Effects of the Freezing-Thawing Cycle Mode on Alpine Vegetation in the Nagqu River Basin of the Qinghai-Tibet Plateau

The freezing-thawing cycle is a basic feature of a frozen soil ecosystem, and it affects the growth of alpine vegetation both directly and indirectly. As the climate changes, the freezing-thawing mode, along with its impact on frozen soil ecosystems, also changes. In this research, the freezing-thawing cycle of the Nagqu River Basin in the Qinghai-Tibet Plateau was studied. Vegetation growth characteristics and microbial abundance were analyzed under different freezing-thawing modes. The direct and indirect effects of the freezing-thawing cycle mode on alpine vegetation in the Nagqu River Basin are presented, and the changing trends and hazards of the freezing-thawing cycle mode due to climate change are discussed. The results highlight two major findings. First, the freezing-thawing cycle in the Nagqu River Basin has a high-frequency mode (HFM) and a low-frequency mode (LFM). With the influence of climate change, the LFM is gradually shifting to the HFM. Second, the alpine vegetation biomass in the HFM is lower than that in the LFM. Frequent freezing-thawing cycles reduce root cell activity and can even lead to root cell death. On the other hand, frequent freezing-thawing cycles increase microbial (Bradyrhizobium, Mesorhizobium, and Pseudomonas) death, weaken symbiotic nitrogen fixation and the disease resistance of vegetation, accelerate soil nutrient loss, reduce the soil water holding capacity and soil moisture, and hinder root growth. This study provides a complete response mechanism of alpine vegetation to the freezing-thawing cycle frequency while providing a theoretical basis for studying the change direction and impact on the frozen soil ecosystem due to climate change.