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

Abstract. Site-level observations have shown pervasive cold season CO₂ release across Arctic and boreal ecosystems, impacting annual carbon budgets. Still, the seasonality of CO₂ emissions are poorly quantified across much of the high latitudes due to the sparse coverage of site-level observations. Space-based observations provide the opportunity to fill some observational gaps for studying these high latitude ecosystems, particularly across poorly sampled regions of Eurasia. Here, we show that data-driven net ecosystem exchange (NEE) from atmospheric CO₂ observations implies strong summer uptake followed by strong autumn release of CO₂ over the entire cold northeastern region of Eurasia during the 2015–2019 study period. Combining data-driven NEE with satellite-based estimates of gross primary production (GPP), we show that this seasonality implies less summer heterotrophic respiration (R_h) and greater autumn R_h than would be expected given an exponential relationship between respiration and surface temperature. Furthermore, we show that this seasonality of NEE and R_h over northeastern Eurasia is not captured by the TRENDY v8 ensemble of dynamic global vegetation models (DGVMs), which estimate that only 52 % of annual R_h occurs during Aug–Apr, while the data-driven estimate suggests 64–70 % of annual R_h occurs over this period. We explain this seasonal shift in R_h by respiration from soils at depth during the zero curtain period, when sub-surface soils remain unfrozen up to several months after the surface has frozen. Additional impacts of physical processes related to freeze-thaw dynamics may contribute to the seasonality of R_h. This study confirms a significant and spatially extensive early cold season CO₂ efflux in the permafrost rich region of northeast Eurasia, and suggests that autumn R_h from subsurface soils in the northern high latitudes is not well captured by current DGVMs.