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).