Afforestation increases glomalin-related soil protein content and enhances its association with stable soil carbon in karst regions

Afforestation is widely regarded as a practical approach to enhance soil organic carbon (SOC) storage. Glomalin-related soil protein (GRSP), primarily produced by arbuscular mycorrhizal fungi, is vital for promoting SOC storage. However, the effects of afforestation on GRSP accumulation in bulk soil and the mineral-associated organic matter (MAOM) fraction, along with its relationship with mineral-associated organic carbon (MAOC), remains poorly understood, particularly in karst soils characterized by high mineral content. This study investigated these dynamics across six sites in the karst region of southwestern China, representing a broad climatic gradient. We collected 30 soil samples (0–10 cm) each from croplands and plantations along the gradient, analyzing SOC and GRSP content in both bulk soils and MAOM fractions. We found that afforestation significantly increased SOC by 58.1 % in bulk soils and 57.8 % in MAOM fractions, while MAOC rose by 63.2 %. The total GRSP content increased by 9.5 % in bulk soils and 10.8 % in MAOM fractions, compared to croplands, associated with higher soil nitrogen content and microbial biomass carbon after afforestation. However, the contribution of GRSP to SOC decreased after afforestation, likely due to increased plant-derived carbon inputs. Notably, GRSP contributed more to soil carbon in the MAOM fractions than in bulk soils across both croplands and plantations. Additionally, GRSP content showed a positively correlation with MAOC and SOC, suggesting a strong association with soil carbon stability. Furthermore, higher mean annual precipitation promoted GRSP accumulation, primary by increasing soil nutrient content. These findings highlight that while afforestation reduces the contribution of GRSP to SOC in karst regions, it enhances overall GRSP content, thereby increasing SOC stability. Furthermore, areas with higher precipitation offer greater potential for SOC stabilization through afforestation, though increased dissolved organic carbon leaching may temper this effect, providing valuable insights for carbon sequestration strategies in karst ecosystems.