Mechanisms and kinetics of nitric and nitrous oxide production during nitrification in agricultural soil

Laboratory experiments were conducted with three California agricultural soils to examine substrate and process controls over temporal variability of NO and N2O production during nitrification, and to quantify the kinetics of HNO2-mediated chemical reactions. Gross NO production rates were highly correlated (r(2) = 0.93-0.97) with calculated concentrations of HNO2, which were shown to originate from autotrophic microbial oxidation of NH4+ to NO2- . Production of NO was not correlated with NH4+ or NO3-, or with the overall nitrification rate. Distinct periods of high NO2- accumulation occurred below critical pH values in each soil, apparently due to inhibition of microbial NO2- oxidation. Data suggest that even during periods of relatively low NO2- accumulation and rapid overall nitrification, HNO2-mediated reactions may have been the primary source of NO. Rate coefficients (k(PNO)) relating NO production to HNO2 concentrations were determined for sterile (lambda-irradiated) soils, and were similar to k(PNO) values in 2 of 3 nonsterile soils undergoing nitrification. Production of N2O was correlated with HNO2 (r(2) = 0.88-0.99) in sterile soils, and with NO2- and NO3- (R-2 = 0.72-0.91) in nonsterile soils. Experiments using N-15 confirmed that dissimilatory NO3- reduction contributed to N2O production even under primarily aerobic conditions. Sterile k(PNO) and k(PN2O) values were correlated (r(2) = 0.90 and 0.82) with soil organic matter content. Overall, the results demonstrate that both steps of the nitrification sequence, together with abiotic reactions involving NO2-/HNO2 need to be considered in developing improved models of NO and N2O emissions from soils.