Litter quality and nutrient availability regulated the direction and magnitude of priming effect of soil organic carbon

Abstract Priming effects (PE) play a pivotal role in regulating soil organic carbon (SOC) dynamics, 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. Here, we conducted a 42-d incubation experiment using seven ¹³C-labelled litters with contrasting C: N ratios added to soils with low and high fertility (LF and HF) under two temperature regimes (23 °C and 33 °C). CO₂ efflux, PE, extracellular enzyme activities, and microbial resource limitations were subsequently quantified. Litter decomposition was higher in HF soils and at 33 °C. Temperature had only a minor influence on PE. The direction of PE was mainly controlled by litter quality: high-quality litter (C: N ratio < 25) led to positive PE, whereas low-quality litter (C: N ratio > 25) led to negative PE. The magnitude of PE was generally greater in HF than in LF. These differences were closely related to microbial resource limitation. In HF, lower nutrient limitation favored preferential use of added substrates and co-metabolism, resulting in an “apparent” PE with relatively small enzyme responses and limited soil-derived CO₂ release. In LF, stronger nutrient limitation promoted enzyme production and nutrient mining, leading to a “real” PE that was more closely tied to litter quality and temperature. Despite the occurrence of PE, litter addition led to a net positive C balance over the 42-d incubation. These findings demonstrate that the direction and magnitude of PE are jointly regulated by litter quality, soil fertility, and temperature through distinct microbial mechanisms, with implications for soil C management under varying environmental conditions. Moreover, our findings highlight that interactions between litter quality and soil fertility are closely associated with coordinated changes in soil enzyme activities and microbial resource limitation, thereby influencing short-term soil carbon balance and providing an integrated perspective for understanding soil carbon cycling under changing environmental conditions.