Macroecology of soil fungi and bacteria in the United States using a data-model integration approach

Soil bacteria and fungi are the ultimate driver of the biogeochemical cycling of carbon (C) and nutrient in soils. We integrated in situ observations of fungal (FBC) and bacterial (FBC) biomass C based on phospholipid fatty acid technique with the CLM-Microbe model for 34 sites of National Ecological Observatory Network (NEON). We first parameterized the CLM-Microbe model by optimizing gross primary productivity (GPP), net primary productivity (NPP), soil organic C of top 1 m, FBC, and BBC. The CLM-Microbe model can reproduce the spatial distribution of fungi and bacteria across sites, with 70% (P<0.001), 63% (P<0.001), and 99% (P<0.001) of variations in FBC, BBC, and FBC:BBC (F:B) ratio explained by the CLM-Microbe model. Both FBC and F:B ratio exhibited significant correlations with latitude, mean annual temperature (MAT), soil temperature (ST), and pH, whereas BBC was significantly associated with bulk density, GPP, and NPP. The FBC, BBC, and F:B ratio showed no significant trend along soil texture, soil moisture, and mean annual precipitation. Then we used principal component analysis method to infer the primary C sources for fungi and bacteria. Fungal biomass C was primarily contributed by dissolved organic matter (DOM), soil organic matter (SOM), and litter at sites dominated by grass and deciduous shrub, whereas it was primarily from litter and SOM at sites dominated by trees (tropical-, template-, and boreal-) and evergreen shrub. Bacterial biomass C was predominately regulated by DOM, SOM, and litter at sites dominated by grass, deciduous shrub, and boreal-trees, whereas it was predominated by litter and SOM at sites dominated by tropical- and temperate-trees and evergreen shrub. In contrast, C loss (microbial lysis) is controlled by climatic factors (e.g., MAT and ST), showing a significant declining trend along latitude. A Mantel test further indicated the predominant role of pH and recalcitrant soil C for FBC, recalcitrant soil C for BBC, and latitude, MAT, recalcitrant soil C, and pH for F:B ratio. By combining in-situ data of soil bacteria and fungi and a microbial-explicit model, this study reported the macroecology of fungal and bacterial biomass C in the United States, which advances our knowledge of microbial ecology and are beneficial for microbial modeling in simulating soil microbial community structure and their roles in global C soil.