Climatic and landscape influences on soil moisture are primary determinants of soil carbon fluxes in seasonally snow-covered forest ecosystems

Abstract

A changing climate has the potential to mobilize soil carbon, shifting seasonally snow-covered, forested ecosystems from carbon sinks to sources. To determine the sensitivity of soil carbon fluxes to changes in temperature and moisture, we quantified seasonal and spatial variability of soil carbon dioxide (CO2) fluxes (N = 746) and dissolved organic carbon (DOC) in leachate (N = 260) in high-elevation, mixed conifer forests in Arizona and New Mexico. All sites have cold winters, warm summers, and bimodal soil moisture patterns associated with snowmelt and summer monsoon rainfall. We employed a state factor approach, quantifying how distal controls (parent material, regional climate, topography) interacted with proximal variability in soil temperature (−3 to 26 °C) and moisture (2–76 %) to influence carbon effluxes. Carbon loss was dominated by CO2 flux (250–1220 g C m−2 year−1) rather than leached DOC (7.0–9.4 g C m−2 year−1). Significant differences in mean growing season CO2 flux were associated with parent material and aspect; differences appear to be mediated by how these distal controls influence primarily moisture and secondarily temperature. Across all sites, a multiple linear regression model (MLR) relying on moisture and temperature best described growing season CO2 fluxes (r2 = 0.63, p < 0.001). During winter, the MLR describing soil CO2 flux (r2 = 0.98, p < 0.001) relied on distal factors including snow cover, clay content, and bulk carbon, all factors that influence liquid water content. Our findings highlight the importance of state factors in controlling soil respiration primarily through influencing spatial and temporal heterogeneity in soil moisture.