1.1 Data collection
Data in our study was collected from Web of Science and Science Direct. We compiled the dataset by searching the literature published from January 2000 to March 2021 with the following key words: “dryland” OR “arid” OR “semi-arid” AND “microbial biomass”. This resulted in a total of 2755 published references, after a duplicated filtering literature retrieved from both database with EndNote references manager software (EndNote X9). We performed a filtering through the abstract and title of each reference, then through the full text to decide whether the article matched our selection criteria. Data was chosen based on the following criterion: (1) the article contained at least the amount of SMBC or SMBN; (2) the amount of SMBC or SMBN was measured by the fumigation extraction method, which is the most widely applied and standardized method for the estimation of SMBC and SMBN (Vance et al., 1987). We use only one method and ignore those based on dilution plate court, microware irradiation, microscopy, phospholipid fatty acid, quantitative PCR or substrate-induced respiration to avoid methodological discrepancies; meanwhile fumigation extraction method was most frequent method we found; (3) experiments were taken under natural conditions (laboratory or greenhouse experiments were excluded), when manipulation experiments were reported, only data from the “control” plots was used; (4) measurements were conducted in dryland ecosystems (AI < 0.65).
Altogether, we found 86 relevant published papers, including 109 observations of SMBC and 79 observations of SMBN from 100 study sites across global dryland regions (Fig. 1 ). A list of articles including climatic and soil information was presented in Appendix. All original data were extracted from the text, tables, figures and appendices of the publications. If data was present graphically, we used the Web Plot Digitizer 4.2 software to digitize the numerical data.