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.