Nitrous oxide production in a forest soil at low temperatures – processes and environmental controls

Recent investigations have highlighted the relative importance of the winter season for emissions of N 2O from boreal soils. However, our understanding of the processes and environmental controls regulating these emissions is fragmentary. Therefore, we investigated the potential for, and relative importance of, N 2O formation at temperatures below 0 °C in laboratory experiments involving incubations of a Swedish boreal forest soil. Our results show that frozen soils have a high potential for N 2O formation and subsequent emission. Net N 2O production rates at −4 °C equaled those observed at +10 to +15 °C at moisture contents >60% of the soil's water-holding capacity. The source of this N 2O was found to be denitrification occurring in anoxic microsites in the frozen soil and temperature per se did not control the denitrification rates at temperatures around 0 °C. Furthermore, both net nitrogenmineralisation and nitrification were observed in the frozen soil samples. Based on these findings we propose a conceptual model for the temperature response of N 2O formation in soils at low temperatures.