Elevated CO₂ consistently increases soil respiration, the primary pathway for the return of organic carbon (C) to the atmosphere, but uncertainties exist about the cause of this increase. Increasing atmospheric CO₂ may increase soil respiration (SR) by increasing plant and microbial C availability via increased photosynthesis and belowground C inputs or by increasing soil water availability via CO₂-induced declines in stomatal conductance and plant water use. Furthermore, other global change factors may affect these drivers of SR, interacting with CO₂ to increase or decrease SR. We therefore took concurrent measurements of SR and volumetric soil water content (VSWC) throughout a growing season in the Biodiversity, CO₂, and N (BioCON) experiment, with the goals of determining the interacting effects of these factors on SR and VSWC and discovering the mechanism(s) behind the effect of elevated CO₂ on SR.

Results/Conclusions

Elevated CO₂ had the largest effects on VSWC and SR, increasing both by as much as 100%. Diversity and N had small, mostly negative effects on VSWC. Surprisingly, diversity did not affect SR. Nitrogen had a small positive effect on SR that was unrelated to its effect on VSWC. On most – but not all – days, the effect of CO₂ on SR increased with the effect of CO₂ on VSWC. Additionally, the effect of CO₂ on SR was largest during dry periods. Thus, our results indicate that while a portion of the large positive response of SR to CO₂ was associated with CO₂-induced increases in VSWC, this association was temporally variable. Yet CO₂-induced changes in soil moisture failed to explain between 40-100% of the CO₂ effect on SR (depending on day and the size of the CO₂ effect on soil moisture). Additionally, elevated CO₂ increased SR across all levels of VSWC, suggesting a role for increased substrate availability in driving the stimulation of SR by elevated CO₂, and providing further support for increased rates of C cycling at elevated CO₂.