Elevated CO₂ stimulates grassland soil respiration by increasing carbon inputs rather than by enhancing soil moisture

It is not clear whether the consistent positive effect of elevated CO 2 on soil respiration (soil carbon flux, SCF ) results from increased plant and microbial activity due to (i) greater C availability through CO 2 ‐induced increases in C inputs or (ii) enhanced soil moisture via CO 2 ‐induced declines in stomatal conductance and plant water use. Global changes such as biodiversity loss or nitrogen ( N ) deposition may also affect these drivers, interacting with CO 2 to affect SCF . To determine the effects of these factors on SCF and elucidate the mechanism(s) behind the effect of elevated CO 2 on SCF , we measured SCF and soil moisture throughout a growing season in the B iodiversity, CO 2 , and N ( B io CON ) experiment. Increasing diversity and N caused small declines in soil moisture. Diversity had inconsistent small effects on SCF through its effects on abiotic conditions, while N had a small positive effect that was unrelated to soil moisture. Elevated CO 2 had large consistent effects, increasing soil moisture by 26% and SCF by 45%. However, CO 2 ‐induced changes in soil moisture were weak drivers of SCF : CO 2 effects on SCF and soil moisture were uncorrelated, CO 2 effect size did not change with soil moisture, within‐day CO 2 effects via soil moisture were neutral or weakly negative, and the estimated effect of increased C availability was 14 times larger than that of increased soil moisture. Combined with previous B io CON results indicating elevated CO 2 increases C availability to plants and microbes, our results suggest that increased SCF is driven by CO 2 ‐induced increases in substrate availability. Our results provide further support for increased rates of belowground C cycling at elevated CO 2 and evidence that, unlike the response of productivity to elevated CO 2 in B io CON , the response of SCF is not strongly N limited. Thus, N limited grasslands are unlikely to act as a N sink under elevated CO 2 .