Phosphorus and water supply independently control productivity and soil enzyme activity responses to elevated CO2 in an understorey community from a Eucalyptus woodland

Aims: While it is well-established that nitrogen (N) availability regulates elevated [CO2] (eCO2) effects on plant growth and soil carbon (C) storage in N-limited environments, there are fewer studies investigating the role of phosphorous (P) supply on such responses in P-limited environments. In this study, we explored whether P fertilization influences the response of plant growth, soil enzyme activity and C fluxes to eCO2, and determined how different levels of water availability regulate these processes.

Methods: We used soil collected from a temperate, P-limited Eucalyptus woodland containing the native soil seed bank to grow a potted replica of local understory communities. We exposed the emerging communities to eCO2 under two contrasting water levels and two levels of P fertilization. We assessed plant biomass allocation, the rhizosphere activity of extracellular enzymes related to C, N and P cycles, and pot-level CO2 fluxes.

Results: The positive effects of eCO2 on plant production and ecosystem C dynamics were strongly constrained by low levels of P availability. Enhanced water supply increased rhizosphere enzyme activity with minor impacts on plant biomass responses to eCO2. Our data also suggest that plant and microbial mechanisms that increase nutrient release from SOM may not be able to overcome this P limitation. Conclusions: While current Earth System Models predict positive feedback responses of terrestrial ecosystems on C storage under eCO2, here we emphasize the importance of accounting for the widespread phenomenon of P-limitation in such responses.