Litter C:N ratios drive priming effects via enzymes and microbial resource limitation in soils of differing fertility, thereby influencing soil carbon sequestration

Background and aims Soil organic carbon (SOC) turnover is strongly influenced by the priming effect (PE), yet the interactive roles of litter quality, soil fertility, and temperature remain unclear. The objective was to determine how responses of SOC mineralization to the carbon to nitrogen (C:N) ratio of litter in different soils and incubation temperature, and which mechanism controls the responses. Methods A 42-day incubation experiment was conducted using seven ¹³ C-labeled litters with contrasting C:N ratios, which were added to high- and low-fertility soils and incubated at 23°C and 33°C. CO₂ efflux, PE, extracellular enzyme activities, and microbial resource limitations were subsequently quantified. Results Litter decomposition proceeded faster in fertile soils. High C:N ratio litter decomposed more rapidly than low C:N ratio litter, reflecting differences in their chemical composition and origin. The direction of PE shifted with litter C:N ratio: low C:N litter induced a positive PE, whereas high C:N litter induced a negative PE. Temperature exerted only minor effects; however, PE was strongly associated with extracellular enzyme activities and microbial C limitation. Soil fertility amplified both litter decomposition and PE, underscoring its pivotal role in regulating SOC dynamics. Litter quality and soil fertility emerged as the dominant regulators of PE, with extracellular enzyme activity mediating the microbial response. Conclusions These findings underscore the importance of microbial traits, enzyme activity, and litter stoichiometry in regulating SOC turnover and provide insights into soil C management under varying environmental conditions.