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

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.