Afforestation Enhances Soil Mineral‐Associated Organic Carbon Accumulation Through Calcium‐Mediated Microbial Pathways in Karst Ecosystems

Mineral‐associated organic carbon (MAOC), protected from rapid microbial decomposition through mineral‐organic associations, is vital for the long‐term stability of soil carbon pools. Afforestation is a critical strategy for enhancing soil carbon storage; however, the mechanisms governing MAOC accumulation across divergent climatic conditions remain unclear, particularly for karst regions, which are ecologically fragile and characterized by high‐calcium (Ca) soils. Here we compared MAOC content and its key drivers between croplands and 20‐year‐old plantations across a broad climatic gradient in the karst region of southwest China. We found that afforestation significantly increased MAOC content in warmer regions (> 18°C) compared with cooler areas (< 15.6°C). Across the entire region, afforestation increased MAOC content by 64% compared to cropland soils. Soil exchangeable Ca, microbial biomass carbon (MBC), and microbial necromass carbon (MNC) were identified as the primary factors influencing MAOC accumulation. Afforestation promoted MAOC accumulation through increasing soil exchangeable Ca, microbial biomass, and microbial necromass. Specifically, two key pathways were identified: (1) afforestation increased soil exchangeable Ca content, which subsequently promoted MBC and MNC contents, thereby driving MAOC accumulation and (2) afforestation directly increased MNC content, facilitating MAOC accumulation. Furthermore, under afforestation, higher temperature and precipitation enhanced soil exchangeable Ca content, thereby promoting MAOC accumulation in warmer and wetter regions. These findings elucidate how afforestation strengthens Ca‐mediated interactions between microbial biomass and necromass to enhance MAOC accumulation in karst ecosystems. Under warmer climates, afforestation in karst regions offers an effective approach to bolstering soil carbon stability, providing a mechanistic framework for optimizing restoration strategies in climatically diverse and vulnerable landscapes.