Introduction
Drylands cover about 41% of the Earth’s land surface and support more than 38% of the total global population, representing a globally important biome of great significance to human welfare (Millennium Ecosystem Assessment, 2005). In the next decades, dryland regions may face less frequent rainfall events and experience more intense drought events due to climate change. As a consequence, a worldwide expansion of drylands is expected for the near future (Gao and Giorgi, 2008; Dai et al., 2013; Feng and Fu, 2013; Berg et al., 2016; Huang et al., 2016). These changes may exacerbate water scarcity, land desertification, and soil degradation, threatening the populations depending on drylands for food production and the associated income. The United Nations Environment Programme (UNEP) defines drylands according to aridity index (AI), which is the ratio of mean annual precipitation to mean annual potential evaporation, and drylands are lands with an AI of less than 0.65 (UNEP, 1992; UN Environment Management Group, 2011). To a certain extent, AI can also indicate whether a certain area is rich in water with a higher value of AI (Wan et al. 2021). In general, soil water content is of particular importance as a driver of microbial community since many microbial metabolism processes such as heterotrophic respiration are affected by soil water (Moyano et al., 2012). Microbial activity and the amount of soil microbial biomass generally increase with soil water content to reach a maximum; but extremely high soil water content can induce oxygen limitation and decrease soil microbial activity (Moyano et al., 2012). Several studies indicated that soil water content may vary with space and time, and the reason is generally correlated closely with AI and air temperature (Delgado-Baquerizo et al., 2013; Wang et al., 2014). Therefore, we may expect negative correlations between the amount of soil microbial biomass and AI, especially in dryland regions where water is often the main limiting factor for soil living organisms (Xu et al., 2017).
A key composition of ecosystem is the soil microbes, which take part in energy flow, nutrient cycling and ultimately ecosystem productivity (Wardle et al., 1998). Soil microbial biomass carbon (SMBC) and nitrogen (SMBN) are indicators of soil microbial activities. In fact, soil microbial biomass (SMB) just accounts for a small fraction of soil weight, and it has been suggested that the amount of SMBC in forest soils is estimated to be less than 5% of the total soil organic carbon stock (Fierer et al., 2009). Nevertheless, SMB is undoubtedly the most active nutrient pool in soils. Likewise, the ratio of SMBC to SMBN also provides evidence and insights to understand soil nutrient cycle and other biochemical characteristics of soil (Bargali et al., 2018; Li et al., 2018). Moreover, the amount of SMBC and SMBN and the ratio of SMBC to SMBN can be used as indicators of soil quality (Schloter et al., 2003), as their high values was generally associated with energetic and nutritious soils. It is attributed to the fact that soil microbes are the drivers of these flows and cycles, because soil microbes have short turnover time and are sensitive to environmental disturbance. A recent study provides evidence that soil fungal communities showed significant differences with the different amount of SMBC, and the amount of SMBC was the prime factor in fungal community structure (Liu et al., 2019). Additionally, some studies also suggest that soil condition was correlated with the amount of SMBC (Zhou et al., 2015). Collectively, the amount of SMB and ratio of SMBC to SMBN are robust indexes to evaluate and understand the underground ecology.
Given different conditions between dryland regions and other terrestrial regions, including differences in above ground productivity, edaphic properties and nutrient cycling, general universal law in terrestrial regions do not represent necessarily a relevant benchmark for the fragile dryland regions. There have been some experiments and studies on the driving factors on the amount of SMB, but the results of these studies are often inconsistent or even completely opposite. The amount of SMBC and SMBN is not only affected by the amount of dead primary products and soil organic matter (Kaiser et al., 1992; Zak et al., 2003), but also regulated by soil environmental conditions, such as soil temperature, pH, oxygen, soil water content and nutrient availability (Gallardo and Schlesinger, 1992; Zhou et al., 2015). However, the relative contributions of these factors are inconsistent among the above studies. Therefore, we still know little about the controlling factors of SMB in dryland regions.
Since most drylands region are relatively infertile and fragile (Reynolds et al., 2007), a better understanding of patterns and drivers of SMBC and SMBN is essential for the sake of maintaining dryland regions ecosystem functioning and services. We hypothesized that in the dryland regions, (1) in global dryland regions, the higher the degree of drought (the lower the aridity index), the lower the amount of SMBC and SMBN, (2) the amount of SMBC and SMBN would be different among ecosystem types, (3) the pattern of SMB in dryland regions would be affected by climatic factors and soil factors.