Magnitude and Pathways of Increased Nitrous Oxide Emissions from Uplands Following Permafrost Thaw

Abstract

Permafrost thawing may release nitrous oxide (NO) due to large N storage in cold environments. However, NO emissions from permafrost regions have received little attention to date, particularly with respect to the underlying microbial mechanisms. We examined the magnitude of NO fluxes following upland thermokarst formation along a 20-year thaw sequence within a thermo-erosion gully in a Tibetan swamp meadow. We also determined the importance of environmental factors and the related microbial functional gene abundance. Our results showed that permafrost thawing led to a mass release of NO in recently collapsed sites (3 years ago), particularly in exposed soil patches, which presented post-thaw emission rates equivalent to those from agricultural and tropical soils. In addition to abiotic factors, soil microorganisms exerted significant effects on the variability in the NO emissions along the thaw sequence and between vegetated and exposed patches. Overall, our results demonstrate that upland thermokarst formation can lead to enhanced NO emissions, and that the global warming potential (GWP) of NO at the thermokarst sites can reach 60% of the GWP of CH (vs ∼6% in control sites), highlighting the potentially strong noncarbon (C) feedback to climate warming in permafrost regions.

Trajectory of Topsoil Nitrogen Transformations Along a Thermo‐Erosion Gully on the Tibetan Plateau

“…The soil C and N concentrations at a depth of 0–15 cm are 145.2 and 13.0 g kg −1 , respectively (Liu, Chen, Zhang, et al, ). The average active‐layer thickness in nonthermokarst areas is estimated at ~0.9 m. Thermokarst landscapes, that is, thermo‐erosion gullies, develop in this permafrost region, with the depth of the active layer within the gully exhibiting no significant difference from that of the nonthermokarst areas (Yang, Peng, Marushchak, et al, ; Yang, Peng, Olefeldt, et al, ).…”

Reduced quantity and quality of SOM along a thaw sequence on the Tibetan Plateau

“…The soil in this area has a silty loam texture with pH of 5.8. Based on GSSI georadar (SIR-20, Laurel, Santa Clara, USA) and thaw-probe measurements, the average active layer thickness (ALT) of this study area is 0.86 m (Yang et al 2018b). The ALT within and outside the thaw feature is 1.05 and 0.83 m, respectively (figure S2).…”

Permafrost nitrous oxide emissions observed on a landscape scale using the airborne eddy-covariance method

“…For the anthropogenic sector, this primarily occurs in the form of enhanced microbial activity in agricultural soils due to an imbalance between nitrogen (N) fertilizer supply and crop uptake (Syakila and Kroeze, 2011). For the natural sector, tropical soils are considered to be the largest source of N 2 O (Zhuang et al, 2012). Meanwhile, N 2 O emissions from permafrost-laden regions have long been assumed to be neg-J.…”

“…Meanwhile, N 2 O emissions from permafrost-laden regions have long been assumed to be neg-J. Wilkerson et al: Permafrost N 2 O emissions observed on landscape scales ligible (Martikainen et al, 1993;Potter et al, 1996) and are ignored in current N 2 O budgets (Anderson et al, 2010;Zhuang et al, 2012). This is largely because higher latitudes are considered nitrogen-limited and biogeochemically inactive relative to the tropics (Zhuang et al, 2012).…”

“…This is reasonable if permafrost N 2 O emissions are truly negligible as is assumed. However, recent in situ measurements of permafrost soils in Russian tundra and northern Finland (Repo et al, 2009;Marushchak et al, 2011) have found several geologic formations that may emit N 2 O fluxes comparable to tropical soil emissions (Zhuang et al, 2012). These formations include bare peat surfaces and thaw-induced permafrost collapse known as thermokarst.…”