3.1 SMB under low vegetation cover level in dryland regions
Our study summarized that in the dryland regions, the amount of SMBC and
SMBN were 358.47 ± 25.45 mg kg-1 and 51.86 ± 4.59 mg
kg-1 in the 0-30 cm soil profile, which were almost
half of the previous terrestrial ecosystems study (SMBC
680.4 mg kg-1and SMBN 105 mg kg-1, Xu et al. 2013). It has been
speculated that the discrepancy might be due to the study regions.
Dryland regions are part of land and just cover about 41% of Earth’s
land surface, which include four main ecosystem types: grassland,
desert, cropland and forest. The total area of these four ecosystem
types accounts for at least 65% of the dryland regions area; meanwhile,
forests which with high vegetation cover only account for 18% of the
dryland regions area. (Millennium Ecosystem Assessment, 2005). Desert
and grassland have lower vegetation cover rate and plant diversity
compared to tropical forest (Schuldt et al., 2019; Conti et al., 2013;
Cook-Patton et al., 2011). Vegetation cover and plant diversity
influence the plant quantity directly and the quality of plant litter
and root exudates which in turn indirectly affect the soil condition
(Drenovsky et al., 2010; Xiao et al., 2017); meanwhile, the microbial
processes of carbon and nitrogen cycles were affected by soil condition
due to the differences in quality and quantity of nutrient (Bargali et
al., 1993; Kara et al., 2008).
Consistent with our hypothesis, the amount of SMBC and SMBN were
significantly different among ecosystem types in dryland regions
(Fig. 5 ). Different ecosystems develop unique patterns of
adaptation, especially under extreme conditions such as drought. Desert
which was the main ecosystem type in drylands region is with sparse
vegetation cover, in turn the soil in desert was lacked of ample root
networks and plant litter to provide enough nutrient to soil microbes
(Yang et al., 2014; Luna et al., 2016; Gang et al., 2012). The reason
for low amount of SMBC and SMBN in the desert was likely a deficiency
available nutrient content by low degree of aboveground vegetation
cover. To some extent, the greater amount of organic matter accumulates
may indirectly affect soil microbial activity and abundance, then soil
becomes more suitable for microbial growth (Shrestha et al., 2008).
According to the plant-soil-microbes theory supported by many studies,
the response among the three parts is significant and sensitive. The
roots-dominated soil usually shows a greater chance of nutrient supply
and cycling, thus enhances the contents of SMBC and SMBN around the root
zone especially in desert soil (Jia et al., 2017). Jia et al. (2017)
found after plant restoration in an arid desert of northern China, the
vegetation cover rate increased and the greater amount of plant litter
enters the soil, which in turn governs the soil organic matter and
nutrients, consequently the content of SMBC significantly increased. In
the grassland ecosystem, grazing was one of the major and special
factors to affect the ecosystem nutrient cycles. There was no doubt that
microbes and animals, such as sheep, competed for ecosystem nutrient.
When grazing was reduced, higher organic matter inputs to soil from root
exudates and plant litter which are more conducive to microbial growth,
and then the contents of SMBC and SMBN increased (Wu et al., 2014). On
the contrary, some researchers found no evidence of a difference between
SMB and land cover, soil quality was important instead (Moghimian et
al., 2017).
3.2 stoichiometry of soil
and soil microbial biomass
The stoichiometry of SMB from open-access papers and our study were
together summarized in the table (Table 2 ). We indicated that
the soil C/N and SMBC/SMBN in dryland regions was 10.69 and 8.73, which
were different compared to other publications. Firstly, the data only
included research from drylands region (aridity index < 0.65)
in our study, concluding grasslands and deserts mainly (accounted for
75.4%); while the data in other studies like Xu et al. (2013) and
Cleveland et al. (2007) were extracted from global region, including all
ecosystem types. Differences in vegetation cover and plant diversity are
the main reasons. Secondly, the sampling depth used in our study was
different from that of the studies at global scale (i.e., 0-30 cm vs.
0-10 cm). There were a lot of studies that show the microbes were
distributed differently in the vertical direction of soil, and the
content in the top layer soil was higher than that of the deep soil (Jia
et al. 2017; Xu et al., 2013).
The ratio of SMBC to SMBN was an indicator of soil stoichiometry balance
and was close related with microbial C use efficiency (CUE) and N use
efficiency (NUE), which was important in predicting efficiency of
nutrient cycling (Mooshammer et al., 2014). The definition of CUE or NUE
is a ration of nutrient allocated to growth over nutrient taken up by
microbial community composition (Manzoni et al., 2012; Sinsabaugh et
al., 2013). High ratio of soil C/N meant soil is rich in available C (N
deficiency), microbes would have high NUE and low CUE, finally lead a
low degree of SMBC/SMBN. In our study, the ratio of soil C/N was 10.69,
which waslower than other previous results, indicated that in dryland
regions, the soil was at a high substrate N sufficient condition. Thus,
it was not surprising that microbes would be adjusted to high CUE, then
lead to a high SMBC/SMBN. This inference was consistent with our
research result, the ratio of SMBC/SMBN in our study was 8.73, which was
higher than other results.
Providing additional evidence that the SMBC/SMBN was higher in dryland
regions, we also found a previous study shown that soil with high water
content usually had more available C than available N. Actually, it
would result in lower SMBC/SMBN. The study reinforced our results.
Dryland regions often were characterized by water deficiency, which is a
plausible explanation for SMBC/SMBN in dryland regions was lower than
global average. To the best of our knowledge, the mechanisms behind this
relationship are not clear, and this might be also related to
differences in microbial communities at different soil depths.