Temporal effects of grassland enclosure on root growth and glomalin-mediated carbon sequestration

Enclosure is one of the key measures for restoring degraded grasslands. The enhancement of carbon sequestration after grassland restoration is directly related to the improvement of regional ecological quality and the achievement of carbon neutrality goals. We conducted an in-situ root growth experiment lasting for four years (6, 12, 24, and 48 months) using the internal growth core method across natural grassland and grasslands with different closure durations (5, 10, 25, and 30 years) in Yunwushan National Nature Reserve. We explored the duration effect on root growth and globulin-related soil protein (GRSP) carbon sequestration in enclosed grasslands. The results showed that in plots with different closure durations, root biomass in growth bags significantly increased with the duration of closure, reaching a peak (0.36 g per bag) in the 25-year closure plot, which was a 125.0% increase compared to that in the natural grassland. The GRSP content increased simultaneously with the root biomass, reaching a maximum value (3.16 g·kg^-1 ) in the 25-year closure plot. Partial least squares path model analysis revealed that root biomass and GRSP drove soil organic carbon sequestration by promoting the accumulation of particulate organic carbon (path coefficients of 0.18 and 0.33, respectively), which in turn affected soil organic carbon sequestration (path coefficient=0.47). Principal component (PC) analysis indicated that the PC1 score was highest in the 25-year closure plot, suggesting that the accumulation of reactive carbon pool reached its optimal level. At this stage, the soil organic carbon and mineral-bound organic carbon conten were also the highest, confirming that 25 years of closure is a key milestone for carbon pool accumulation. We recommended that grasslands in this region be moderately utilized after 25 years of closure to accelerate nutrient cycling and further enhance the carbon sink potential of the grasslands.