can contribute to eutrophication of freshwater. Phosphorus based manure management is an option to reduce WEP and thereby reduce agricultural P runoff. In P based manure management, manure is applied to meet the P needs of a crop or not to exceed a given soil test level. Because P base manure management does not supply enough nitrogen (N) to meet the needs of the crop, addition fertilizer N needs to be applied. Fertilizer N applied to soils may increase the rate of mineralization of organic matter and lowers soil pH and therefore may affect the solubility of soil inorganic and organic P pools. The extent to which this may affect WEP or plant P availability is not known. Thus, laboratory and greenhouse studies were conducted to determine the effects of P based manure management on WEP and on short-term P plant availability. Phosphorus based manure management had no significant effect on the shift of organic P to WEP, but the increased acidity due to urea hydrolysis and subsequent nitrification of ammonia had a significant effect on the solubilization of P form the Ca-bound IP pool, thereby increasing WEP. This could be a significant consideration where Ca-bound IP dominates IP, P based manure management is implemented and increased WEP is subject to export to surface waters via runoff.