Sources of nitrous oxide and fate of mineral nitrogen in sub-Arctic permafrost peat soils.

Nitrous oxide (N₂O) emissions from permafrost-affected terrestrial ecosystems have received little attention, largely because they have been thought to be negligible. Recent studies, however, have shown that there are habitats in subarctic tundra emitting N₂O at high rates, such as bare peat surfaces on permafrost peatlands. The processes behind N₂O production in these high-emitting habitats are, however, poorly understood. In this study, we established an in situ ¹⁵N-labelling experiment with the main objectives to partition the microbial sources of N₂O emitted from bare peat surfaces (BP) on permafrost peatlands and to study the fate of ammonium and nitrate in these soils and in adjacent vegetated peat surfaces (VP) showing low N₂O emissions. Our results confirm the hypothesis that denitrification is mostly responsible for the high N₂O emissions from BP surfaces. During the study period denitrification contributed with ~79% of the total N₂O emission in BP, while the contribution of ammonia oxidation was less, about 19 %. However, nitrification is a key process for the overall N₂O production in these soils with negligible external nitrogen (N) load because it is responsible for nitrite/nitrate supply for denitrification, as also supported by relatively high gross nitrification rates in BP. Generally, both gross N mineralization and gross nitrification rates were much higher in BP with high N₂O emissions than in VP, where the high C/N ratio together with low water content was likely limiting N mineralization and nitrification and, consequently, N₂O production. Also, competition for mineral N between plants and microbes was additionally limiting N availability for N₂O production in VP. Our results show that multiple factors control N₂O production in permafrost peatlands, the absence of plants being a key factor together with inter-mediate to high water content and low C/N ratio, all factors which also impact on gross N turnover rates. The intermediate to high soil water content which creates anaerobic microsites in BP is a key N₂O emission driver for the prevalence of denitrification to occur. This knowledge is important for evaluating future permafrost -N feedback loops from the Arctic. [ABSTRACT FROM AUTHOR]