Hot spots for nitrous oxide emissions found in different types of permafrost peatlands

Abstract

Recent findings on large nitrous oxide (N 2 O) emissions from permafrost peatlands have shown that tundra soils can support high N 2 O release, which is on the contrary to what was thought previously. However, field data on this topic have been very limited, and the spatial and temporal extent of the phenomenon has not been known. To address this question, we studied N 2 O dynamics in two types of subarctic permafrost peatlands, a peat plateau in Russia and three palsa mires in Finland, including also adjacent upland soils. The peatlands studied have surfaces that are uplifted by frost (palsas and peat plateaus) and partly unvegetated as a result of wind erosion and frost action. Unvegetated peat surfaces with high N 2 O emissions were found from all the studied peatlands. Very high N 2 O emissions were measured from peat circles at the Russian site (1.40 AE 0.15 g N 2 O m À2 yr À1 ). Elevated, sparsely vegetated peat mounds at the same site had significantly lower N 2 O release. The N 2 O emissions from bare palsa surfaces in Northern Finland were highly variable but reached high rates, similar to those measured from the peat circles. All the vegetated soils studied had negligible N 2 O release. At the bare peat surfaces, the large N 2 O emissions were supported by the absence of plant N uptake, the low C : N ratio of the peat, the relatively high gross N mineralization rate and favourable moisture content, together increasing availability of mineral N for N 2 O production. We hypothesize that frost heave is crucial for high N 2 O emissions, since it lifts the peat above the water table, increasing oxygen availability and making it vulnerable to the the physical processes that may remove the vegetation cover. In the future, permafrost thawing may change the distribution of wet and dry surfaces in permafrost peatlands, which will affect N 2 O emissions.

Contrasting denitrifier communities relate to contrasting N2O emission patterns from acidic peat soils in arctic tundra

“…Nitrification is suggested to be the main source of N 2 O in well-aerated soils with a water-filled pore space of o60% (Conrad, 1996;Pihlatie et al, 2004). However, a water-filled pore space of 70-80% in cryoturbated peat circles, nitrate concentrations in the mM range, correlation of high water contents with high N 2 O emission and a C-to-N-ratio of B25 suggest denitrification rather than nitrification as the primary source of N 2 O (Pihlatie et al, 2004;Repo et al, 2009;Marushchak et al, 2011). Indeed, anoxic microcosms at in situ pH with cryoturbated peat soil showed an immediate production of N 2 O from endogenous nitrate, and 75% of the initial nitrate-N was recovered in N 2 O, indicating denitrification rather than dissimilatory nitrate reduction of non-denitrifiers (DNR) (Figure 1a).…”

“…mulated nitrite (Smith 1983;Tiedje, 1988). Nitrite is virtually absent in cryoturbated peat (Repo et al, 2009) (Repo et al, 2009;Marushchak et al, 2011).…”

“…Denitrification as major source of N 2 O in cryoturbated peat Cryoturbated acidic peat circles are 'hotspots' of N 2 O emission in the arctic permafrost region, which was previously regarded as an insignificant source of N 2 O (Denman et al, 2007;Repo et al, 2009;Marushchak et al, 2011). Denitrification, dissimilatory nitrate reduction to ammonium, nitrification and chemodenitrification are potential sources of N 2 O in soils (Smith, 1983;Conrad, 1996;Bremner, 1997;van Cleemput, 1998;Kresovic et al, 2009).…”

“…The global warming potential of N 2 O is 300-fold higher than that of CO 2 on a 100-year basis, and the atmospheric concentration of N 2 O increased from 270 to 319 ppb from 1750 to 2005 (Forster et al, 2007). Agricultural and pristine tropical soils are well-recognized major sources of N 2 O, whereas the importance of arctic peatlands as sources of N 2 O is just emerging (Denman et al, 2007;Repo et al, 2009;Marushchak et al, 2011).…”

“…Repeated freezing and thawing of the cryoturbated soil leads to breakdown of soil aggregates, renders decomposable organic carbon more easily accessible to microbes and may thereby activate the microbial community including N 2 O producers (Mørkved et al, 2006;Sharma et al, 2006). Thus, cryoturbated peat circles represent acidic 'hotspots' of microbial N 2 O emission in the tundra (Repo et al, 2009;Marushchak et al, 2011).…”