Long-term effects of CO2 on N2 fixation and ecosystem-level nutrient budgets

Background/Question/Methods

Nitrogen often constrains plants’ ability to fix and sequester C under elevated CO₂, and in nitrogen-limited systems, N fixation is the only means of acquiring “new” N. Therefore, CO₂ effects on N fixation are important to understand in the context of ecosystem function and climate change. Nitrogen fixation can itself be limited by the availability of P, Fe, Mo and V, but few studies have explored the effect of CO₂ on these elements in concert.
We present results on stand-level rates of N fixation and nutrient budgets for a Florida scrub oak community exposed to 12 years of elevated CO₂. Nitrogen fixation rates initially increased under high CO₂ relative to ambient CO₂ plots, but over time this positive effect of CO₂ on N fixation declined, and there was no CO₂ effect on N fixation during final three years of the experiment. We hypothesized that elevated CO₂ depressed or had no effect on N fixation over time due to lower availability of the elements needed for nitrogenase activity, P, Fe, Mo and V via alterations to soils or changes in element interactions in soil.
To test our hypothesis, we determined the stand-level budgets of a broad suite of plant nutrients and metals, including P, Fe, Mo and V. We quantified element pools in the aboveground plant community, roots, N fixer (Galactia elliottii) above ground biomass and root nodules, and soil fractions to 3m depth. We also determined N-fixation rates using the isotope dilution method.

Results/Conclusions
Nutrient concentrations in Galactia leaves were generally lower under elevated CO₂ (K and Mn were exceptions), either due to a dilution by elevated levels of non-structural C under elevated CO₂ or due to increased competition for these elements with oaks. From these measurements it appears that the decline in the response of N fixation to elevated CO₂ over time was not related to changes in foliar or soil element concentrations. Our data suggest that in this system the direction and magnitude of CO₂ effects on N fixation rates significantly vary over time, and this is likely due to factors other than nutrient limitation.