Mineralization of organic nitrogen from farm manure applications

This study aimed to quantify the amount of nitrogen (N) mineralized from the organic fraction of farm manures under field conditions. Nine different farm manures were stripped of their ammonium-N content prior to soil incorporation and establishment of ryegrass at two sites in England. Grass N uptake and nitrate-N leaching were measured for five consecutive seasons and compared with an untreated control, with the sum of N uptake + leaching (net of the control) used as an estimate of the amount of organic N mineralized from the applied manures. The amount mineralized was related to thermal time (cumulative day degrees above 5 degrees C-CDD), with two distinct phases - an initial phase up to 2300 CDD (c.18 months under UK climatic conditions) where mineralization proceeded at rates ranging between 0.005 and 0.027% mineralized/CDD and a slower phase at >2300 CDD, where rates were negligible at <0.001% mineralized/CDD. There was no difference between soil types, both being light-textured (<20% clay), but there were differences between manure types depending on the manure C: organic N ratios. For pig slurry and layer manure (C: organic N = 9-12: 1), up to 70% of the organic N was mineralized, compared to 10-30% mineralization from the cattle slurry and straw-based farmyard manures-FYMs (C: organic N = 10-21: 1). The relationships derived provide a useful tool for predicting both the amount and timing of manure N release, with important implications for both crop N uptake and leaching risk.