A Synthesis of the Effects of Density Regulation and Mixed-Tree Transformation on Soil Organic Carbon Dynamics in Chinese Fir Plantations

Chinese fir (Cunninghamia lanceolata) is one of the most important fast-growing timber species in southern China and plays a critical role in regional carbon sequestration and timber production. Soil organic carbon (SOC) is a key component of the terrestrial ecosystem carbon pool, and its content, composition, and stability directly affect soil fertility, ecosystem service functions, and the ability to cope with climate change. This review summarizes the mechanisms by which density regulation and conifer–broadleaf mixed forest management affect the content, fractions and stability of SOC in Chinese fir plantations. Density regulation changes stand structure, litterfall, and roots, which can impact soil microbial activity, litter decomposition, and mineralization of soil organic matter. Conifer–broadleaf mixed planting and broader mixed-forest reconstruction, through introducing functionally distinct tree species, can optimize stand microenvironments, increase species diversity, improve litter quantity and quality, and diversify root exudates. These changes further regulate soil organic carbon (SOC) accumulation and its physicochemical stability. Based on the latest literature reports, we demonstrate that mixed-species stands with a moderate broadleaf proportion significantly enhance SOC sequestration relative to pure stands, driven by improved litter quality and soil pH neutralization that promote microbial necromass formation and aggregate-associated carbon stabilization. Optimal density regulation complements these benefits by facilitating understory development and root carbon input. Current research indicates that both density reduction and species mixing, as two independent silvicultural measures, can individually enhance soil organic carbon (SOC) stability in Chinese fir plantations. This review identifies key research gaps and provides theoretical foundations for carbon-oriented sustainable management of Chinese fir plantations.