Abstract
Summary Laboratory experiments were conducted with three California agricultural soils to examine substrate and process controls over temporal variability of NO and N 2 O production during nitrification, and to quantify the kinetics of HNO 2 ‐mediated chemical reactions. Gross NO production rates were highly correlated ( r 2 = 0.93–0.97) with calculated concentrations of HNO 2 , which were shown to originate from autotrophic microbial oxidation of NH 4 + to NO 2 − Production of NO was not correlated with NH 4 + or NO 3 – , or with the overall nitrification rate. Distinct periods of high NO 2 – accumulation occurred below critical pH values in each soil, apparently due to inhibition of microbial NO 2 – oxidation. Data suggest that even during periods of relatively low NO 2 – accumulation and rapid overall nitrification, HNO 2 ‐mediated reactions may have been the primary source of NO. Rate coefficients (k PNO ) relating NO production to HNO 2 concentrations were determined for sterile (λ‐irradiated) soils, and were similar to k PNO values in 2 of 3 nonsterile soils undergoing nitrification. Production of N 2 O was correlated with HNO 2 ( r 2 = 0.88–0.99) in sterile soils, and with NO 2 – and NO 3 – ( R 2 = 0.72–0.91) in nonsterile soils. Experiments using 15 N confirmed that dissimilatory NO 3 – reduction contributed to N 2 O production even under primarily aerobic conditions. Sterile kPNO and kPN2O 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 NO 2 – /HNO 2 need to be considered in developing improved models of NO and N 2 O emissions from soils.