2.1 SMB among different drought classes
In our study, the average contents of SMBC and SMBN in global dryland regions were 358.47 ± 25.45 mg kg-1 and 51.86 ± 4.59 mg kg-1, respectively. At drought level, the amount of SMBC and SMBN did vary among different drought classes, which both increased from arid to semi-arid to dry sub-humid drought classes (Fig. 2 ). The amount of SMB in arid drought class was significantly less than the other two classes, but there was not a significantly difference between semi-arid drought and dry sub-humid drought classes. The relationship between SMBC and SMBN is a crucial parameter to determine soil stoichiometry balance, which is closely related with the microbes’ use efficiency. Our study showed that the relationship between the amount of SMBC and SMBN was significantly positive (Fig.3 ).