Introduction

Microorganisms play an essential role in soil carbon (C) and nutrient biogeochemistry and pose dramatic impacts on various ecosystem processes, including organic matter mineralization, soil formation, and nutrient availability (Högberg et al. 2001; Rillig & Mummey 2006; Turner et al. 2013; Crowther et al. 2019). Eventually, the ultimate fate of soil C is primarily driven by soil microbes (Schimel & Schaeffer 2012). Although the critical roles of soil microbes in global C and nutrient cycling have been widely recognized (Falkowski et al. 2008; van der Heijden et al.2008), the research on biogeographic distribution of fungi and bacteria is still in its infancy. Furthermore, microbial community structure is an important factor controlling C and nutrient biogeochemistry as bacteria and fungi differ in enzyme production (e.g., lignocellulose and cellulose), C use efficiency, and biomass stoichiometric ratios (Caldwell 2005; Six et al. 2006; Mouginot et al. 2014), and respond differently to multiple global change factors (Rousk & Bååth 2007a; Rousk et al. 2009). Therefore, biogeographic patterns of bacteria and fungi provide pivotal information for understanding microbial contributions to global C and nutrient biogeochemistry.
Geographic distribution of microbes is driven by a suite of abiotic and biotic factors (Martiny et al. 2006; Hanson et al. 2012). Previous studies have investigated the controlling factors on microbial diversity and functions, including soil organic C (SOC), climate, and vegetation (de Vries et al. 2012). Soil moisture (SM), soil organic matter quality, and soil pH are among the key effectors influencing soil microbial community composition (Fierer & Jackson 2006; Eskelinen et al. 2009; Brockett et al. 2012; Dinget al. 2015). Although these findings provide valuable information for local to regional environmental drivers and proxies of soil microbial community structure, we still lack a holistic and quantitative understanding of soil microbial biogeography and different microbial groups at the global scale. In particular, the lack of clear quantitative understanding of bacterial and fungal biogeography and their controls hinder the explicit incorporation of microbial mechanisms into climate models (DeLong et al. 2011; Wieder et al.2013; Xu et al. 2014).