Microbial carbon use efficiency as a driver of subsoil carbon storage across contrasting European land uses

Subsoils below 30 cm depth store a large fraction of soil organic carbon, yet the biological mechanisms controlling carbon stabilization at depth remain poorly understood. In particular, the role of microbial carbon use efficiency (CUE)—the balance between microbial growth and respiration—in regulating subsoil carbon storage has rarely been addressed across contrasting land uses and management practices. Within the DeepHorizon project, subsoil samples were collected from 40 core sites representing major European land uses, including arable land, grassland, forest, and semi-natural ecosystems. We hypothesize that management practices promoting continuous carbon inputs and stable physical conditions at depth favor higher microbial efficiency and greater carbon retention. We quantified microbial respiration and substrate-induced activity using MicroResp, and estimated microbial carbon use efficiency by combining respiration measurements with microbial growth derived from the ¹⁸O–H₂O method. We assessed how CUE varies across land uses, soil types, climatic gradients, and land management practices, and how this variability relates to subsoil carbon stocks and organic matter characteristics. This work provides a mechanistic basis to explain spatial variability in deep soil carbon storage and will directly feed into the process-based models developed in DeepHorizon project, improving their ability to simulate subsoil carbon dynamics under future land-use and climate scenarios.