Effects of elevated atmospheric carbon dioxide on soil nitrogen availability

Vegetation and soils may sequester a portion of the CO2 that is accumulating in Earth's atmosphere if elevated CO2 stimulates plant growth on a widespread scale. The effect of elevated CO2 on plant growth depends in part on soil nutrient availability, which may be altered by the effects of elevated CO2 on plants. Elevated CO 2 could stimulate soil microbial activity and biomass production if it increases the availability of organic carbon to microbes or if it reduces plant water use and thereby reduces drought stress on soil microbes. Increased soil microbial activity and biomass could increase soil nitrogen availability to plants by accelerating the release of nitrogen from soil organic matter (i.e., nitrogen mineralization).; To test the hypothesis that low C availability limits the activity and biomass of soil microbes in North Carolina piedmont loblolly pine forests, I performed an assay of C, N, and P limitation of microbial respiration and biomass in mineral soil and forest floor samples collected in three loblolly pine stands near Durham, North Carolina. To test the hypothesis that elevated CO2 Will stimulate microbial activity and biomass, I measured microbial biomass, nitrogen mineralization and soil moisture in three experiments in which plants were exposed to elevated CO2.; I found that microbial activity and the size of the biomass carbon pool in the mineral soil under loblolly pine forests were limited by the availability of organic C. Elevated CO2 caused a 40% increase in microbial biomass N on one of two dates on which I collected soil samples in an experiment in which loblolly pines were grown in open-top chambers on homogenized soil at ambient or elevated CO2 for three years. Elevated CO2 had no statistically significant effects on microbial biomass or N mineralization in an Andropogon virginicus (L.)-dominated old field or in a 13-year-old loblolly pine (Pinus taeda L.) forest, and elevated CO2 did not significantly alter soil moisture in any of the experiments. My results suggest that elevated CO2 may increase microbial biomass and accelerate soil N cycling in loblolly pine forests growing on highly disturbed soils.