Global change factors reshape the links between litter properties, decomposers, and decomposition in mature oak forests

1.) Increasing atmospheric CO2 concentrations alongside more frequent and severe droughts are key global change factors impacting litter decomposition and global carbon cycles. However, we have a poor understanding of how both drought and elevated CO2 impact interactions between initial litter chemical properties and abiotic and biotic components of the decomposition environment, especially under field conditions. 2.) We close this gap by testing how drought and elevated atmospheric CO2 concentrations modify rates of litter decomposition by acting through two pathways, the decomposition environment and initial litter properties. For this analysis, we used two long-term manipulative field experiments in mature oak woodlands: the Birmingham Institute of Forest Research Free-Air CO2 Enrichment (BIFoR FACE) experiment and the Oak Observatory at the Observatoire de Haute-Provence (O3HP) drought experiment that reduces seasonal precipitation. In each experiment, litter bags were deployed in a reciprocal transplant design whereby control and experimental litter (i.e. litter derived from plots under either drought or elevated CO2 conditions) were incubated in control and experimental plots at the respective experiment. We measured litter mass loss, functional groups, C:N ratios, moisture content, and microbial and mesofauna communities. 3.) We found that drought and elevated CO2 concentrations modified mass loss rates through different pathways. In the drought experiment, litter placed in droughted conditions decomposed slower as the decomposition environment shaped mass loss rates more than initial litter properties. By contrast, both decomposition environment and initial litter properties impacted litter C:N, functional groups, and microbial communities during the experiment. In the elevated CO2 experiment, experimental litter derived from elevated CO2 plots corresponded with slower mass loss over the first three harvests as the decomposition environment did not impact litter properties or decomposers. 4.) Synthesis: We found that drought and elevated atmospheric CO2 can modify mass loss rates through distinct pathways. Further research is needed to disentangle the relative roles of climate and decomposers with decomposition rates under drought and elevated CO2, but our experiment highlights that litter decomposition will be reshaped by global change factors through multiple mechanisms.