Soil Acidification Decouples Microbial Biomass Production From Necromass Accumulation in Apple Orchards via Enhanced Mineral Association

ABSTRACT Extensive chemical fertilization has caused severe soil acidification in China, particularly in orchard ecosystems. Microbial necromass carbon (MNC) plays a pivotal role in soil organic carbon (SOC) accumulation, yet how it responds to acidification and contributes to SOC remains poorly understood. Using a space‐for‐time substitution approach across apple orchards on the Jiaodong Peninsula, we revealed a fundamental decoupling between microbial biomass production and necromass accumulation driven by acidification. Specifically, acidification suppressed microbial biomass but enhanced MNC accumulation, a paradox primarily explained by a shift in carbon stabilization mechanisms: clay‐mineral protection became the dominant process, as directly evidenced by a significant increase in mineral‐associated organic carbon (MAOC). This decoupling undoubtedly enhanced the necromass accumulation coefficient (NAC), which was driven by microbial adaptations including higher enzyme activities (β‐xylosidase and acid phosphatase) and a community shift toward fungi. However, the increase in stabilized carbon (MAOC and MNC) could not compensate for substantial losses of labile carbon (e.g., particulate organic carbon), leading to an overall decline in SOC. As a result, the contribution of MNC to SOC increased significantly, a process regulated by soil pH both directly and indirectly through properties such as clay content and the C/N ratio. The concomitant decrease in the SOC:clay ratio indicated reduced mineral saturation under acidification, further highlighting the critical role of mineral protection in sequestering microbial‐derived carbon. Our findings demonstrate that acidification reconfigures the soil carbon cycle by strengthening mineral‐mediated stabilization pathways, thereby enhancing SOC stability despite a net reduction in total carbon stocks.