Patterns and drivers of anaerobic nitrogen transformations in sediments of thermokarst lakes

Abstract

Significant attention has been given to the way in which the soil nitrogen (N) cycle responds to permafrost thaw in recent years, yet little is known about anaerobic N transformations in thermokarst lakes, which account for more than one‐third of thermokarst landforms across permafrost regions. Based on the N isotope dilution and tracing technique, combined with qPCR and high‐throughput sequencing, we presented large‐scale measurements of anaerobic N transformations of sediments across 30 thermokarst lakes over the Tibetan alpine permafrost region. Our results showed that gross N mineralization, ammonium immobilization, and dissimilatory nitrate reduction rates in thermokarst lakes were higher in the eastern part of our study area than in the west. Denitrification dominated in the dissimilatory nitrate reduction processes, being two and one orders of magnitude higher than anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA), respectively. The abundances of the dissimilatory nitrate reduction genes (nirK, nirS, hzsB, and nrfA) exhibited patterns consistent with sediment N transformation rates, while α diversity did not. The inter‐lake variability in gross N mineralization and ammonium immobilization was dominantly driven by microbial biomass, while the variability in anammox and DNRA was driven by substrate supply and organic carbon content, respectively. Denitrification was jointly affected by nirS abundance and organic carbon content. Overall, the patterns and drivers of anaerobic N transformation rates detected in this study provide a new perspective on potential N release, retention, and removal upon the formation and development of thermokarst lakes.

Denitrification dominates dissimilatory nitrate reduction across global natural ecosystems

“…Denitrification, anammox, and DNRA can occur simultaneously under anaerobic conditions, but their relative contribution to nitrate reduction are highly variable because of their different preferences for substrate availability and other environmental factors (Li et al., 2019; Mao et al., 2023; McTigue et al., 2016). This study first revealed that, at the global scale, the contribution of denitrification to dissimilatory nitrate reduction increased significantly with longitude, while the contribution of DNRA decreased (Figure 3a).…”

Microbial nitrogen and phosphorus co‐limitation across permafrost region

“…The organic C pool in 0–3 m of soils is ~15.3–46.2 Pg (Ding et al, 2016; Wang et al, 2021). Due to the continuous climate warming, the alpine permafrost is rapidly degrading, characterized by the thickening of the active layer (~2 cm year −1 ; Wang et al, 2022) and the widespread occurrence of thermokarst landscape such as thermo‐erosion gully (Yang et al, 2021) and thermokarst lake (Mao et al, 2023; Wei et al, 2021). The permafrost degradation was predicted to result in ~122 Tg (1 Tg = 10 12 g; under Representative Concentration Pathway 4.5, a moderate scenario) and ~300 Tg (under Representative Concentration Pathway 8.5, a worst‐case scenario) of permafrost C release across the plateau by 2099 (Liu et al, 2022).…”

“…The organic C pool in 0-3 m of soils is ~15.3-46.2 Pg (Ding et al, 2016;Wang et al, 2021). Due to the continuous climate warming, the alpine permafrost is rapidly degrading, characterized by the thickening of the active layer (~2 cm year −1 ; Wang et al, 2022) and the widespread occurrence of thermokarst landscape such as thermo-erosion gully (Yang et al, 2021) and thermokarst lake (Mao et al, 2023;Wei et al, 2021). The permafrost…”

Priming effect stimulates carbon release from thawed permafrost

“…The dominant permafrost types on the Tibetan Plateau are discontinuous (50%–90%) and sporadic (10%–50%) permafrost (Wang et al, 2022; Zhang et al, 2008), which account for 55.4% and 41.4%, respectively, of the total permafrost area of the region (Figure S1). Over recent decades, the plateau has undergone accelerated soil warming (Ran et al, 2018) and extensive permafrost degradation (Mao et al, 2023; Ni et al, 2021; Xu et al, 2023). Permafrost temperature at a depth of 10 m has increased at an average rate of 0.02–0.78°C decade −1 from 2004 to 2018 (Zhao et al, 2021).…”