Drought stress increases total C released from roots

BACKGROUND AND AIMS: Root exudation and rhizodeposition are critical pathways for plant carbon (C) allocation to soil, influencing soil organic C stability and ecosystem functioning under changing climates. However, the effects of drought stress on these below-ground processes remain poorly understood. We provide an updated and comprehensive assessment of the current state of knowledge on how drought stress affects root-derived C fluxes (root exudates, rhizodeposition) across diverse functional types and experimental setups to direct future research. METHODS: We conducted a meta-analysis to quantify how drought stress affects total C and other compound classes in root exudation and rhizodeposition. KEY RESULTS: Our analysis included 40 data points spanning diverse functional types, ecosystems, sampling methods and experimental conditions. We observed that root-released total C significantly increased in response to drought stress. Among compound classes, carbohydrates and organic acids also increased in response to drought stress, suggesting these compound classes may underlie total C responses in root exudation and rhizodeposition. The variables that explained the most variance in total C response ratio were ecosystem, flowering type, cotyledon type, functional type and drought intensity. CONCLUSIONS: To direct future research, our analysis identified knowledge gaps, particularly the need for studies to examine trees and shrubs, field-based research, broader drought intensity ranges, and measurements of total C, compound classes and specific compounds when possible. Improving our understanding of root-derived C responses to stress is crucial for predicting terrestrial C cycling dynamics and ecosystem function under increased drought frequency and intensity.