Rate of denitrification and stoichiometry of its products in fluvo‐aquic Cambisols for sterilized and unsterilized incubations

Abstract

There has been an increase in studies suggesting that both biotic and abiotic (i.e. chemical) denitrification in soil contribute to nitrate (NO3−) loss and nitrogenous gas (NO, N2O and N2) emissions, and that rates of reaction and stoichiometry of products depend on soil pH. However, their relative importance and product composition have not been addressed simultaneously, limiting our understanding on their in‐situ coupling pathways and mechanisms of response to changes in soil pH. Here, we present changes in NOx− (i.e. NO3− and NO2−) concentrations and nitrogenous gas amounts during 15 days of anaerobic incubations of sterilized and unsterilized fluvo‐aquic Cambisols after the addition of NO3− and adjustments to pH. Nitrate losses in sterilized soil were 47.7, 27.5 and 52.9 mg N kg−1 at pH 6.5, 7.1 and 8.3, respectively, accounting for 90.0, 79.5 and 179% of the corresponding values for unsterilized soil. The rates of nitrate reduction increased with pH in sterilized soil, whereas the largest rate in unsterilized soil was at the lowest pH (6.5). Moreover, the rates of reaction of the stepwise reductions from nitrite (NO2−) to N2 tended to increase with pH in both sterilized and unsterilized soils. These results verify the potential importance of abiotic denitrification to the nitrogen cycle in alkaline soils and demonstrate the difference between abiotic and biotic nitrate reductions in responses to changes in soil pH. These findings improve our ability to assess the coupling mechanisms and eco‐environmental influence of biotic and abiotic denitrification in circumneutral and alkaline soils.Highlights Abiotic denitrification is an important pathway for nitrate loss in circumneutral and alkaline fluvo‐aquic soils. Abiotic and biotic nitrate reduction respond differently to changes in soil pH. Stepwise reduction rates from NO2− to N2 increased with pH in both sterilized and unsterilized soils. Abiotic nitrate reduction should be incorporated in biotic and abiotic denitrification coupling in alkaline soils.