CO₂ concentration and water availability alter the organic acid composition of root exudates in native Australian species

Purpose: Root exudation of organic acids (OAs) facilitates plant P uptake from soil, playing a key role in rhizosphere nutrient availability. However, OA exudation responses to CO₂ concentrations and water availability remain largely untested. Methods: We examined the effects of CO₂ and water on OA exudates in three Australian woodland species: Eucalyptus tereticornis, Hakea sericea and Microlaena stipoides. Seedlings were grown in a glasshouse in low P soil, exposed to CO₂ (400 ppm [aCO₂] or 540 ppm [eCO₂]) and water treatments (100% water holding capacity [high-watered] or 25–50% water holding capacity [low-watered]). After six weeks, we collected OAs from rhizosphere soil (OA_rhizo) and trap solutions in which washed roots were immersed (OA_exuded). Results: For E. tereticornis, the treatments changed OA_rhizo composition, driven by increased malic acid in plants exposed to eCO₂ and increased oxalic acid in low-watered plants. For H. sericea, low-watered plants had higher OA_exuded per plant (+ 116%) and lower OA_rhizo per unit root mass (–77%) associated with larger root mass but fewer cluster roots. For M. stipoides, eCO₂ increased OA_exuded per plant (+ 107%) and per unit root mass (+ 160%), while low-watered plants had higher citric and lower malic acids for OA_rhizo and OA_exuded: changes in OA amounts and composition driven by malic acid were positively associated with soil P availability under eCO_2. Conclusion: We conclude that eCO₂ and altered water availability shifted OAs in root exudates, modifying plant–soil interactions and the associated carbon and nutrient economy.