Nitrification controls N2O production rates in a frozen boreal forest soil

The winter season has been identified as a significant contributor to N2O emissions from boreal soils, but our understanding of the processes regulating these emissions is fragmentary. We investigated potential N-sources and pathways involved in N2O formation in a frozen boreal forest soil by labeling soil samples with 15N-containing substrates, and measured rates of 15N2O/15N2 formation under both oxic and anoxic conditions. Our results showed that all N2O produced in the frozen samples originate from denitrification, but the rate-limiting factor is NO3− availability, which is largely governed by nitrification. This suggests that N2O formation in frozen boreal soils may be sustained for a prolonged period of time, but is governed by a delicate balance of the O2 regime.