3.3 Factors affecting SMB
Accurately predicting SMBC and SMBN response to drought requires more comprehensive insights into the influence of edaphic properties and environmental conditions. In our study, we found the amount of SMBC and SMBN varied significantly among drought types (Fig. 2 ). Recent study suggested that increasing drought and global change have pronounced negative effects on the SMB in terrestrial ecosystems (Wan et al., 2021), which was in line with our results. Thakur et al. (2015) found that soil microbes were sensitive to soil water content, even in a short-term drought, microbial physiology and plant diversity would change a lot. Similarly, drought’s legacy effect to shrubland lasting for two years after the drought ended has been observed by Hinojosa et al. (2019) in a post-fire experiment, drought mainly did have an important impact on soil processes. The amount of SMBC increased but SMBN decreased with drought, and in total the drought decreased the amount of SMB (Hinojosa et al., 2019).
In our study, pH insignificantly affected the amount of SMBC and SMBN but had a significantly negative correlation with the ratio of SMBC to SMBN (Table 1). The soil pH has been regarded as a crucial factor for the growth of soil microbial communities. A study found that bacteria grow better in slightly alkaline soil, while most fungi prefer acidic soil (Ma et al., 2016). The mainstream view is that compared to bacteria, fungi characterized by higher growth efficiency and slower degradation rate. Thus, the soil rich in fungi would accumulate higher contents of SMBC and SMBC than which rich in bacteria (Six et al., 2006). Several publications confirmed that soil pH was the main factor regulating the microbial community structure and biomass content (Liu et al., 2014; Cao et al., 2017). Moreover, Cao et al., 2017 showed pH was a significant driver to regulate the size and abundance of the microbial community and Mendes confirmed that pH had a strong relationship with the bacterial community structure in south-eastern region of the Amazon (Mendes et al., 2015).
Temperature played an important role in soil microbes carbon and nitrogen cycling. Moreover, opposite responses along temperature gradient were observed in our results, temperature had a positive correlation with the amount of SMBC but was negative with SMBN (Table.1). The results were not totally consistent with a prior study that SMBC and SMBN showed positive response (3.61% and 5.85% respectively) to warming (Xu et al., 2017). Water loss due to rising temperature might inhibit microbial growth, which eventually decrease the contents of SMBC and SMBN, especially in water-limited ecosystem (i.e., grassland). Therefore, the influence of temperature on microbes has both direct and indirect effects, and the final change of microbial biomass was the result of multiple processes. In general, it was difficult to summarize a universally suitable law about the microbes’ adaptation to temperature. The main reasons for this finding were, firstly, the increase of temperature would enhance the activity of soil microbes, which would promote the development of more organisms, then increase the soil microbial biomass of the community. However, increasing temperature can contribute to greater decomposition of SOC, and cause the loss of soil carbon pool. SOC was the main nutrient source for soil microbes, and the loss of SOC directly leads to the decrease of nutrient source for microbes. Altogether, these finding emphasized the importance of environmental factors in regulating the microbes.