Soil respiration in a grassland is enhanced under elevated CO2 but is unchanged by elevated CO2 concentration or watering supply

Soils contain the largest pool of terrestrial organic carbon (C) and play a key role in the global C cycle. Terrestrial systems release CO₂ to the atmosphere through soil respiration (R_s), the sum of heterotrophic respiration and autotrophic respiration. On an annual basis, the flux of C to the atmosphere via R_s is 10 times greater than the amount of C released through anthropogenic sources. Thus, it is essential we understand environmental drivers of this massive C flux for accuracy of Earth System models.

Soil respiration and decomposition generally are enhanced under elevated CO₂ (eCO₂) due to indirect shifts in soil C input and water availability. Plants reduce stomatal aperture under eCO₂, which results in decreased transpiration and increased soil moisture. Increased moisture may provide greater diffusion of substrate to microbes and enhance plant biomass, while eCO₂-enhanced photosynthesis may stimulate R_s through increased plant biomass and C substrate to soil microbes. However, it is currently unclear what contribution of the CO₂ effect on R_s is attributed to moisture and what is attributed to C inputs.

We sought to disentangle the impact of these two factors on C cycling in TasFACE2, a unique experiment which exposes a temperate grassland to three levels of CO₂ and explicitly controlled water availability. We measured plant biomass production, root decomposition rates and R_s over one year. Both R_s and root decomposition rates were substantially greater under eCO₂ compared to ambient CO₂. Surprisingly, there were no differences between 475 μmol CO₂ mol^-1 and 550 μmol CO₂ mol^-1 despite differences in plant biomass between CO₂ levels. Although CO₂ had a substantial effect on R_s and mass loss, there was no impact of soil moisture on these processes and no interaction between CO₂ and water supply was present. This is surprising as we expected the CO₂ effect on R_s to be greatest in dry soils due to reduced transpiration rates under eCO₂. Further, we expected decomposition and R_s rates to increase with CO₂ level as a result of greater soil C inputs and relative moisture levels. These findings suggest in this grassland, decomposition and R_s are sensitive to the CO₂-effect on soil moisture and C input but are insensitive to differences between 475 μmol CO₂ mol^-1 and 550 μmol CO₂ mol^-1 and a range of soil moisture conditions.