Repeated litter inputs promoted stable soil organic carbon formation by increasing fungal dominance and carbon use efficiency

Plant carbon (C) input via litter decomposition is essential for forming soil organic C (SOC). Previous studies have investigated the fate of litter C through one-time litter addition, while litter input could be recurrent, e.g., gradual litter input from above-ground plant material in diverse ecosystems. This paper investigates how litter input’s frequency (single vs. repeated) and amount (low vs. high) affect the SOC formation and the associated microbial community composition and activity in a controlled microcosm experiment. After incubating the 13C-labeled litter (Stipa krilovii) for 80 days with the fresh field soil collected from a semi-arid steppe in Inner Mongolia, China, we measured newly formed SOC, microbial biomass C (MBC), and microbial phospholipid fatty acid (PLFA) indicated by 13C-labeling. The repeated litter addition resulted in 19.92% greater and 12.45% more efficient SOC formation than the single addition. Repeated litter addition also increased C use efficiency (CUE, + 76.71%), indicating more substrate C allocated for microbial growth than respiration. It also enhanced the fungi to bacteria ratio by 47.40% (based on 13C-PLFA) and maintained relatively stable microbial assimilation of litter C (based on 13C-MBC). These findings were consistent regardless of litter amounts. Our results indicate that the uninterrupted litter input to the soil in natural ecosystems benefits SOC formation and highlights soil fungi’s critical role in promoting SOC formation. This study suggests that incorporating plant C input frequency into SOC dynamic models may improve the predictions of C cycling.