The impact of elevated carbon dioxide on nitrogen fixation and ecosystem level element cycling

The magnitude of climate effects due to increasing carbon dioxide (CO 2) in Earth's atmosphere will be determined in part by the response of terrestrial plants and soils to a high CO2 world. Increased plant productivity under elevated CO2 could lead to increased soil C storage. However, every organism involved in the conversion of atmospheric CO2 to soil C requires nitrogen (N) and a suite of other macro and micronutrients to maintain their metabolic and reproductive requirements. Nitrogen limitations to plant productivity under elevated CO2 could be relieved if N2 fixing plant-microbe symbioses increase N inputs via higher N2 fixation, but this effect is dependent on the availability of non-N nutrients in soil. Plant nutrients other than N could be mobilized from soil pools to plant biomass under elevated CO2 via increased uptake. Lastly, some naturally occurring chemical elements are toxic to organisms (i.e., Pb, Cd, U), but it is unknown if elevated CO2 changes the distribution of these elements from soils to plants. Determining CO2 effects on potentially toxic elements is an important contribution towards an integrative approach to global change and environmental pollution research.;Nutrient dilution is a commonly cited effect of elevated CO2. I used meta-analysis (Chapter 1) to test for elevated CO2 effects on plant nutrient concentration (B, Ca, Cu, Fe, K, Mg, Mn, P, S and Zn among 4 plant functional groups and 2 levels of N fertilization). CO2 reduced the concentration of plant nutrients (6.6 % across nutrients and plant groups), but the reduction is less than expected (18.4 %) from carbohydrate accumulation alone.;I also utilized a field study in Florida that increased CO2 to twice-ambient levels to test the above hypotheses about element cycling and global change. After 11 years of elevated CO2 exposure, the pools of Na, Al, S, V, Zn and Mo in plant biomass (above and below ground pools) were greater under elevated CO2. I observed a net gain in whole-ecosystem S (plants and soluble S pools in soils) under elevated CO 2. My calculations of nutrient redistribution from soluble soil pools to plant biomass suggest that the increase in plant Na, V, Zn and Mo is greater than the decline of those elements in soils (Chapter 2). I measured significant positive correlations between N2 fixation rates and leaf Mg and K concentration, but negative correlations with fixation and legume ( Galactia elliottii) Al and Fe (Chapter 3). Root nodules, the site of N2-fixation, had 1--2 orders of magnitude higher concentrations of elements relevant to fixation and plant nutrition, suggesting these structures are resource "hot spots". However, I observed an overall trend for lower N 2 fixation rates in both CO2 treatments over time (Chapter 3). The Florida experiment also demonstrated that CO2 enrichment causes lower concentrations of potentially toxic elements in oaks ( Quercus myrtifolia), but due to the large growth response of these plants to high CO2, they accumulate potentially toxic elements in biomass, with corresponding declines in the extractable element pools in surface soils.