Sensing winter soil respiration dynamics in near-real time

Abstract:

Some of the largest reductions in seasonal snow cover are projected to occur in temperate latitudes. Limited measurements from these ecosystems indicate that winter soil respiration releases as much as 30% of carbon fixed during the previous growing season. This respiration is possible with a snowpack that insulates soil from ambient fluctuations in climate. However, relationships among snowpack, soil temperature, soil moisture, and winter soil respiration in temperate regions are not well-understood. Most studies have infrequently sampled soil respiration and its drivers, and most measurements have been limited to the soil surface. We made near-real time, continuous measurements of temperature, moisture, and CO₂ fluxes from the soil profile, through the snowpack, and into the atmosphere in a deciduous forest of New Hampshire, USA. We coupled these data with daily sampling of snow depth and snow water equivalent (SWE). Our objectives were to continuously measure soil CO₂ production (P_soil) and CO₂ flux through the snowpack (F_snow) and to compare F_snow and P_soil with environmental drivers. We found that F_snow was more dynamic than P_soil, changing as much as 30% over several days with shifting environmental conditions. Multiple regression indicated that SWE, air temperature, surface soil temperature, surface soil CO₂ concentrations, and soil moisture at 15 cm were significant predictors of F_snow. The transition of surface temperature from below to above 0°C was particularly important as it represented a phase change from ice to liquid water. Only air temperature and soil moisture at 15 cm were significant drivers of P_soil, where higher moisture at 15 cm resulted in lower P_soil rates. Time series analysis showed that F_snow lagged 40 days behind P_soil. This lag may be due to slow CO₂ diffusion through soil to overlying snow under high moisture conditions. Our results suggest that surface soil CO₂ losses are driven by rapid changes in snow cover, surface temperature, and surface moisture while winter soil CO₂ production is driven by subsurface moisture conditions. Our finding that subsurface moisture regulated winter soil respiration demonstrates the transformative power of sensors; winter measurements of soil CO₂ concentrations and moisture levels would not be possible with traditional methods.