A pan-arctic synthesis of nongrowing season respiration: Key drivers and responses to a changing climate

Air temperature across the Arctic has been increasing twice as fast as the global average, with the greatest warming occurring during the nongrowing season (NGS; late fall, winter, early spring). Warming during the NGS may have a profound impact on belowground carbon storage because microbial respiration continues when soils are frozen, while photosynthetic uptake has, in large part, ceased. While NGS respiration is recognized as an important component of annual arctic carbon balance, in situ measurements of NGS CO₂ fluxes from soils are relatively sparse. As a result, both the sign and magnitude of the current and future arctic carbon balance remain highly uncertain. To address this uncertainty, we examine the drivers of NGS soil respiration using 1.) a newly-established network of soil respiration sensors located at ten sites across Alaska and Western Canada, and 2.) a synthesized dataset of NGS CO₂ fluxes that span the northern high latitude region. We upscaled the pan-Arctic synthesis data to estimate current NGS CO₂ fluxes across the permafrost region and project NGS CO₂ fluxes under two future climate scenarios (RCP 4.5 and 8.5).

Temperature had the greatest influence on NGS CO₂ flux, followed by gross primary productivity (GPP) and landcover type. There was a positive relationship between NGS flux and leaf area index, enhanced vegetation index, and soil organic carbon, all pointing to the key role of substrate availability on NGS respiration. Another important driver of NGS respiration is the availability of unfrozen water, which is a function of soil temperature and texture; soil moisture was positively related to NGS CO₂ flux, and percent sand was negatively related to flux. There was a significant increase in pan-arctic NGS CO₂ fluxes under both climate scenarios, but NGS CO₂ fluxes were substantially reduced with climate mitigation: Compared to current emissions, there was a 21% projected increase in NGS CO₂ flux by 2100 under RCP 4.5 and 41% increase under RCP 8.5. Increased CO₂ emissions during the NGS may shift the Arctic from a carbon sink to a source, but the magnitude of this response will be highly dependent on future climate change mitigation efforts.