Publications

This chapter discusses the mechanisms through which elevated CO2 can cause changes in soil nitrogen cycling. Rising atmospheric CO2 could alter soil nitrogen (N) cycling, shaping the responses of many terrestrial ecosystems to elevated CO2. Increased carbon input to soil through increased root growth, altered litter quality, and increased soil water content through decreased plant water use in elevated CO2 can all affect soil N transformations and thus, N availability to plants. Nitrogen limits net primary productivity (NPP) in many terrestrial ecosystems, so changes in N availability to plants will influence NPP in an elevated CO2 environment. Furthermore, changes in NPP will alter carbon uptake by the terrestrial biosphere, and thus, feed back to rising atmospheric CO2. Elevated CO2 could also influence the processes that regulate N inputs to and losses from ecosystems —N fixation, gaseous N losses, and N leaching. Such changes could alter ecosystem nitrogen stocks and thus, nitrogen available to support NPP. Additionally, soil emissions of N20 contribute to the greenhouse effect and stratospheric ozone destruction, and emissions of NOx contribute to photochemical smog and acid rain. Thus, by altering soil nitrogen cycling, elevated CO2 could cause other changes in atmospheric chemistry.

We report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO₂ during the first 14 months of experimental treatment. Elevated CO₂ stimulated above-ground growth, nitrogen mass, and root nodule production of the nitrogen-fixing vine, <em>Galactia elliottii</em> Nuttall. During this period, elevated CO₂ reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO₂ did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH₄⁺ immobilized per unit microbial N) measured over a 24-h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO₂ best explains these changes. These results demonstrate that atmospheric CO₂ concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long-term feedbacks to the growth responses of plants to elevated CO₂. Greater nitrogen fixation and reduced leaching could stimulate nitrogen-limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant-available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time.

Most studies on the effects of elevated CO₂ have focused on the effects on plant growth and ecosystem processes. Fewer studies have examined the effects of elevated CO₂ on herbivory, and of these, most have examined feeding rates in laboratory conditions. Our study takes advantage of an open-top CO₂ fertilization study in a Florida scrub-oak community to examine the effects of elevated CO₂ on herbivore densities, herbivore feeding rates, and levels of attack of herbivores by natural enemies. Higher atmospheric CO₂ concentration reduced plant foliar nitrogen concentrations, decreased abundance of leaf-mining insect herbivores, increased per capita leaf consumption by leafminers, and increased leafminer mortality. As suggested by other authors, reduced foliar quality contributed to the increase in herbivore mortality, but only partly. The major factor increasing mortality was higher attack rate by parasitoids. Thus increasing CO₂ concentrations may reduce the survivorship of insect herbivores directly, by reducing plant quality, but also indirectly, by changing herbivore feeding and eliciting greater top-down pressure from natural enemies.