Microbial nitrogen transformations in response to treated dairy waste in agricultural soils

Dairy wastes are commonly applied to croplands as N fertilizers, but the dynamics of N release and transformations during the growing season are difficult to predict. We compared N mineralization kinetics and examined microbial N transformations in soil receiving dairywaste compost vs. lagoon effluent. Mineralization kinetics was examined with a 70-d laboratory incubation, and a first-order model was used to derive mineralization parameters. Measurements of N transformations were conducted with N-15 pool dilution techniques in silage corn field plots that were unfertilized or fertilized with ammonium sulfate, lagoon effluent, or compost at two rates equivalent to 100 or 200 kg available N ha(-1). The N mineralization potential was higher and the first-order rate constant was lower in soil receiving compost than lagoon effluent. Approximately 6% of compost N was mineralized within 2.5 mo; in contrast, up to 90% lagoon effluent organic N was released. However, silage yield was greatest in the compost treatment, showing that synchronization of N availability is as important as the amount mineralized. The field N-15 measurements indicated that microbial NO3- consumption was negligible despite the treatments. Microbial NH4+ immobilization in soil receiving dairy wastes was similar to that in soil unfertilized or fertilized with inorganic N. Soil treated with the high-rate compost had the highest rates of mineralization and nitrification, which led to the highest soil N03 accumulation. Our observations suggest that peak plant demand is met by the compost N; however, its high N mineralization potential makes the management of dairy compost a difficult task.