Soil amendments reshape alpine meadow microbial communities across degradation gradients by modulating nutrient availability

Climate change and overgrazing have led to the continued degradation of alpine meadows on the Qinghai–Tibet Plateau, thereby severely impairing the ecosystem services provided by these grasslands. Soil amendments are an important strategy for restoring degraded grasslands; however, the regulatory effects of different amendment measures across varying degradation gradients and the underlying microbial response mechanisms remain unclear. This study systematically investigated the effects of three soil amendments (humic acid, superabsorbent polymer, and Bacillus subtilis ) on soil physicochemical properties and microbial community structure in mildly, moderately, and severely degraded alpine meadows. The results indicated that the regulatory effects of soil amendments are significantly dependent on the degradation gradient: In the severe degradation stage, amendments significantly increased available nutrients, lowered pH, and alleviated alkalization, whereas their regulatory effects were limited in the mild degradation stage. Amendments selectively altered the abundance of microbial groups involved in organic matter degradation, nitrogen cycling, and stress-tolerant symbiosis. Humic acid demonstrated the optimal regulatory effect in the mild degradation stage, whereas superabsorbent polymers and Bacillus subtilis were more suitable for restoration in the moderate and severe degradation stages. Redundancy analysis indicated that bacterial communities were primarily subject to deterministic selection by soil pH, available potassium, and organic matter gradients, with the intensity of environmental selection increasing with increasing degradation severity; fungal communities exhibited significant environmental responses only in the moderate degradation stage, while stochastic processes dominated community assembly in the mild and severe degradation stages. In summary, alpine meadows at various stages of degradation require differentiated remediation strategies. Precise, stage-specific remediation tailored to degradation severity can facilitate synergistic restoration of the soil–microbial system, providing a theoretical basis and practical guidance for restoring degraded alpine meadows on the Qinghai–Tibet Plateau.