3.2 Soil nutrient and stoichiometric characteristics
Soil degradation was one of the important characteristics of desertification. With the development of desertification, the degradation of soil physical and chemical factors had gradually increased (Wang et al., 2018). Related studies have found that, with the succession and development of desertification, and the content of soil organic matter and total N decrease significantly (Meng et al., 2018). In this study, from TS, TSD to TD, the total porosity, total N and soil organic matter were significant decreased, and soil bulk density and pH were significant increased. But the total P, total K and available P were in order of TSD < TD < TS. Due to the long-term warming and drying of the climate, the succession of temperate steppe to Temperate Steppe Desert has been induced. The vegetation type had evolved from herbaceous to semi-shrub (Wu et al., 2017). The decrease in vegetation litter amount and the decrease in decomposition rate (decomposing bacteria activity decreased) were co-leading of the reduced soil carbon, nitrogen, and phosphorus nutrients, which turned the changes of soil pH, porosity and bulk density. At the same time, soil changes had a negative effect on vegetation, which was caused the heterogeneity absorption of trace elements by vegetation (Tao et al., 2019). Soil ecological stoichiometric characteristics (C/N, C/P and N/P) have a strong regulatory effect on the carbon fixation process in terrestrial ecosystems (Zhang et al., 2016), which is an important parameter to measure soil quality (Liu et al., 2021), reflecting the ability of soil to release nitrogen and phosphate mineralized nutrients. The optimal C/N required for microbial life activities was 25 (Tessier and Raynal, 2019), which is the most favorable ratio for the conversion of organic carbon. In this study, from TS, TSD to TD, C/N rate was decreased, and C/N of TS, TSD to TD was 15.70, 10.13 and 5.80, respectively. That was, TS was good for microbial life activities, but TSD and TD was bad for microbial life activities. Soil C/P ratio was an important indicator of soil phosphorus mineralization capacity, which could measure the potential of soil organic matter mineralization to release phosphorus or absorb phosphorus fixation (Zhang et al., 2018). In this study, from TS, TSD to TD, C/N was decreased, and C/P of TS, TSD to TD was 63.17, 10.04 and 2.98, respectively. The N/P ratio was used as an index for determining the nutrient factors that limit productivity, and N/Pīš¤10 and N/P>20 were used as indicators to evaluate the productivity of vegetation limited by nitrogen or phosphorus(Li et al., 2018). In this study, from TS, TSD to TD, C/N was decreased, and N/P of TS, TSD to TD was 4.02, 1.00 and 0.53, respectively. That was, productivity was increasingly limited by soil nitrogen from TS, TSD to TD.