Sensitivity of Soil Respiration to Variability in Temperature and Moisture in a Semi-Arid Grassland

Soil surface CO₂ efflux or soil respiration (R_s) is a key component of the carbon cycle in grassland ecosystems. Understanding and quantifying the response of soil respiration to environmental variables is of critical importance in assessing the impacts of the changing climate on the ecosystem carbon cycle. The impact of wet and dry soil conditions on soil respiration and its sensitivity to temperature is assessed using continuous automated soil chamber measurements of R_s in 2015 over a semi-arid temperate grassland located on the Audubon Research Ranch in south western Arizona, USA (Ameriflux id: US-Aud). This NOAA’s surface budget network (SEBN) site received below-normal precipitation in July–September of 2015, resulting in a soil water deficit during the growing season. When soil water content (SWC) at the 5 cm depth was > 0.08 m³m^-3, soil respiration was positively correlated to soil temperature(T_s) at the 2 cm depth. Below this threshold value of SWC, R_s was largely decoupled from T_s, dropping to less than half of its maximum value during wet soil conditions. Similar patterns were exhibited by eddy covariance measurements of nighttime ecosystem respiration. During periods of soil water stress, the sensitivity of Rs to temperature (dR_s/dT_s) is substantially reduced, resulting in Q₁₀<2. The results indicate that R_s was less influenced by soil temperature during drought conditions at this semi-arid grassland, and have a negative feedback on R_s, mostly above ~18 °C.