Freeze–thaw cycle frequency affects root growth of alpine meadow through changing soil moisture and nutrients

Alpine meadows grow in alpine regions and play an important role in the production and life of alpine regions. As a unique feature of alpine regions, freeze–thaw cycles (FTCs) affect the growth of alpine meadows. However, with climate change, the change in the freeze–thaw cycle frequency (FTCF) has become obvious. These changes affect the content and distribution of soil moisture and nutrients, as well as the growth of roots in the alpine meadow. Therefore, based on the analysis of FTCF in the Nagqu River Basin, the characteristics of soil moisture, nutrients, and alpine meadow roots are analyzed, thus revealing the influence mechanism of FTCF on the root growth of alpine meadows. The results highlight three major findings. (1) Compared with the low-frequency mode (LFM), the moisture at 0–20, 20–40, and 40–60 cm in the high-frequency mode (HFM) has decreased by 30.74%, 52.89%, and 47.52%, respectively. Additionally, in HFM the contents of soil hydrolysable nitrogen (HN), available K (AK), and microbial biomass carbon (MBC) at the same depth are lower than those in LFM. (2) The original distribution of soil moisture at 0–60 cm has gradually increased from the surface to the bottom. However, with the increase in FTCF, the distribution of the soil moisture now means that the soil moisture at the surface (0–20 cm) and the deeper layers (40–60 cm) is higher than that in the middle (20–40 cm). (3) With the increase in FTCF, the growth mode of alpine meadow roots has changed from vertical extension to horizontal divergence; the distribution range of roots has changed from 0–40 cm to 0–20 cm; the length, surface area, and volume of 0–0.5 mm roots have increased by 20.95 cm, 1.90 cm(2), and 0.014 cm(3); and the corresponding specific gravity has increased by 9.09%, 13.50%, and 12.14%, respectively. This study provides a theoretical basis for predicting the growth mode of alpine meadow roots in the Nagqu River Basin under the influence of climate change and provides guidance for protecting the ecology of alpine regions and mitigating and solving global climate change.