N2O, NO and NO2 fluxes from a grassland: Effect of soil pH

Fluxes of N2O, NO and NO2 between grassland and the atmosphere were measured over 1 y using three plots which have been maintained at a constant pH of 3.9, 5.9 and 7.6 over many years. Net fluxes of N2O and NO were always from the soil to the atmosphere, whilst those of NO2 were invariably from atmosphere to soil. Mean fluxes of N2O decreased appreciably with increasing acidity, whilst NO fluxes showed little dependence on pH, with the highest mean flux from the plot at pH 5.9. Sterilization of soil cores by autoclaving reduced N2O emissions almost to zero at all pH values, but residual production of NO was found, even at low pH. Increasing the pH of unsterilized soil cores from pH 3.9–6.5±0.5 led to a reduction in NO and especially N2O fluxes. It was concluded that the microbial community of the soil had adjusted to the low pH and was responsible for the entire production of N2O and much of the NO release. Chemodenitrification is also responsible for some NO production, especially at low pH.