Microbial-mediated decline in mineral-associated organic carbon following rice terrace abandonment in subtropical China

Rice terraces (RT) play a crucial role in sustaining soil carbon sequestration and ecosystem services, yet face increasing abandonment. Particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) differ markedly in their formation and stability. A carbon fraction perspective helps reveal the impacts of terrace abandonment on soil organic carbon (SOC) and its microbial mechanisms, which remain poorly understood. This study investigated six typical RT and their paired abandoned terraces (mainly converted to upland or forest) in subtropical China. It evaluated the effects of RT abandonment on SOC and its underlying drivers. The results showed that RT abandonment significantly reduced SOC content and shifted its composition, primarily due to declines in MAOC, while POC remained unchanged. MAOC decline was primarily driven by reductions in microbial necromass carbon, particularly that derived from fungi. Moreover, in the 0-20 cm soil layer, variations in MAOC were primarily regulated by intensified microbial carbon limitation, along with declines in microbial biomass and carbon use efficiency. In the 40-60 cm soil layer, changes in MAOC were more strongly influenced by enhanced microbial co-limitation by carbon and phosphorus and an increased dominance of K-strategist microbial communities. These findings suggest that RT function as stabilizers of SOC, whereas abandonment weakens their capacity to maintain soil carbon stability. In particular, changes in MAOC are strongly regulated by microbial nutrient limitation and life-history strategies. Therefore, preventing terrace abandonment and implementing sustainable management strategies are critical for sustaining soil carbon stability.