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.