Litter biota and quality as drivers of litter decomposition in mature oak forests subject to drought or elevated CO2

Increasing atmospheric CO2 concentrations alongside more frequent and severe droughts are key global change factors impacting litter decomposition and global carbon cycles. Yet, we have a poor understanding of how these perturbations impact interactions between initial litter properties and decomposition environment. We tested how drought and elevated atmospheric CO2 concentrations modify litter decomposition via litter properties and decomposition environment using two separate, long-term manipulative drought or elevated CO2 field experiments in mature oak woodlands. Litterbags were deployed in a reciprocal transplant design within each experiment, where we measured litter mass loss, C-biochemistry, C:N ratios, moisture content, and decomposer communities. We found that litter placed in droughted plots decomposed slower than in control plots and experimental litter derived from elevated CO2 plots decomposed slower over the first three harvests compared to control litter. Under drought, litter mass loss rates and C:N ratio was regulated by initial litter properties and the decomposition environment, while elevated CO2 impacted mass loss via changes in initial litter properties. We show that drought and elevated atmospheric CO2 can modify the decomposability of litter prior to litterfall and during the subsequent decomposition, highlighting the need to disentangle their individual and interactive effects to better predict how global change factors influence decomposition. HighlightsO_LIDrought impacted litter mass loss more than elevated CO2 conditions C_LIO_LIDecomposition environment shaped mass loss with drought but not elevated CO2 C_LIO_LITreatment-effects on litter properties is observable after decomposition begins C_LIO_LIDecomposers and litter properties still impact decomposition at middle-to-late stages C_LI