Root exudates increase microbial biomass but decrease diversity and richness: a meta-analysis

While plant root exudates play a crucial role in regulating soil microbial communities, their complex composition, low concentration and degradability make it difficult to determine their effects on microbial biomass, diversity and structure. In this study, we integrated 3174 observations from 121 publications to systematically assess the effects of root exudate addition on soil microbial communities and their differential response mechanisms. The results indicated that exudates significantly increased microbial biomass (total microbial biomass: +18.65%; bacteria biomass: +31.35%; fungi biomass: +37.73%) but decreased α-diversity (−6.17%), richness (−9.16%) and evenness (−10.09%), while also altered the community structure. Natural exudates, with their complex composition and higher nutrient content, had a stronger effect on promoting microbial biomass, whereas synthetic exudates led to the overgrowth of certain bacterial taxa, suppressing microbial community diversity. The responses of soil available nitrogen content were identified as key predictors of microbial biomass, while soil pH was an important predictor of microbial richness and evenness. Meanwhile, the carbon input rate from root exudates significantly suppressed α-diversity and altered microbial community structure. Additionally, the C:N ratio of exudates played a critical role in regulating β-diversity and community structure. This study highlights the importance of exudate chemical properties, experimental conditions and soil physicochemical characteristics, offering a new perspective on the mechanisms regulating microbial communities by root exudates.