Rhizosphere effects of woody plants on soil biogeochemical processes: A meta-analysis

Rhizosphere effects control soil biogeochemical cycling. However, quantitative assessment of rhizosphere effects on belowground processes such as carbon and nitrogen cycling remains rare, especially for woody species in field experiments. Here, we conducted a meta-analysis to synthesize the rhizosphere effects of woody plants (185 species from 128 sites) on belowground processes (20 variables), including soil carbon and nutrient pools, microbial biomass, enzyme activities, and carbon and net nitrogen mineralization rates. We also explored the effects of plant mycorrhizal types (arbuscular mycorrhizal, AM vs. ectomycorrhizal, ECM), leaf traits (evergreen vs. deciduous, broadleaf vs. coniferous), bulk soil properties and climatic factors on these rhizosphere effects. Our results indicated that the rhizosphere effects were positive on all variables except soil pH and phenol oxidase activity. Most rhizosphere effects did not vary significantly between mycorrhizal types, but the effect on soil nitrate-N was notably lower under ECM-associated species than under AM-associated species. Rhizosphere effects increased carbon and net nitrogen mineralization rates by 38% and 40% on average, and the variation was mainly affected by bulk soil properties (e.g., pH, soil organic carbon and available nitrogen concentrations) rather than plant functional types and climatic factors. Collectively, these results provide quantitative evidence that rhizosphere effects influence belowground carbon and nutrient cycling in forest ecosystems, which are helpful for the development of ecosystem models that explicitly consider plant-soil interactions in the rhizosphere.