Coupling of soil organic carbon and inorganic carbon: A conceptual review

Soil constitutes one of the largest active carbon reservoirs at the Earth’s surface and contains two functionally distinct yet interconnected pools: soil organic carbon (SOC) and soil inorganic carbon (SIC). Although SOC-SIC interactions are increasingly recognized as important for soil carbon storage and climate feedbacks， the two pools have often been examined separately. This conceptual review synthesizes current understanding of SOC-SIC coupling from a process-oriented narrative perspective， focusing on their fundamental characteristics， spatial distribution patterns， coupling pathways， and environmental controls. SOC is commonly partitioned into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)， which differ in source， microbial accessibility， and persistence. POC is generally plant-derived， relatively labile， and readily decomposed， whereas MAOC is more strongly protected by mineral associations and usually has longer residence times. SIC， dominated by carbonate minerals， accumulates mainly in arid and semi-arid regions and is regulated by hydroclimatic conditions， soil pH， Ca/Mg chemistry， and acidification. Existing evidence indicates that SOC-SIC coupling is mediated by three interacting pathways: biogeochemical transformations linking SOC mineralization with carbonate dissolution and precipitation; physicochemical mediation involving carbonate equilibria， mineral protection， and divalent cations; and microbial regulation of organic carbon transformation and carbonate reactions. Calcium may serve as a key mediator by promoting cation bridging， organo-mineral associations， microbial colonization， and carbonate formation. These intrinsic pathways are further shaped by external and edaphic controls， including climate， land use， human activities， soil texture， and parent material， which regulate soil pH， moisture， microbial activity， mineral composition， and carbonate dissolution-precipitation dynamics. Future studies should integrate isotope tracing， spectroscopic techniques， long-term observations， and modeling to link microscale reactions with soil profile， ecosystem， and regional carbon dynamics. A clearer conceptual understanding of SOC-SIC coupling may improve soil carbon accounting and assessment of soil carbon responses to environmental change.