Abstract
Abstract. The contribution of soil heterotrophic respiration to the boreal–Arctic carbon (CO2) cycle and its potential feedback to climate change remains poorly quantified. We developed a remote-sensing-driven permafrost carbon model at intermediate scale (∼1 km) to investigate how environmental factors affect the magnitude and seasonality of soil heterotrophic respiration in Alaska. The permafrost carbon model simulates snow and soil thermal dynamics and accounts for vertical soil carbon transport and decomposition at depths up to 3 m below the surface. Model outputs include soil temperature profiles and carbon fluxes at 1 km resolution spanning the recent satellite era (2001–2017) across Alaska. Comparisons with eddy covariance tower measurements show that the model captures the seasonality of carbon fluxes, with favorable accuracy in simulating net ecosystem CO2 exchange (NEE) for both tundra (R>0.8, root mean square error (RMSE – 0.34 g C m−2 d−1), and boreal forest (R>0.73; RMSE – 0.51 g C m−2 d−1). Benchmark assessments using two regional in situ data sets indicate that the model captures the complex influence of snow insulation on soil temperature and the temperature sensitivity of cold-season soil heterotrophic respiration. Across Alaska, we find that seasonal snow cover imposes strong controls on the contribution from different soil depths to total soil heterotrophic respiration. Earlier snowmelt in spring promotes deeper soil warming and enhances the contribution of deeper soils to total soil heterotrophic respiration during the later growing season, thereby reducing net ecosystem carbon uptake. Early cold-season soil heterotrophic respiration is closely linked to the number of snow-free days after the land surface freezes (R=-0.48, p<0.1), i.e., the delay in snow onset relative to surface freeze onset. Recent trends toward earlier autumn snow onset in northern Alaska promote a longer zero-curtain period and enhanced cold-season respiration. In contrast, southwestern Alaska shows a strong reduction in the number of snow-free days after land surface freeze onset, leading to earlier soil freezing and a large reduction in cold-season soil heterotrophic respiration. Our results also show nonnegligible influences of subgrid variability in surface conditions on the model-simulated CO2 seasonal cycle, especially during the early cold season at 10 km scale. Our results demonstrate the critical role of snow cover affecting the seasonality of soil temperature and respiration and highlight the challenges of incorporating these complex processes into future projections of the boreal–Arctic carbon cycle.
Highlights
Warming in the northern high latitudes (> 50◦ N) is occurring at roughly twice the global rate and has trigged a series of changes in boreal and Arctic ecosystems, including earlier and longer growing seasons, widespread soil thawing, and permafrost degradation (Jeganathan et al, 2014; Liljedahl et al, 2016), with large impacts on the regional carbon cycle (McGuire et al, 2016)
Our results show that earlier snow melting, associated with spring warming, enhances soil heterotrophic respiration throughout the growing season, leading to a reduction in net carbon uptake later in the growing season in Alaska (Fig. S12)
We developed a remote-sensing-driven permafrost carbon model at an intermediate scale (∼ 1 km) to evaluate the sensitivity of the seasonal and annual carbon (CO2) cycle and soil respiration to snow cover changes across Alaska during the recent two decades (2001–2017)
Summary
Warming in the northern high latitudes (> 50◦ N) is occurring at roughly twice the global rate and has trigged a series of changes in boreal and Arctic ecosystems, including earlier and longer growing seasons, widespread soil thawing, and permafrost degradation (Jeganathan et al, 2014; Liljedahl et al, 2016), with large impacts on the regional carbon cycle (McGuire et al, 2016). Satellite remote sensing data sets over the past several decades indicate reductions of 0.8–1.3 d decade−1 in the duration of the annual frozen period in the northern high latitudes (Kim et al, 2015) and ∼ 3–4 d decade−1 in the snow cover duration across the Northern Hemisphere, mostly due to spring snow cover reduction (Hori et al, 2017; Bormann et al, 2018) Strong warming in both spring and fall has significantly reduced snow cover during the shoulder seasons; there is large spatial variability across the region, partly due to more variable snow cover conditions during fall and winter (Brown and Derksen, 2013; Hori et al, 2017). How the boreal–Arctic carbon cycle responds to such changes remains to be understood
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
