Elevated CO2 does not change the rhizosphere effect on gross nitrogen and phosphorus mobilization in a mature Eucalyptus woodland

Sufficient nutrient availability is a prerequisite for enhanced plant and ecosystem production under elevated atmospheric carbon dioxide levels. While previous research has focused on nitrogen limited ecosystems, young forests and at one nutrient at a time, the present study simultaneously investigated gross soil nitrogen and phosphorus dynamics in a mature Eucalyptus woodland with phosphorus limitation, exposed to elevated CO 2 (EucFACE). Neither gross N mineralization rate, gross nitrification rate nor the exchange rate of inorganic P between soil solid phase and soil solution were affected by elevated CO 2 . However, we observed that rates were usually higher in the presence of living roots, compared to root-free soil. Also, the soil content of inorganic N and P was unaffected by elevated CO 2 . These results are partly unexpected, as we hypothesized an increased P mobilization under elevated CO 2 . Our results suggest that it is the limited amount of mobilizable P rather than the competition between soil microorganisms and plants for available P that may constrain plant uptake of P and tree growth at the site. The soil microorganisms likely already utilize all mobilizable P at ambient CO 2 conditions, and the remaining soil P is bound too strongly for mobilization. Our study highlights the complexity of P limited soil systems and their response to eCO2 as well as identify research gaps including seasonal eCO2 interactions and the value of integrating dynamics of multiple nutrients to reveal the trajectory of change. • First study that simultaneously investigated gross soil N and P dynamics under eCO 2 • Neither gross N mineralization, nor P mobilization were affected by eCO 2 at EucFACE • The limited amount of mobilizable P may constrain plant uptake of P and tree growth