Gap-Size-Dependent Responses of Soil Organic Carbon Fractions in Natural Pinus koraiensis Forests of Northeast China

Abstract Purpose As a key forest management tool in near-natural forestry, canopy gaps offer significant potential to restore low-productivity forests. However, the effects of canopy gap size on soil organic carbon (SOC) and its physical fractions remain poorly understood. In this study, we investigated the responses of SOC and its physical fractions to canopy gaps of three sizes in natural Korean pine forests of northeastern China. Methods Soil samples were collected along a gradient extending from closed-canopy stands to gap edges, expanded gap zones, and gap centers. Soil organic carbon, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) were measured together with canopy closure, environmental variables and other soil conditions. Results The results showed that SOC per unit area decreased significantly in large and medium-sized gaps by 14.11% and 12.07%, respectively, relative to closed-canopy stands, whereas small gaps increased SOC by 26.49%. MAOC decreased from closed stands toward gap centers in large and medium-sized gaps but increased overall in small gaps. The relative contribution of MAOC to SOC increased with gap size. RDA and SEM analyses further revealed that canopy closure indirectly regulated SOC fractions through its effects on soil temperature, electrical conductivity, and pH. Conclusions Overall, this study elucidates the mechanisms by which gap formation enhances soil organic carbon stability and demonstrates the potential of small gaps to increase soil carbon sequestration. Therefore, integrating small-canopy-gap management into sustainable forest management practices may provide an effective approach to simultaneously enhance soil organic carbon stability and forest carbon sequestration.