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

Area of Science:

• * Soil science and microbial ecology
• * Plant physiology and ecosystem dynamics
• * Biogeochemical cycles

Background:

• * Root carbon (C) exudation is crucial for nutrient cycling, decomposition, and ecosystem function.
• * Limited quantitative understanding exists regarding C allocation to root exudates under changing water availability.
• * Root exudates significantly influence soil organic matter formation and ecosystem resilience.

Purpose of the Study:

• * To review and synthesize the importance of exudate C fluxes in forest ecosystems.
• * To quantify mass-specific exudation rate (SER), total exudation rate (TER), and root exudate fraction (REF).
• * To examine drought effects on root exudation and identify research priorities.

Main Methods:

• * Literature review and synthesis of existing studies on root exudation.
• * Quantitative summary of SER, TER, and REF from published data.
• * Analysis of drought impacts on exudation rates and C allocation.

Main Results:

• * Average SER <1 mg C gdry root-1 day-1, TER 3.8 Pg C year-1, REF 1-17% of net primary production.
• * Drought often increases SER and REF (1.2- to 11-fold), but TER remains constant, indicating stable absolute C input.
• * Exudate-affected soil volumes are largest in tropical forests and smallest in boreal forests.

Conclusions:

• * Root exudation is a significant C flux, with drought altering its dynamics but not necessarily total soil input.
• * Spatiotemporal variations and biome-specific responses to drought require further standardized research.
• * Understanding exudation is critical for predicting ecosystem C dynamics, soil formation, and climate change adaptation.