Rhizosphere carbon enrichment

The rhizosphere, the narrow soil zone influenced by living roots, is a major locus of carbon input and microbial transformation, but global patterns of rhizosphere effects on cropland soil carbon pools remain poorly resolved. Here, we synthesized 478 paired observations comparing rhizosphere and bulk soils from 93 field and pot studies in cropland systems worldwide. Using multilevel random-effects meta-analysis, we quantified rhizosphere effects on soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), total nitrogen (TN), and key enzyme activities, and then used environmental covariates to identify predictors of effect-size variation. On average, rhizosphere soils contained 9.5% more SOC, 44.7% more DOC, 23.5% more MBC, and 7.9% more TN than adjacent bulk soils, together with higher activities of most measured carbon- and nutrient-acquiring enzymes. The larger responses of DOC and MBC than SOC suggest that rhizosphere influences are expressed more strongly in active and microbial carbon pools than in total SOC. Effect sizes varied substantially with land-use type, soil depth, cropping system, and crop species. Soil texture, particularly silt content, and background SOC were the strongest predictors of spatial variation in rhizosphere effect size. As an application of the fitted model, projected effect sizes across China’s croplands were generally higher in paddy than in upland systems. These results show that rhizosphere soils are systematically enriched in carbon relative to adjacent bulk soils across croplands, but that the magnitude of enrichment is strongly context dependent.