Increasing CO₂ concentrations have been shown to limit soil nitrogen availability in terrestrial ecosystems, thereby limiting plant growth. Because changes in nitrogen availability can affect the composition of available nitrogen forms, we examined effects of CO₂ concentrations on nitrogen levels in soil pore water. We measured nitrate, ammonium and dissolved organic nitrogen concentrations in grassland plots that were exposed to a gradient of CO₂ concentrations, ranging from 250 ppm to 500 ppm. Additionally, soils from the following three series: Austin (a mollisol), Houston (vertisol), and Bastrop (alfisol), were used in this experiment in order to determine how soil differences may impact ecosystem responses to changing CO₂ concentrations. Soil pore water samples were taken from suction lysimeters installed to a depth of 15 cm in all three soil types and across the CO₂ gradient. 

Results/Conclusions

Preliminary results show distinct differences in dissolved inorganic nitrogen concentrations among the soil types. Specifically, nitrate concentrations were lowest in the Bastrop soils (a sandy soil), and highest in the Houston soils (a clay loam). However, ammonium concentrations were greatest in the Austin soils, a clay loam. Initially no significant effect of CO₂ concentration was seen in total nitrogen concentrations, but the CO₂ effect became more pronounced as the growing season progressed, and plant demand for nitrogen increased. Our initial results indicate that the response of soil nitrogen to increasing CO₂ may differ among soil types. The results of this experiment provide a greater understanding of how soil systems will react to environmental changes, such as rising CO₂ concentrations, which is vital to effectively manage these grassland ecosystems.