Management of Microbial Nitrate Production in Agricultural Soils

Nitrate (NO3-) is of central importance in the internal soil nitrogen (N) cycle. While animal wastes and nitrification inhibitors have been used in modern agriculture for decades, their effects on soil NO3- concentrations in relation to microbial NO3- production have not been well characterized. The objective of this research was to determine microbial NO3- production in relation to ammonium (NH4+) availability and nitrifier population activity in agricultural soils receiving animal wastes or nitrification inhibitors. Several laboratory and field studies assessed the impacts of variously treated dairy wastes and the effects of repeated long-term use of a nitrification inhibitor, nitrapyrin, on microbial NO3- production and soil NO3- concentrations in Utah agricultural soils. The various process rates of N mineralization, nitrification, and microbial N immobilization were determined in laboratory and field systems using 15N isotope dilution techniques. Nitrification potentials were used simultaneously to measure the nitrifier population size and activity. Microbial NO3- immobilization was not observed in the laboratory and field experiments. The lack of microbial NO3- consumption indicates that nitrification was the primary process controlling soil NO3- concentrations. Nitrifiers were not weaker competitors than heterotrophs for utilizing soil NH4+; about 50% of the NH4+ mineralized was used by nitrifiers. Low carbon availability may have limited heterotrophic microbial growth, thereby minimizing the heterotrophic microbial consumption of NH4+ and NO3-. Effects of dairy wastes on soil NH4+ availability depend on the treatment systems of dairy wastes and their application rates. The N mineralization potentials were approximately 5% of the organic N in dairy-waste compost versus 90% of the organic N in dairy waste digested anaerobically. Dairy-waste compost at appropriate application rates did not increase nitrification rates, nitrification potentials, or soil NO3- concentrations for several months following application. However, even stabilized dairy-waste compost led to high nitrification rates and potentials and elevated soil NO3- concentrations when it was applied at an excessive rate (i.e., 100 Mg dry wt. ha-1). In a dryland wheat agroecosystem, repeated use of nitrapyrin for 8 years had a 2-year residual effect observed as lower nitrification potentials in soils with a history of nitrapyrin use compared to soils without that history