Investigating the sensitivity of soil heterotrophic respiration to recent snow cover changes in Alaska using a satellite-based permafrost carbon model

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.

Soil Respiration Phenology Improves Modeled Phase of Terrestrial net Ecosystem Exchange in Northern Hemisphere

“…The result is that the NEE and RECO cycles both peak in early July (Table S2 in Supporting Information ) but the RECO peak is broader, consistent with Yi et al. (2020). In mid‐to‐late summer, the RECO flux at the NH sites is substantially reduced due to SM (i.e., substrate diffusion) limits (not shown).…”

“…The new, vertically resolved SOC model is similar to that of Yi et al. (2020): $$$\frac{\partial }{\partial t}{C}_{i}(z)={\mathcal{R}}_{i}(z)-{k}_{i}{C}_{i}(z)+\frac{\partial }{\partial z}\left(D(z)\frac{\partial {C}_{i}}{\partial z}\right)$$$ where $${\mathcal{R}}_{i}(z)$$ represents inputs (and transfers) to SOC pool i at depth z and D ( z ) is the vertical diffusivity of SOC. Diffusivity is taken to be 2 × 10 −4 m 2 yr −1 , after Yi et al.…”

“…Diffusivity is taken to be 2 × 10 −4 m 2 yr −1 , after Yi et al. (2020) for nonpermafrost soils. Each soil layer or depth, z , contains the same three SOC pools, which are the same pools in the baseline NRv8.3 and the other experiments.…”

Multi-year observations reveal a larger than expected autumn respiration signal across northeast Eurasia

“…We use the soil carbon decomposition model developed in Yi et al (2015Yi et al ( , 2020 to simulate the contribution of soil at different depths to total R h and NEE fluxes. The soil decomposition model uses multiple litter and SOC pools to characterize the progressive decomposition of fresh litter to more recalcitrant materials, which include three litterfall pools, three SOC pools with relatively fast turnover rates, and a deep SOC pool with slow turnover rates.…”

“…The litterfall carbon inputs were first allocated to the three litterfall pools depending on the substrate quality of litterfall component and then transferred to the SOC pools through progressive decomposition. We then model the profile of the carbon pools through accounting for the vertical carbon transport (Yi et al, 2020). A constant diffusivity rate was assigned to permafrost (5.0 cm 2 yr −1 ) and non-permafrost (2.0 cm 2 yr −1 ) regions within the top 1 m soil, and then linearly decreased to 0 at the 3 m below surface (Koven et al, 2013).…”