Introduction
”Desertification” refers to land degradation caused by climate change and human activities in arid, semi-arid and semi-humid arid regions (Liu, 1998), which is a global environmental problem that plagues human development, and one of the current ecological research hotspots (Gao et al., 2019). Relevant studies have shown that the global arid area is 6.1×108 hm2, among which China has 2.97×108 hm2, and the arid areas have different degrees of desertification. From 1984 to 1991, the global land desertification accelerated at an annual rate of 3.4% (Office of Desertification Prevention and Control, Ministry of Forestry of the People’s Republic of China, 1994). With global warming, the disorderly development of resources and the urbanization process, desertification may become a major ecological problem and challenge in the 21st century (Li et al., 2019; Rong, 2019). Therefore, understanding the rules of desertification, that is, the change characteristics and mutual relationship of vegetation and soil, has important ecological significance for slowing down the desertification process and vegetation restoration and reconstruction in desertified areas.
Qilian Mountain is located on the eastern edge of the Qinghai-Tibet Plateau in China, adjacent to the Mongolian Plateau and the Loess Plateau (Gao et al., 2019). The Qilian mountains is the intersection of the Qinghai-Tibet Plateau, the Loess Plateau and the Inner Mongolia Plateau, with a fragile ecological environment, and is also one of the most sensitive regions for global climate change (Wang et al., 2001). Grassland is the largest vegetation type in Qilian Mountains (Li et al., 2019). Therefore, the health of the grassland ecosystem in the Qilian Mountains is not only related to the water source security in the Hexi region, but also related to the ecological security and social stability in northwest China (Yao et al., 2018). Related studies have found that the climate change in the eastern part of Qilian is warm and humid, while the central and western regions is warm and dry (Rong, 2019). Therefore, climate change poses a serious challenge to the ecology of the Qilian Mountains. The warming and humidification of the climate in the eastern region had caused the degradation of alpine meadows and the shift of the snow line, while the warming and drying of the climate in the central and western regions led to the succession of temperate steppe to temperate desert steppe and temperate desert (Rong, 2019). Succession of grassland desertification will lead to a series of grassland ecological changes, such as changes in vegetation community structure, soil nutrients, microbial composition, etc. (Rong, 2019). Tang et al. (2016) found that the effects of desertification on total phosphorus and available phosphorus were not significant, and desertification decreased productivity in the desert steppe as a result of direct changes to soil physical properties, which can directly affect soil chemical properties. Li et al. (2006) found that soil organic matter is also reduced with desert development, which leads to destruction of the stability of soil physical structure and nutrient content, such as progressive N, P and K loss in surface and subsoil layers, and in response to changes in soil properties, vegetation altered as regards species composition, species diversity, coverage, structure and life-form (Tang et al., 2016). Therefore, a systematic understanding of the vegetation and soil change characteristics in the process of grassland desertification is of great significance to the restoration of desert grassland.
At present, most of the research on grassland desertification focused on the degradation of desert grassland. According to the degradation standard, the desert grassland is divided into several degradation gradients to study the relationship between vegetation community structure and soil nutrients (Rong, 2019; Li et al., 2006; Tang et al., 2016). Because grassland desertification is a process of long-term succession of grassland under warm and dry climate, this study used the method of replacing time with space, and selects three types grassland (temperate steppe, temperate steppe desert and temperate desert) as research objects replacing desertification succession. The characteristics changes and relationship of vegetation and soil factors were analyzed during the succession of temperate steppe to temperate desert, which could provide theoretical basis and data support for curbing grassland desertification and inducing forward succession of desert grassland.