2.2 SMB among different ecosystem types
In order to conduct vegetation-level analysis, we aggregated the
original data into four ecosystem types: desert, grassland, forest and
cropland. On the whole, the drought degree of the desert ecosystem was
significantly higher, because its aridity index was significantly lower
than that of the other three ecosystems (Fig. 4 ). At ecosystem
level, the amount of SMB and the ratio of SMBC to SMBN varied among
different ecosystem types. The amount of SMBC in desert was
significantly lower than those of other ecosystem types, and the
remaining three ecosystem types were not significantly different from
each other (Fig. 5 ). The amount of SMBN had same order as SMBC;
desert was also the lowest one, but there was a significant difference
between grassland and cropland (Fig. 5 ). The ratio of SMBC to
SMBN showed a completely different pattern between cropland and the rest
three types. Cropland ecosystem had the highest ratio of SMBC to SMBN
(Fig. 5 ).
2.3 Factors affecting the SMB
Based on pearson correlation analysis, the amount and ratios of SMB were
controlled by a variety of factors (Table 1 ). Mean annual
temperature, soil bulk density and soil C/N had positive relationship
with the amount of SMBC. Opposite relationships were observed for SMBN.
There was no significant difference between environmental factors and
the amount of SMB. The ratio of SMBC to SMBN had a strong and negative
correlation with soil properties as pH (p < 0.05) and
sand fraction (p < 0.05); whereas, soil bulk density
and soil C/N did not differ. Soil bulk density and soil C/N increased
with aridity index, while mean annual temperature, soil sand fraction
and soil pH decreased with it.