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.