AN APPROACH FOR ESTIMATING A DECAY SERIES FOR ORGANIC NITROGEN IN ANIMAL MANURE

Animal manures are an important source of N for crop production. Effic ient use of manure is necessary to increase N recycling, eliminate exc essive purchase of fertilizer N, and reduce N loss to the environment. An estimate of the annual rate of organic N mineralization in manure is a necessary prerequisite to efficient manure N management. A minera lization or decay series for the organic N in dairy manure was estimat ed for corn (Zea mays L.) production from several field experiments ov er a 5- to 8-yr period. Four rates of sidedressed fertilizer N (0, 56, 112, and 224 kg N ha-1) were superimposed over several spring-applied manure treatments ranging from 0 to 168 Mg ha-1 yr-1. Ammonia volatil ization from manure was encouraged, to allow for an assessment of the fertilizer N equivalence of the more stable organic N fraction. Based on silage dry matter yield and N uptake, a decay series of 0.16, 0.10, 0.03, 0.03 and 0.02, and 0.21, 0.09, 0.03, 0.03 and 0.02, respectivel y, described the fertilizer N equivalence of organic N. The first numb er in the decay series estimates the fraction of organic N that is ava ilable for crop use during the year of application, and the second to fifth numbers are estimates of available N from residual N in Years 2 through 5. An independent set of field data was used to evaluate the a ccuracy of the decay series. The predicted decay series, based on dry matter yield (R2 = 0.16) was more variable than the corresponding deca y series based on N uptake (R2 = 0.72). The non-N effects of manure we re insignificant, and therefore yield and N uptake were directly relat ed to the N contribution.