Quantifying the timescales over which exogenous and endogenous conditions affect soil respiration

Understanding how exogenous and endogenous factors and above-ground-below-ground linkages modulate carbon dynamics is difficult because of the influences of antecedent conditions. For example, there are variable lags between above-ground assimilation and below-ground efflux, and the duration of antecedent periods are often arbitrarily assigned. Nonetheless, developing models linking above- and below-ground processes is crucial for estimating current and future carbon dynamics. We collected data on leaf-level photosynthesis (A_sat) and soil respiration (R_soil) in different microhabitats (under shrubs vs under bunchgrasses) in the Sonoran Desert. We evaluated timescales over which endogenous and exogenous factors control R_soil by analyzing data in the context of a semimechanistic temperature-response model of R_soil that incorporated effects of antecedent exogenous (soil water) and endogenous (A_sat) conditions. For both microhabitats, antecedent soil water and A_sat significantly affected R_soil, but R_soil under shrubs was more sensitive to A_sat than that under bunchgrasses. Photosynthetic rates 1 and 3 d before the R_soil measurement were most important in determining current-day R_soil under bunchgrasses and shrubs, respectively, indicating a significant lag effect. Endogenous and exogenous controls are critical drivers of R_soil, but the relative importance and the timescale over which each factor affects R_soil depends on above-ground vegetation and ecosystem structure characteristics.