Abstract

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.

Highlights

  • The Arctic and sub-Arctic regions store more than 50% of the Earth’s soil carbon (C) pool (1330–1580 Pg) (Schuur et al, 2015)

  • Total N2O and 15N-N2O fluxes followed approximately similar seasonal patterns across all bare peat surfaces (BP) plots, with the highest N2O fluxes measured between day of the year (DOY) 210 and 215 (Figure 1 20 and 3c)

  • 15 The N2O emission rates from the BP surfaces were high, as hypothesized, while N2O emissions from vegetated peat surfaces (VP) were negligible throughout the sampling period

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Summary

Introduction

The Arctic and sub-Arctic regions store more than 50% of the Earth’s soil carbon (C) pool (1330–1580 Pg) (Schuur et al, 2015). Soils world-wide are important N2O sources responsible for 60% of the global emissions (IPCC, 2013), it has traditionally been suggested that N2O emissions from Arctic soils are negligible because of their low concentrations of mineral N (Ma et al, 2007; Takakai et al, 2008; Siciliano et al, 2009; Goldberg et al, 10 2010). This generalization has been challenged by the identification of hotspots of N2O on raised permafrost peatlands (Repo et al, 2009; Marushchak et al, 2011) and by measurements of high N2O concentrations in upland tundra soils facing thermokarst formation following permafrost thaw (Abbott & Jones, 2015). The bare peat environment can be considered conducive to microbial N2O production both in nitrification and denitrification

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