The relative controls of temperature, soil moisture, and plant functional group on soil CO₂ efflux at diel, seasonal, and annual scales

Soil respiration (R_soil) is a dominant, but variable, contributor to ecosystem CO₂ efflux. Understanding how variations in major environmental drivers, like temperature and available moisture, might regulate R_soil has become extremely relevant. Plant functional-type diversity makes such assessments difficult because of the confounding influence of varied plant phenology and influences on soil microhabitats. We used automated measurement systems to quantify R_soil in the three microhabitats (under mesquites, under bunchgrasses, and in intercanopy soils) that result from mesquite encroachment into grasslands to inform our understanding of diel R_soil patterns in response to changes in temperature, seasonal variations in R_soil in response to varied soil moisture and plant phenology, and the contribution of each microhabitat to total ecosystem-scale R_soil. We detected a counterclockwise hysteretic response of R_soil to soil temperature, such that up to 100% greater fluxes were observed in the afternoon/evening than the morning for the same temperature. Phenological differences influenced ecosystem-scale R _soil in that mesquites were physiologically active months before bunchgrasses and R_soil rates under mesquites were greater and elevated longer in response to rains. Cumulative annual R_soil was 412, 229, and 202 g C m^-2 under mesquites, bunchgrasses, and intercanopy spaces, respectively. Extrapolating to the ecosystem-scale using cover estimates within the site's eddy covariance footprint illustrated that average mesquite R_soil contributed 46% to overall ecosystem-scale R_soil, though mesquite composed only about 35% of the site. As grasslands transition to shrub dominance, the contribution of R_soil to net ecosystem flux will likely increase, potentially offsetting presumed greater CO₂ uptake potential of woody plants.