Soil amendment incorporation increases organic carbon by improving soil agglomerate and soil microbial biomass carbon in the alpine grassland

Soil amendments aiming to enhance soil quality and bolster carbon sequestration have been extensively investigated. However, the specific impacts of diverse soil amendment types on soil total organic carbon content (TOC), soil aggregate and the growth of ryegrass remain largely unexplored, particularly within the unique context of alpine grassland soils in northwest Sichuan. For this, four soil amendments (CK: no soil amendment, CM: cattle manure 2000 kg ha−1, CS: straw amendment 12,000 kg ha−1 and MS: mushroom substrate 18,000 kg ha−1) were applied to alpine grassland soils over a 2‐year duration, conducted in situ during 2017 and 2018, to investigate the influences of these soil amendments on 0–30 cm soil of TOC, total nitrogen (TN), microbial biomass carbon (MBC), soil aggregation, the above‐ground biomass (DMA) and root traits of ryegrass. Compared to CK, the above‐ground biomass exhibited an average of 348.78% in MS, 287.18% in CS and 115.54% in CM, all reaching statistical significance (p < .05). In the topsoil (0–10 cm), the large soil aggregate rate (LSAR > 0.25 mm) showed a significant increase in CM, CS and MS, particularly in 2018, compared to CK. Our findings further indicated that the improvement in alpine grassland LSAR > 0.25 mm was correlated with a rise in TOC by over 69.89% and MBC by more than 27.14%. The MS treatment resulted in a significant increase in above‐ground biomass and TRL (total root length), while also increasing the levels of TN, MBC and soil aggregates (0.25 ~ 0.5 mm) within the 0–10 cm soil. A similar result of CS treatment was observed to increase the total chlorophyll content and RD (root diameter), as well as an increase in SWC and TOC levels. The TN, MBC, TOC and LSAR contributed 44.77%, 20.87%, 6.46% and 6.45% for ryegrass growth. The SEM indicated that soil amendments promote the growth of ryegrass by improving soil agglomerate and increasing MBC, TOC and TN. Our analysis revealed that ryegrass biomass production was limited by soil nutrients in the alpine grassland of northwest Sichuan. The study also highlights the potential impact of soil amendments on future management practices, contributing to a more comprehensive understanding of the subject.