Tree carbon allocation to root exudates: implications for carbon budgets, soil sequestration and drought response.

Root carbon (C) exudation plays a central role in nutrient acquisition, microbially mediated organic matter decomposition and many other critical ecosystem processes. While it is well known that roots respond strongly to belowground resources, we have a limited quantitative understanding about C allocation to exudates and its fate in soil under changing water availability. This review synthesizes the importance of exudate C fluxes, summarizes studies quantifying mass-specific exudation rate (SER), total exudation rate (TER) and root exudate fraction (REF; the proportion of TER in a plant's C allocation), examines drought effects and highlights key research priorities to advance the understanding of C allocation to exudates in forest ecosystems. On average, SER is often <1 mg C gdry root-1 day-1, TER is 3.8 Pg C year-1 and REF varies between 1 and 17% of net primary production. Spatiotemporal variations in exudation, including seasonal and daily patterns and subsoil exudation, remain critical knowledge gaps. We show that many studies report a 1.2- to 11-fold increase in SER and REF in response to drought. However, TER often remains unchanged, suggesting that absolute exudate C inputs to the soil may stay constant under drought conditions. Disentangling the individual impacts of soil and air drought as well as drought legacy impacts on ecosystem C dynamics are overlooked aspects. By estimating the differences in rhizosphere formation and exudation across various forest biomes, we find that exudate-affected soil volumes are highest in tropical forests and lowest in boreal forests. While current research emphasizes significant C allocation from the canopy to soil via exudates, understanding exudation dynamics and biome-specific responses to drought by using standardized protocols is essential. Expanding these insights is critical for comprehending the role of root exudates in soil organic matter formation, ecosystem resilience and adaptation to climate change.