Long-term biocrust restoration enhances microbial carbon use efficiency but shifts soil organic carbon sequestration pathways.

Enhancing soil organic carbon (SOC) sequestration is a critical indicator of success in dryland landscape restoration. However, how microbial carbon (C) use efficiency (CUE) regulates microbial necromass C (MNC) formation and its contribution to SOC during long-term biocrust restoration remains unclear. We investigated a biocrust restoration chronosequence (0 (mobile dunes), 15, 25, 38, 44, 61, and 69 yr) in the Tengger Desert, China. CUE metrics (18O-H2O tracing) combined with amino sugar biomarkers were used to elucidate the time-dependent mechanisms of microbial-driven SOC stabilization. Our results showed that biocrust recovery increased SOC contents by 6- to 14-fold relative to mobile dunes. Concurrently, biocrust development ameliorated microhabitat limitations, triggering a 20- to 34-fold in microbial CUE over 15-69 yr, which in turn elevated MNC content by 8- to 10-fold. Crucially, the relative contribution of MNC to the SOC pool was substantial (26.5%-32.2%) during early restoration (15 yr), but this contribution declined to ∼18% in late stages (69 yr), likely owing to the diversification of SOC sources and the reduced net accumulation efficiency of MNC. Furthermore, the SOC pool was consistently dominated by bacterial rather than fungal necromass, highlighting the bacterial C pump as the primary engine for dryland soil C accrual. Overall, these findings indicate that the long-term recovery of biocrust involves an efficient yet capacity-limited microbial C sequestration mechanism, which provides important insights for macro-level dryland management such as parameterizing C models and evaluating ecological restoration outcomes.