Contribution of Glomalin-Related Soil Protein to Soil Organic Carbon Following Grassland Degradation and Restoration: A Case from Alpine Meadow of Qinghai–Tibet Plateau

Glomalin-related soil proteins (GRSP) are an important microbial carbon source for soil organic carbon (SOC) and can also protect SOC by promoting the formation of soil aggregates. However, there is a lack of systematic research on how the contribution of GRSP to SOC changes during grassland degradation and restoration. This study analyzed the changes in SOC, total glomalin-related soil protein (GRSPt), easily extractable glomalin-related soil protein (GRSPe) contents, and the ratios of GRSPe/SOC and GRSPt/SOC at different aggregate fractions in the 0–10 cm and 10–20 cm soil layers during the process of grassland degradation and restoration (from natural Sogong grass patches→degraded bare soil patches→transitional weed patches→naturally restored Sogong grass patches/artificially restored grass patches), to explore the contribution of GRSP to SOC at the aggregate scale during grassland succession. (1) With grassland degradation, the mean weight diameter (MWD) and the contents of SOC and GRSP in all aggregate fractions significantly decreased ( p < 0.05); the natural restoration method was more effective in improving MWD than the artificial restoration method; for the SOC content in large aggregates and the GRSPt and GRSPe contents in different aggregate fractions, the artificial restoration method was more effective than the natural restoration method. (2) The contents of GRSPe and GRSPt in all aggregate fractions were significantly and linearly positively correlated with SOC content ( p < 0.01). Moreover, during grassland degradation and restoration, the correlation between GRSPt and SOC in large aggregates first increased and then decreased. Notably, the correlation between GRSP and SOC in all aggregate fractions was significantly higher under the natural restoration method compared to the artificial restoration method. (3) During grassland degradation and restoration, the contents of GRSPe and GRSPt in the aggregate fractions of the 0~10 cm soil layer showed a clear decrease and increase, respectively. The change patterns of GRSPe/SOC and GRSPt/SOC were opposite to each other. Redundancy analysis revealed that total nitrogen (TN) was the factor that explained the highest variance in GRSP content, SOC content, and the GRSPe/SOC ratio across the aggregate fractions, while total phosphorus (TP) was the factor with the strongest explanatory power for the GRSPt/SOC ratio. This study found that the process of grassland degradation and restoration significantly altered the MWD, GRSP content in different aggregate fractions, SOC content, and the contribution of GRSP to SOC, with the contribution of GRSP to SOC showing an opposite trend to the change in GRSP content. Moreover, TN and TP were the main factors influencing GRSP changes. This study provides a scientific basis for assessing the carbon sequestration potential and selecting restoration methods for degraded grasslands.