Forests in central Europe are likely to experience an increase in drought frequency and intensity. The resulting change in availability and quality of organic matter will further affect the soil microbial community composition and activity, with a largely unknown effect on carbon cycling in soils. An important aspect of carbon cycling is the carbon use efficiency, CUE, which is the ratio of carbon incorporated in microbial biomass compared to carbon respired. Because of their high sensitivity to central metabolism, the naturally occurring hydrogen isotopes composition of microbial lipids could offer a novel tool for a mechanistic understanding of CUE in soil. The overarching goal of this Swiss National Science Foundation project, DRIER, is to determine how central metabolic processes involved in carbon cycling by soil microbial communities change with drought. First, we will test the suitability of δ²H values of phospholipid-derived fatty acids (PLFAs) as a proxy for net soil microbial metabolism in laboratory experiments. We will then evaluate how drought and drought-associated organic matter inputs impact soil microbial metabolism, community composition, and gene expressions in mesocosms. Finally, we will determine how soil microbial metabolism changes during a long-term water exclusion experiment in a natural forest at the Swiss Canopy Crane II site https://ppe.duw.unibas.ch/en/sccii/