Seasonality of soil CO2 efflux in a temperate forest: Biophysical effects of snowpack and spring freeze-thaw cycles

Changes in characteristics of snowfall and spring freeze-thaw-cycle (FTC) events under the warming climate make it critical to understand biophysical controls on soil CO2 efflux (RS) in seasonally snow-covered ecosystems. We conducted a snow removal experiment and took year-round continuous automated measurements of RS, soil temperature (T5) and soil volumetric water content at the 5 cm depth (W5) with a half-hour interval in a Chinese temperate forest in 2010-2011. Our objectives were to: (1) develop statistical models to describe the seasonality of RS in this forest; (2) quantify the contribution of seasonal RS to the annual budget; (3) examine biophysical effects of snowpack on RS; and (4) test the hypothesis that an FTC-induced enhancement of RS is jointly driven by biological and physical processes. Empirical RS-T5-W5 models explained 65.3-94.1% of the variability in the RS data, but the number of the regression terms and their coefficients varied with season. This indicates that the model should be fitted to the seasonal data sets separately to explicitly describe the seasonality of RS. The RS during the winter, spring FTC period, and growing season contributed 5.7%, 3.5%, and 91.1%, respectively, to the total annual RS. The relative enhancement of RS due to snowpack and FTCs averaged 3.4 and 2.5, respectively. The snowpack-induced enhancement of RS exponentially increased with T5 (R2 = 0.83) and snow depth (R2 = 0.16), while the FTC-induced enhancement of RS exponentially decreased with T5 (R2 = 0.45) and W5 (R2 = 0.67). These results suggest that the snowpack-induced enhancement mainly results from the snow-depth-dependent insulation of soil from low air temperatures, while the FTC-induced enhancement is dominantly driven by biological processes. Accumulatively, the snowpack and spring FTCs made a minor net contribution (2.3% and 1.2%, respectively) to the annual RS budget.