Abstract
Soil respiration (i.e. from soils and roots) provides one of the largest global fluxes of carbon dioxide (CO2) to the atmosphere and is likely to increase with warming, yet the magnitude of soil respiration from rapidly thawing Arctic-boreal regions is not well understood. To address this knowledge gap, we first compiled a new CO2 flux database for permafrost-affected tundra and boreal ecosystems in Alaska and Northwest Canada. We then used the CO2 database, multi-sensor satellite imagery, and random forest models to assess the regional magnitude of soil respiration. The flux database includes a new Soil Respiration Station network of chamber-based fluxes, and fluxes from eddy covariance towers. Our site-level data, spanning September 2016 to August 2017, revealed that the largest soil respiration emissions occurred during the summer (June–August) and that summer fluxes were higher in boreal sites (1.87 ± 0.67 g CO2–C m−2 d−1) relative to tundra (0.94 ± 0.4 g CO2–C m−2 d−1). We also observed considerable emissions (boreal: 0.24 ± 0.2 g CO2–C m−2 d−1; tundra: 0.18 ± 0.16 g CO2–C m−2 d−1) from soils during the winter (November–March) despite frozen surface conditions. Our model estimates indicated an annual region-wide loss from soil respiration of 591 ± 120 Tg CO2–C during the 2016–2017 period. Summer months contributed to 58% of the regional soil respiration, winter months contributed to 15%, and the shoulder months contributed to 27%. In total, soil respiration offset 54% of annual gross primary productivity (GPP) across the study domain. We also found that in tundra environments, transitional tundra/boreal ecotones, and in landscapes recently affected by fire, soil respiration often exceeded GPP, resulting in a net annual source of CO2 to the atmosphere. As this region continues to warm, soil respiration may increasingly offset GPP, further amplifying global climate change.
Highlights
The northern permafrost region holds over 50% of the global soil organic carbon (SOC) pool and approximately one trillion tonnes of carbon in the top 3 m of soil alone (Hugelius et al 2014, Meredith et al 2019)
Soil respiration was largest in summer and peak daily-averaged respiration was often observed in July (SI figure 3), the warmest month
T-test significance for monthly flux averages is shown in SI figure 3 and seasonal flux patterns according to biome and flux location are shown in SI figure 4
Summary
The northern permafrost region holds over 50% of the global soil organic carbon (SOC) pool and approximately one trillion tonnes of carbon in the top 3 m of soil alone (Hugelius et al 2014, Meredith et al 2019). Arctic air temperatures have increased rapidly (Box et al 2019), rising 2.7 ◦C (annual average) and 3.1 ◦C (October–May) between 1971 and 2017. This warming has increased the length of the non-frozen season (Kim et al 2012) and has deepened soil thaw (Luo et al 2016) in Alaska and Canada. Soil warming can increase microbial activity (Natali et al 2014) and may result in large amounts of soil carbon being released into the atmosphere, predominantly as carbon dioxide (CO2; Schuur et al 2015, Turetsky et al 2020). The seasonality and magnitude of soil respiration are influenced by soil temperature, soil water content, root activity, and microbial-community access to SOC (Bond-Lamberty et al 2004, Schuur et al 2009, Nagano et al 2018)
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