Recent investigations have highlighted the relative importance of the winter season for emissions of N(2)O 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(2)O formation at temperatures below 0 degrees C in laboratory experiments involving incubations of a Swedish boreal forest soil. Our results show that frozen soils have a high potential for N(2)O formation and subsequent emission. Net N(2)O production rates at -4 degrees C equaled those observed at +10 to +15 degrees C at moisture contents >60% of the soil's water-holding capacity. The source of this N(2)O 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 degrees C. Furthermore, both net nitrogen-mineralisation and nitrification were observed in the frozen soil samples. Based on these findings we propose a conceptual model for the temperature response of N(2)O formation in soils at low temperatures.