Effects of Root Exudation on Soil Carbon Formation and Loss: Insights from Artificial Root-Soil Systems

Abstract:

Although root exudates comprise a substantial fraction of plant carbon inputs to soils, their effects on soil carbon dynamics are still poorly understood. Root exudates may prime the decomposition of soil organic matter (SOM), but they also contribute to the formation of SOM via microbial anabolism. The balance between SOM formation and loss in response to root exudation is likely dependent upon characteristics of the microbial community and abiotic properties of a particular soil. However, these factors are frequently confounded in natural ecosystems, making it difficult to disentangle their relative contributions.

Here, we describe a novel ‘synthetic soil’ approach with artificial roots to independently manipulate soil mineralogical properties (high, medium, and low-activity clays); the composition of the soil microbial community (bacterial-dominated and fungal-dominated); and rates of root exudation. We traced the fate of simulated root exudates over a six-month laboratory microcosm incubation, examining changes in soil CO₂ fluxes, the microbial biomass, and particulate and mineral-associated soil organic matter pools. We found that root exudation enhanced soil C losses only in bacterial-dominated microbial communities in soils with low clay sorption capacity. These results emphasize that the impacts of root exudates on belowground carbon cycling are context-dependent and underscore the need for a more nuanced understanding of rhizosphere dynamics.