Nitrification controls N2O production rates in a frozen boreal forest soil

The winter season has been identified as a significant contributor to N 2 O emissions from boreal soils, but our understanding of the processes regulating these emissions is fragmentary. We investigated potential N-sources and pathways involved in N 2 O formation in a frozen boreal forest soil by labeling soil samples with 15 N-containing substrates, and measured rates of 15 N 2 O/ 15 N 2 formation under both oxic and anoxic conditions. Our results showed that all N 2 O produced in the frozen samples originate from denitrification, but the rate-limiting factor is NO 3 − availability, which is largely governed by nitrification. This suggests that N 2 O formation in frozen boreal soils may be sustained for a prolonged period of time, but is governed by a delicate balance of the O 2 regime.