Carbon Mobility and Nitrogen Transformation in Forested and Agricultural Ecosystems.

The potential for terrestrial carbon sequestration under forested and agricultural management is being explored as a means to mitigate greenhouse gas emissions. Urea fertilizer is believed to result in hydrolysis and solubilization of carbon compounds and to thereby enhance the subsurface mobilization of carbon. However, urea degradation may liberate nitrous oxide, a potent greenhouse gas, thus negating any positive impacts of carbon sequestration. Nonreactive tracer bromide and urea fertilizer were applied to a 2 m x 2 m x 3 m instrumented pedon in a forested watershed in Oak Ridge, TN in the spring of 2008. Sampling typically occurred during stormflow under wet conditions, and controlled irrigation was applied during dry conditions. Previously, several years of background data were collected in the absence of applied urea and Br. The liberation of gaseous nitrogen compounds were monitored above the soil surface using non-flow through, non-steady state chambers. Soluble carbon, sulfate, phosphate, pH, Br, ammonia, and nitrate, were monitored in subsurface fritted glassware solution samplers designed to collect flow from either macropores/mesopores (30 cm bubbling pressure) or micropores (500 cm bubbling pressure). This strategy allows identification of compounds actively mobilizing through preferential flowpaths (macropores) versus compounds becoming sequestered into the matrix (micropores) of the soil. Water content and matric potential were monitored within several depths within the pedon using TDR and tensiometers, respectively. The experiment will assess the potential for enhanced subsurface carbon mobilization and sequestration associated with urea application. Outcomes will be compared with preliminary results from an experiment in which ammonium nitrate and Br were applied to switchgrass production fields, and in which the migration of carbon and nitrogen compounds were monitored in the subsurface.