Natural seasonal freeze-thaw processes influenced soil quality in alpine grasslands: Insights from soil functions

Effects of freeze-thaw cycles on nutrient cycling and microbial activity have been well documented in laboratory simulations; however, field evidence remains insufficient, and studies regarding their effects on soil quality index (SQI)—as evaluated by soil functions that are influenced by multiple soil properties—are scarce. Therefore, we conducted spatiotemporal paired soil profile surveys along a freeze-thaw intensity gradient covering six grassland types. Results are as follows: 1) After a seasonal freeze-thaw event, soil properties across the 0–80 cm profile changed by 0.96%–31.02% (physical), −34.29%–44.04% (chemical), and −70.46%–272.97% (biological), with change rates varying across soil layers. 2) A function-based framework was employed to assess SQI_0–30 under freeze-thaw conditions, and the reliability of the function indices and SQI_0–30 was validated. 3) Compared to pre-freezing levels, post-thawing water retention and regulation index changed negligibly (+5.31%), carbon sequestration index remained stable (+2.52%), and the primary productivity index declined noticeably (−9.43%). Conversely, the nutrient supply and cycling index increased notably (+23.89%) due to elevated total potassium, catalase activity, and urease activity. The biodiversity provision index improved substantially (+95.63%) owing to increased dissolved organic carbon. Collectively, the SQI_0–30 increased evidently by 11.78%. 4) These alterations were associated with different freeze-thaw indicators, and the daily freeze-thaw temperature difference at 0–10 cm during the “freezing→frozen→thawing” period explained 55% of the SQI_0–30 change, surpassing impacts of meteorological factors (precipitation, air temperature, and snow depth). Our study suggests that natural seasonal freeze-thaw events can raise alpine grassland soil quality, with varied functional responses. The identified soil indicators and functions sensitive to freeze-thaw cycles facilitate the research on seasonal dynamics of alpine grassland soil and its multi-objective management, and the quantitative relationships with freeze-thaw indicators provide new insights for regional soil mapping in frozen areas under climate change.