POTENTIAL FOR DENITRIFICATION UNDER CHANGING LAND USE AND CLIMATE

Based on periodic measurements of discharge and solute concentrations made at multiple streams, we have estimates of NO₃^- flux from the agricultural uplands to the coastal lagoons of Virginia’s Eastern Shore that range between 1.4 x 10⁵ and 3.8 x 10⁵ kg yr^-1 for the period 2001 to present. This flux in comparison with reported fertilizer usage indicates that about 5 to 18% of applied N reaches the lagoons. After considering plant uptake, we estimate that 70 to 90% of the remaining N is removed from the N-containing groundwater as it discharges to streams. Our earlier work has shown that in many small streams that empty into the lagoons, denitrification is responsible for the removal of the inorganic N that exists almost entirely as NO₃^-. N₂O is an intermediate in the denitrification process, and, in open systems, some fraction of the N₂O leaks from the zone of denitrification to the atmosphere where it is an important greenhouse gas. We sought to evaluate the potential effects of likely future changes that might alter the magnitude of denitrification. Experimental incubations using vertical-flow columns of NO₃^--enriched artificial groundwater moving through sediment cores collected from a stream on Virginia’s Eastern Shore were used to test the influence of temperature and NO₃^- concentration on the removal of NO₃^- and emission of N₂O during denitrification. The amount of denitrification and the amount of N₂O efflux increased with rising temperatures between 5° and 25°C but increased much more strongly with rising antecedent NO₃^- concentrations between 3.5 and 18 mg NO₃^--N L^-1. Using the temperature-response data, we estimate that an annual increase of 2°C in the stream sediment could increase N₂O flux by about 16% in these sediments. Moreover, the response to antecedent NO₃^- concentration suggests that increased N inputs through fertilizer or animal waste application at the land surface could generate 40% increase in N₂O for every 10% increase above current levels of reactive nitrogen use. Thus, the combined effects of changes in land use and climate may increase emission of N₂O to the atmosphere well beyond current levels. To reach desired climate-change outcomes along with improved water quality, the balance of NO₃^- removal and N₂O emission must be considered in any mitigation strategy.