Temperature and moisture effects on greenhouse gas emissions from deep active-layer boreal soils

Abstract. Rapid climatic changes, rising air temperatures, and increased fires are expected to drive permafrost degradation and alter soil carbon (C) cycling in many high-latitude ecosystems. How these soils will respond to changes in their temperature, moisture, and overlying vegetation is uncertain but critical to understand given the large soil C stocks in these regions. We used a laboratory experiment to examine how temperature and moisture control CO₂ and CH₄ emissions from mineral soils sampled from the bottom of the annual active layer, i.e., directly above permafrost, in an Alaskan boreal forest. Gas emissions from 30 cores, subjected to two temperatures and either field moisture conditions or experimental drought, were tracked over a 100-day incubation; we also measured a variety of physical and chemical characteristics of the cores. Gravimetric water content was 0.31 ± 0.12 (unitless) at the beginning of the incubation; cores at field moisture were unchanged at the end, but drought cores had declined to 0.06 ± 0.04. Daily CO₂ fluxes were positively correlated with incubation chamber temperature, core water content, and percent soil nitrogen. They also had a temperature sensitivity (Q₁₀) of 1.3 and 1.9 for the field moisture and drought treatments, respectively. Daily CH₄ emissions were most strongly correlated with percent nitrogen, but neither temperature nor water content was a significant first-order predictor of CH₄ fluxes. The cumulative production of C from CO₂ was over 6 orders of magnitude higher than that from CH₄; cumulative CO₂ was correlated with incubation temperature and moisture treatment, with drought cores producing 52–73 % lower C. Cumulative CH₄ production was unaffected by any treatment. These results suggest that deep active-layer soils may be sensitive to changes in soil moisture under aerobic conditions, a critical factor as discontinuous permafrost thaws in interior Alaska. Deep but unfrozen high-latitude soils have been shown to be strongly affected by long-term experimental warming, and these results provide insight into their future dynamics and feedback potential with future climate change.