From plants to minerals: depth-dependent controls on microbial carbon use efficiency across the global

Microbial carbon use efficiency (CUE) represents a key trait linking microbial metabolism to soil carbon (C) cycling. While subsoils store over 50% of total soil C and are supposed to be more vulnerable to global change than topsoils, the patterns and controls of CUE in subsoils remain unclear, limiting predictions of whole-profile soil C dynamics. Here, we estimated CUE in topsoils (n = 814) and subsoils (n = 379) worldwide using an enzyme-based stoichiometric model and identified dominant drivers in each layer. We found that subsoil CUE was significantly higher than topsoil CUE, indicating a greater allocation of assimilated C to microbial biomass relative to respiration in deeper soils. Topsoil CUE was primarily influenced by vegetation-derived C inputs, whereas subsoil CUE was strongly constrained by mineral protection and soil physicochemical conditions, which suggests subsoil CUE may be less sensitive to global change than previously assumed. Global prediction revealed a poleward increase in CUE across layers, highlighting high soil C retention potential at high latitudes. This geographical pattern also implies that high-latitude soil C is vulnerable and may experience accelerated loss under ongoing climate warming.