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.