Effectiveness of Long‐Term Enclosure on Restoration of Leymus chinensis Grassland in Inner Mongolia: Focus on Soil Carbon Storage and Fractions

ABSTRACT Grazing exclusion is widely applied to restore degraded grasslands. However, the effects of long‐term enclosure on vegetation and soil properties in semi‐arid grasslands remain poorly understood. We compared vegetation attributes, soil organic carbon (SOC) and its fractions, and other soil properties among grasslands enclosed for 41 years (UG41) and 21 years (UG21) and that under continuous grazing in a typical steppe in central Inner Mongolia, China. Compared with long‐term grazing, enclosure significantly increased plant above‐ and belowground biomass by 28%–80% ( p < 0.05), and enhanced SOC by 42%–62% ( p < 0.05). However, no further increase was observed either in biomass or SOC stocks under UG41 relative to UG21. Enclosure also reduced sand content but increased silt content in the top 20 cm soil layer, and increased free iron and aluminium oxide contents. Structural equation modelling indicated that enclosure‐enhanced belowground biomass was important in promoting particulate organic carbon (POC) content, which contributed substantially to SOC storage. Compared with grazing, both UG41 and UG21 increased POC and mineral‐associated organic carbon (MAOC) contents in the top 30 cm soil layer; and increase of MAOC was slower than POC from UG21 to UG41, resulting a lower MAOC/SOC ratio, that is, a lower SOC stability under UG41 than UG21. These findings indicate that short‐term enclosure has strong restoration effects by enhancing vegetation production and SOC stocks and stability, while extremely long‐term enclosure more than 21 years has limited capacity to further enhancement in semi‐arid steppe grasslands. This study provides insights into optimising enclosure duration for restoring degraded grasslands.