Climate projections point at prolonged dry spells in summer across Europe We postulate that by desiccation of topsoil, microbial activity therein ceases unless countered by water supply through capillary rise from groundwater We firstly aim at the experimental verification of this phenomenon in cropland soils We innovate by creatively making use of existing natural gradients in groundwater depth within sloping agricultural fields We in situ monitor soil moisture and decomposition of 13C-labelled organic matter and soil organic matter in a substantial set of agricultural fields across several lighter soil textural classes We use these datasets to assess if coupling of biogeochemical and hydrological models significantly improves simulation of soil organic matter dynamics We simulate impact of drier summers and lowered groundwater on C-cycling for a large number of cases