Cover crop presence and soil depth shape microbial CUE and soil organic matter priming

Diverse cover crops (CCs) have been proposed as an agronomic strategy to enhance carbon sequestration in deeper soil layers. However, it remains unclear how key, counteracting microbial-mediated processes such as carbon use efficiency (CUE) and priming of old soil organic matter (SOC), which promote and reduce SOC stocks, respectively, respond to increasing CC diversity. We investigated the effects of CC richness on microbial CUE and its relationship with priming effects (PE) at two soil depths (0–10 cm and 20–30 cm). Vegetation effects on CUE were strongly depth-dependent, with lower CUE in topsoil than in subsoil. In contrast, increasing CC richness had no detectable effect on CUE at either depth. Priming effects showed a similar depth dependency: vegetated plots exhibited stronger priming in subsoil than in topsoil, but priming was not related to CC richness. Overall, our results reveal pronounced depth-dependent interactions between cover crops and microbial processes governing soil carbon cycling. They suggest that the presence of cover crops, rather than optimizing species richness, is the dominant factor regulating microbial CUE and priming. Because both processes are simultaneously enhanced in subsoils, further research is needed to resolve their net effect on SOC persistence and the potential for long-term carbon sequestration under cover cropping.