How will root exudation respond to climate change across plant species and ecosystems?

Root exudation, the export of low-molecular weight organic carbon (C) compounds from living plant roots into soil, is an important biogeochemical process that links plant and soil C pools. Because changes in root exudation rate and root exudate composition can impact soil C dynamics over short timescales, understanding the response of root exudation to climate change is relevant for predicting future soil C stocks. However, the response of root exudation to climate change could vary depending on the plants in the ecosystem, the local environment, and the acting climate change driver(s). Here, I synthesize data collected from five whole-ecosystem climate change experiments in the United States. I show that warming drives strong but taxon-specific responses of root exudation rate and root exudate composition, and that the direction of this response varies depending on whether the soil or air is being warmed. Negative root exudation responses to soil warming suggest that enhanced soil nutrient mineralization under warming reduces exudate demand, whereas strong positive responses of exudation to air warming suggest that greater productivity increases exudate C supply. Furthermore, I show that elevated CO2 does not induce a consistent increase in root exudation across species and ecosystems, contrary to predicted responses based on source-sink dynamics. I provide evidence that null or negative CO2 effects on root exudation may be due to trade-offs with C allocation to mycorrhizal fungi. Using artificial root exudate experiments, I show that the effects of climate change on exudation rates are likely to interact with climate change-induced shifts in soil microbial community composition to regulate soil C dynamics. I suggest that increases in root exudation due to warming are likely to induce soil C losses which may be partially offset by changes to the soil microbial community, while elevated CO2 effects on root exudation are more likely to scale with root biomass responses.