Global magnitude of rhizosphere effects on soil microbial communities and carbon cycling in natural terrestrial ecosystems

The rhizosphere is one of the most dynamic interfaces on the Earth. Understanding the magnitudes of rhizosphere effects (RE, difference in bio-physicochemical properties between rhizosphere and bulk soils) on soil microbial communities and their moderators is important for studying on below-ground carbon (C) cycling. A comprehensive meta-analysis was conducted to quantify the REs on soil microbial biomass, community structure, respiration, and C-degrading enzymes. We found that REs on soil C and nutrients, total microbial biomass, the abundance of specific microbial groups, fungi to bacteria ratio, respiration, and C-degrading enzymes were positive, but the magnitudes were varied with biomes, plant functional types, and mycorrhizal types. REs on microbial biomass, respiration, and C-degrading enzymes increased with the increase of mean annual temperature and mean annual precipitation, but decreased with the increase of soil clay, C, nitrogen (N), and phosphorus (P) contents. The REs on microbial biomass and respiration also increased as the REs on soil C:N:P increased. Compared with bulk soil, per unit rhizosphere soil C supported more microbial biomass, per unit of which respired more C, leading to faster C decomposition in rhizosphere. Our findings indicate that the increase in microbial biomass, co-metabolism induced by labile and energy-rich organic C of root exudates, and overflow respiration induced by stoichiometric imbalance together contribute to the enhanced C decomposition in rhizosphere. The global pattern of REs on soil microbial communities is critical to revealing the plant-microbe-soil interactions in terrestrial ecosystems.