3.3 Soil enzymes characteristics
Soil enzymes decomposed complex organic compounds into smaller organic compounds and inorganic nutrients (Grandy et al., 2007). As the most active and sensitive components in soil, soil enzymes played an important role in the nutrient cycle of soil and the supply of nutrients needed for plant growth, and were an important index for evaluating soil quality of different grassland types (Verónica et al., 2007). Its activity and mainly affected by soil physical and chemical properties, soil microorganisms and vegetation litters (Cui et al., 2019). In ours research, with the increase of soil depth, urease, alkaline phosphatase and sucrase enzyme activity decreased and catalase enzyme activity increased. From TS, TSD to TD, the alkaline phosphatase, catalase and sucrase enzyme activity were decreased. Because the soil surface layer accumulates more litter and humus than the depth soil layer, the organic matter content was high and the nutrient source was sufficient, which was conducive to the growth and reproduction of microorganisms. meanwhile water and heat in topsoil better and ventilation conditions, plant hair root and litter turnover rapid, which further accelerated the microbial growth and metabolism, making the soil Urease, Alkaline phosphatase and Sucrase enzyme activity decreased with the increase of soil depth, but the Catalase enzyme activity increased.
3.4 The evolution mechanism of grassland desertification succession
This research used space instead of time to analyze the evolution of grassland vegetation and soil in the process of grassland desertification, in the western Qilian Mountains- eastern Qinghai-Tibet Plateau during the global warming process. Based on the steppe spatial distribution pattern, the evolution from temperate steppe to temperate desert, rainfall decreased and temperature raised, that was, climate change was ”warming and drying” (Rong, 2019). The complex of altitude as a regional hydrothermal factor also showed a decreasing trend (from temperate steppe to temperate desert was from 2800m to 1500m) (Xu et al., 2021). In the grassland ecosystem, above-ground vegetation was a sensitive indicator of climate change (Zhou et al., 2020), and prioritized the perception of climate change and adjusted its growth characteristics to adapt to environmental changes. For example, in the process of grassland desertification, grassland vegetation coverage, biomass and diversity index decreases, and the height of the grass layer increased (Fig.2), and plant species evolved from herbaceous plants to lignified shrubs to reduce water evaporation and adapt to rising temperatures, which was a survival strategy for plants (Zhou et al., 2020). When vegetation changed, soil, as a substrate and nutrient donor for plants, would also change, such as physical structure (increased bulk density, decreased porosity), decreased chemical nutrients (carbon, nitrogen, and phosphorus), and decreased soil enzyme activities (Fig2) (Wang et al., 2018). Meanwhile, changes in soil physical structure, nutrient content and enzyme activity will adversely affect plants and affect their growth. The ”coordination” and ”antagonism” of grassland plants and soil promote the flow of material and energy in the grassland ecosystem to adapt to changes in the environment (He et al., 2021). The results could provide theoretical basis and data support for curbing grassland desertification and inducing forward succession of desert grassland.