Advancing the spring rest-grazing time until the critical period when soil thaws promotes soil recovery and bacterial diversity in alpine meadows

The management strategy of the Qinghai–Tibet Plateau (QTP) alpine meadows is “conservation + use”. Scientific management of grazing and sustainable use of alpine meadows is essential for maintaining the ecological health of the region. An optimal rest-grazing period in spring can protect the pasture and prevent trampling during soil freeze-thawing and plant re-greening stages. The treatment methods investigated in this study were: 1) rest-grazing when soil surface began to thaw until the grass withered (R1); 2) rest-grazing when soil thawing depth was >10 cm until the grass withered (R2); 3) rest-grazing when plant upper ground re-greening (coverage reached 30–40%) until the grass withered (R3); 4) rest-grazing when re-green plant coverage reached 80% until the grass withered (R4); and 5) rest-grazing when dominant plant height was approximately 5 cm until the grass withered (R5, traditional rest-grazing, CK). The soil physicochemical properties, enzyme activities, and microbial diversity of each treatment were determined. The results demonstrated that advancing the time of rest-grazing increased the soil water content (SWC) and decreased the soil temperature (ST). There was no significant change in the total soil nutrients between the treatments. However, there were significant differences in the available nutrients and enzyme activities (p < 0.05). Compared with R5, R1 significantly increased SWC by 15.90–56.22%, NO3––N content increased by 7.91%–7.94%, available phosphorus (AP) content increased by 5.3–15.0%, and soil organic matter (SOM) content increased 1.40 –7.72%. R1 and R2 had higher soil β-glucosidase (BG) activity and alkaline phosphatase (ALP) activity, while R3 and R4 had higher sucrose (SUC) activity. R3, R4, and R5 increased soil bacterial metabolism and energy conversion; in particular, it promoted the carbon cycle, resulting in a significant increase in SUC activity (R4 > R3 > R5). The abundance and diversity of soil bacteria showed that R1 was significantly higher than R2, R3, and R4, but not significantly different from R5, while R5 enhanced soil spatial heterogeneity. Thus, the optimal period for grazing in alpine meadows on the QTP is before soil thawing. This treatment could increase the utilization of available nutrients, SOM content, and soil bacterial diversity.