Carbon and nitrogen co-cycling dynamics following discontinuous permafrost thaw in Northwest Territories, Canada

Abstract:

With ongoing global permafrost decay there has been a concerted research effort to quantify changes in carbon (C) fluxes following thaw. However, much less emphasis has been placed on characterizing changes in other nutrient cycles, like nitrogen (N) — despite the intrinsic link between C and N dynamics. To address this knowledge gap, we evaluated concomitant changes in C and N pool quantity and quality as drivers of CO₂ production in thawed boreal permafrost soils. Permafrost cores from eight locations across southern Northwest Territories (NWT), Canada were subdivided by depth and incubated for over 2 yrs, quantifying changes in porewater leachate chemistry and CO₂ production rates, as well as initial soil C and N stocks. This work was contextualized via four years of in-situ annual active layer thickness measurements from 124 permafrost sites in the same region. We found abrupt changes in porewater C and N quantity and quality following experimental thaw across all depths, with dissolved organic carbon and dissolved nitrogen concentrations 160% and 70% above mean values respectively. Over time the quality of dissolved C and N pools significantly declined, which paired with evidence of sustained microbial mineralization. Piece-wise structural equation modeling revealed that, contrary to expectations, CO₂ trends were predominantly predicted by initial soil carbon content. Our field-based frost probing showed that active layer thickness increased by an average of 13 cm/yr, with the greatest change occurring in sites more recently burned by wildfire. Using our incubation data to project C losses, we estimate that up to 0.01 g C/m²/day/cm thaw were released during the growing seasons between 2016 – 2019 in southern NWT due to surface permafrost thaw. Taken together this research details the first-order mechanics that link C and N in boreal soils across soil depths, confirming that initial soil C stocks are an important predictor of potential CO₂ losses following permafrost thaw.