Influences of Soil Temperature, Soil Moisture and Canopy Photosynthesis on Diurnal and Seasonal Dynamics of Soil Respiration.

Soil respiration is the major pathway by which terrestrial ecosystems are losing carbon to the atmosphere. It is important to understand the role soil respiration playing in regulating atmospheric CO₂ concentration for developing policies and practices to mitigate climate change.  The majority of previous studies that investigated soil respiration have focused on unmanaged ecosystems. However, managed agroecosystems experience numerous transitory events associated with crop production during the growing season which can cause rapid changes in the soil environment that can result in soil CO₂ emissions that can be several orders of magnitude greater than the typical undisturbed background rates.  In this study, we investigated the role of soil moisture, soil temperature and canopy photosynthesis on diurnal and seasonal dynamics of the soil respiration from a biomass sorghum (Sorghum bicolor) field in the Southern Great Plains.  Soil respiration was measured at half-hourly intervals using two automated chambers (Model LICOR 8100A). Soil temperature and soil volumetric water content (VWC) were measured at 5 cm depth at half-hourly intervals.  Atmospheric CO₂ fluxes were measured using an eddy covariance flux tower and half-hourly net ecosystem CO₂ exchange (NEE) and gross canopy photosynthesis were computed. On average, soil respiration rates measured using the chamber closer to the plants were 46% higher than the respiration rates measured using the chamber in between the plant rows which could be the result of higher root respiration. Soil temperature was positively correlated with soil respiration (p < 0.01).  The temperature sensitivity of soil respiration (Q₁₀) was 2.07 during off-season and 1.53 during the growing season. Rainfall and irrigation significantly enhanced soil respiration. We also observed a positive correlation between soil respiration and canopy photosynthesis (p < 0.01). The results of this study suggest that soil temperature, VWC and canopy photosynthesis have major influence on determining the rates of respiration.