4.1 Effects of grazing on community characteristics
Grazing is a continuous and highly complex disturbance that can change the characteristics of grassland plant communities, such as the biomass and species diversity. In this study, community biomass and litter decreased as the grazing intensity increased, which was consistent with the results of Sun et al. (2013). The coverage of plant communities fluctuated greatly by year, especially in 2018 (wet year). The coverage under the heavy grazing treatment was 156% higher than that under the no grazing treatment, but the biomass under the heavy grazing treatment was 45% lower than that under the no grazing treatment. This result seems to be contradictory and may be due to the following reasons. 1) Precipitation can explain 55-86% of the variation in a plant community (Bai et al., 2012). During the analysis of the precipitation data, a large amount of continuous precipitation before the sampling period was noted. Continuous precipitation can lead to a surge in plant growth with relatively little interspecific competition under heavy grazing conditions, which is referred to as the "opportunism" strategy. During the process of observing the community coverage in the field, the coverage of these "opportunistic" plants was also measured, and the results showed an increased coverage in the heavily grazed communities. The individual moisture contents of these "opportunistic" plants increased; however, the biomass that accumulated in the community under heavy grazing was still lower than that under no grazing. 2) Most of the dominant species in the desert steppe are perennial clumped grasses. Although most of these species were consumed by the high-intensity livestock, a small number of leaves remained. Vegetation coverage is the proportion of the vertical projection area of vegetation to the surface area. During the observation of community coverage, although these perennial grasses contained very short leaves (very small biomass), their coverage accounted for a large proportion of the community coverage.
Westoby (1998) assumed that community succession requires special rainfall events, such as rare heavy rains, to drive community structure changes. This theory explains our research results very well because there was no significant difference in the community structure between the grazing and the no grazing treatments in 2016 and 2017, but the no grazing treatment had a significantly different structure than the heavy grazing treatment in 2018. Grazing makes the grassland appear to be low competition, increases the plant sites for plant seeds, improves the reproduction and distribution ranges of grazing-tolerant species and maintains species richness and livestock feeding in a delicate temporary steady state (Liu et al., 2017). In addition, the desert steppe is dry and rainless, the community structure is simple and the number of species is small. Thus, there was no significant difference in species richness among the grazing treatments. However, due to the continuous large amount of precipitation during the peak plant growth period in 2018, the "opportunistic" plants under the grazing treatments grew in large quantities and destroyed the relative balance between the other grassland species and livestock. The competitive disadvantage of the other grassland species coupled with the high-intensity feeding of livestock resulted in a decline in species richness under the grazing treatments, especially the heavy grazing treatment, in 2018.
4.2 Effects of grazing on plant and soil C, N and P pools
Previous studies have suggested that for the studied species, nutrients are less affected by the environment and are relatively stable. Grazing changes the species composition, thereby altering the community stoichiometry because species have different nutrient components (Ritchie et al., 1998; Bardgett and Wardle, 2003). This theory supports our research to a certain extent; for example, in 2016 and 2017, the plant C contents were the highest under the no grazing treatment, followed by the moderate grazing and heavy grazing treatments, while there were no significant differences in C contents in 2018. In addition, these results may be due to the characteristics of desert grassland plants and their adaptation to grazing disturbances. The most prominent climatic characteristics in arid areas are the scarcity of precipitation and the large inter-annual and inter-seasonal variations. In drought years, the leaf area and leaf width showed a decreasing trend. In contrast, the leaf area and leaf length showed a preferential growth characteristic in wet years (Balota et al., 2008; Picotte et al., 2009). Most of the plants in the desert steppe belong to radical leaf- and stem-free supporting structures. Therefore, there was no significant difference in the leaf C content under the abundant precipitation in 2018. In 2016 and 2017, due to intensive heavy grazing, plants usually avoided grazing by reducing their height, which changed their C assimilation and accumulation abilities.
In addition, the results showed that plants under the heavy grazing treatment had higher N and P contents in plant leaves than those under the no grazing and moderate grazing treatments. We propose the following potential mechanisms to explain these phenomena. Plants induce livestock to feed on plant tissues, organs or populations with low P and N contents (Sun et al., 2014). Sheep feeding not only stimulates the growth of pasture plants but also promotes the redistribution of N and P to young organs, thus increasing the N and P contents in living plants under grazing treatments, leading to the maximum average N and P contents under heavy grazing conditions. This phenomenon implies that plants manipulate the nutrient cycle by altering the diet of livestock to a certain extent. In the desert steppe, a large amount of precipitation will significantly increase the water content of plant leaves. The daily evaporation of the desert steppe is very high, which means that large amounts of precipitation will increase the evaporation capacity of plants per unit time (the transpiration tension is strengthened). Higher transpiration rates will promote the synthesis of more N-rich transporters in roots for transporting nutrients to compensate for the plant leaves. To support the rapid growth of plants, ribosomes must rapidly synthesize protein, which means that plants must allocate more P to rRNA, which improves growth rates and increases the N and P contents in plants.
By analysing the soil N content, it was found that the soil N content under the heavy grazing treatment was higher than that under the no grazing treatment, and this is consistent with Bai et al. (2012). This result indicates that N is in a circulating state in heavy grazing ecosystems. This may be because grazing increases C-rich root exudates that stimulate microbial activity and transformation, ultimately leading to an increased availability of soil nutrients to plants (Bardgett et al., 1998; Hamilton and Frank, 2001). These results imply that grazing can accelerate the soil N cycle in desert grasslands. First, grazing can increase nutrient cycling by stimulating microbial activity by importing fresh plant litter and animal excreta (Holland, Cheng & Crossley, 1996; Hamilton & Frank, 2001). Second, grazing may induce plant avoidance strategies to produce higher concentrations of defence compounds to reduce nutrient losses (hobbie 1992; Ritchie, Tilman & Knops, 1998). These theories support our results to some extent. Further, research has shown that the C:N ratio in soil can reveal the relationship between the transformation of carbon and nitrogen during soil biological decomposition. When the soil C:N ratio was between 5.6 and 11.3, the soil microbial biomass began to increase, and soil N mineralization increased significantly. When the C:N ratio was between 15.3 and 20.6, the soil microbial biomass increased rapidly, organic matter decomposition weakened and mineralized N was released (Gundersen et al., 1998). In this study, compared with the no grazing treatment, the soil C:N ratios under the heavy grazing and moderate grazing treatments were in the range of 5.6-11.3, which further verified that grazing accelerated the flow and transformation of N in the soil.
Based on the analysis of the C, N and P contents in plants, we propose that plants under grazing conditions, especially heavy grazing, may adapt to a severe living environment in the following ways. Under long-term grazing stress, plants can induce livestock to eat specific plant tissues and organs or plant species with low N and P contents. To continue to grow and reproduce, the surviving plants must remain in a relatively stable maintenance state (because the rapid growth of plants requires the synthesis of a large amount of proteins and RNA, which requires them to have high N and P contents). Partial correlation analysis revealed that precipitation was positively correlated with plant N and P contents, suggesting that precipitation was the "trigger" that disrupted the equilibrium. In the soil, accelerated N cycling could meet the N needs for plant growth, and precipitation was positively correlated with the C and N contents in the 0-20 cm soil layer, suggesting that precipitation is an important factor driving the changes in grazing ecosystems. Our results further validate the hypothesis that grazing and precipitation are important indicators of community biomass and species richness.