CO 2 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 CO 2 concentrations and water availability remain largely untested. Methods We examined the effects of CO 2 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 CO 2 (400 ppm [aCO 2 ] or 540 ppm [eCO 2 ]) 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 eCO 2 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 , eCO 2 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 eCO 2 and altered water availability shifted OAs in root exudates, modifying plant–soil interactions and the associated carbon and nutrient economy.