Long-term fate of nitrogen from annual feedlot manure applications

Animal wastes are a valuable soil amendment, but when rates of application exceed crop nitrogen (N) requirements, N can leach into groundwater or be lost to the atmosphere. The prediction of optimum manure application rates, however, is complicated by the mineralization of organic N that accumulates with repeated manure applications over many years. The objective of this study was to determine the N balance in soils receiving long-term, repeated manure applications, as influenced by application rate and moisture regime. A N balance was constructed for a site at Lethbridge, AB, in which various rates of 1- to 2-yr-old manure (up to 180 Mg ha(-1) yr(-1)) have been applied annually to irrigated and nonirrigated Chernozemic (Typic Haploboroll) clay loam since 1973. Under nonirrigated conditions, all of the N applied in manure was accounted for by crop uptake, soil organic N (Kjeldahl N), and soil NO3-N. Losses of N via leaching or volatilization were small. Under irrigation, however, particularly at higher rates of manure application, appreciable amounts of N were lost by leaching and volatilization. The proportion of manure N mineralized was independent of application rate and irrigation regime. During a period of almost 20 yr, about 56% of the N applied in manure was mineralized. This estimate permits more quantitative estimation of manure application rates that exploit the agronomic value of manure without risk of adverse environmental effects under similar climatic conditions.