No CO2 fertilization effect on plant growth despite enhanced rhizosphere enzyme activity in a low phosphorus soil

Aims: Accurate predictions of plant responses to elevated CO2 (eCO2) levels require a better understanding of uptake and allocation of resources that affect growth. While it is well-established that plants can increase C transfer belowground to increase access to nutrients in N-limited environments, there is less studies investigating the role of these adaptations in P-limited systems. In this study, we aimed to characterise plant nutrient use and acquisition strategies under future atmospheric [CO2] scenarios, under P-limited conditions and determine how different levels of water availability regulate these strategies. Methods: We exposed four grass species commonly found in the understory of P-limited Australian Eucalyptus woodlands to eCO2 over a period of 14 weeks, under two contrasting water levels. We assessed a suite of root morphological and chemical traits and the rhizosphere activity of extracellular enzymes related to C, N and P cycles, along with changes in plant allocation patterns and use efficiency of these nutrients. Results: Elevated CO2 effects on root functional attributes were species-specific, but clear trends towards increased phosphatase activity were observed across species, leading to a lower C:P ratio. In contrast, water supply affected root morphological attributes, but interactions between CO2 and water levels on functional traits were minor. Greater water availability also stimulated microbial activity in the rhizosphere, but without observable changes in the relative demand for N and P relative to C. Conclusions: Despite changes in rhizosphere processes, eCO2 did not lead to increased plant biomass regardless of water supply, suggesting primary nutrient limitation and a lack of positive rhizosphere feedbacks to plant growth.