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Microbial biogeography through the study of the assembly rules of microbes has the potential to yield ecological information that is generalizable for a wide range of microorganisms. Here we performed a large-scale field investigation of the distribution patterns of microbes across the arid and semi-arid grassland ecosystems covering an area of 690,000 km² in northern China. Soil microbial abundance and community composition were examined through quantifying microbial phospholipid fatty acids (PLFAs) at fifty sampling sites along environmental gradients. A multi-model inference analysis identified soil organic carbon (SOC) as a key driving factor for microbial biomass and quantified its effect. Structural equation models (SEM) were further fitted to the data to provide a better mechanistic resolution of direct and indirect pathways that connected PLFAs and environmental variables. The SEM analysis also …