Effects of Increasing pH on Nitrous Oxide and Dinitrogen Emissions from Denitrification in Sterilized and Unsterilized Forest Soils

Abstract

Denitrification, as an important part of the soil nitrogen cycle, is widely considered to be a major source of nitrous oxide (N2O). Both biotic and abiotic denitrification processes contribute significantly to soil N2O emission, especially under acidic conditions. Increasing soil pH was found to suppress N2O emissions from denitrification, while the underlying mechanism remains uncertain. In this study, we incubated fresh forest soil anaerobically after increasing soil pH and adding nitrate (NO3−) under both sterilized and unsterilized conditions. The dynamic changes of NO3−, nitrite (NO2−), N2O and dinitrogen (N2) were monitored continuously during the 15 days of incubation. The results showed that nitrate reduction rates increased with soil pH in both sterilized and unsterilized soils, with the former having higher rates. The obvious production and consumption of nitrite were found at pH 7.1, rather than at pH 5.5, especially in sterilized soils. In both sterilized and unsterilized soils, accumulative emission of N2O and N2O-N/(N2O+N2)-N product ratios decreased significantly with increasing pH, while N2 showed the opposite trend. In sterilized soils, N2O was the dominant end gas product, accounting for 40.88% and 29.42% of the added nitrate at pH 5.5 and 7.1, respectively. In unsterilized soils, N2 was the only final gas product at pH 7.1 (59.34% of the added nitrate), whereas N2O dominated at pH 5.5 (26.67% of the added nitrate). Our results here showed that increasing soil pH promoted the conversion of N2O to N2 under both sterilized and unsterilized conditions, and highlighted the potential importance of abiotic denitrification on N2O emission.