Ecosystem responses to rising atmospheric CO2: feedbacks through the nitrogen cycle
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.