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 [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)). ResultsFor 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.) ConclusionWe conclude that eCO(2) and altered water availability shifted OAs in root exudates, modifying plant-soil interactions and the associated carbon and nutrient economy.