Influence of atmospheric CO2 enrichment on nitrous oxide flux in a temperate forest ecosystem


  • Rebecca L. Phillips,

  • Stephen C. Whalen,

  • William H. Schlesinger


Long-term exposure of native vegetation to elevated atmospheric carbon dioxide (CO2) is expected to increase the water content and the input of labile carbon (C) to soil, which could stimulate nitrification and denitrification and enhance nitrous oxide (N2O) emissions. We measured N2O fluxes for 2 years in a Pinus taeda forest that was continuously enriched 200 μL L−1 CO2 above the ambient atmospheric CO2 concentration (∼560 μL L−1) beginning 16 months prior to our study. Soil treated with elevated CO2 showed higher N2O emissions at low winter temperatures than the ambient CO2 control. Conversely, soil treated with elevated CO2 showed lower N2O emissions at high summer temperatures than the control soil. Annual N2O fluxes, however, were similar between treatments (∼6600 μg m−2). Factors that influence denitrification and N2O production were investigated in the laboratory using intact soil core incubations. Nitrate additions (0.17 mg KNO3−N g−1 ) to intact soil cores during laboratory incubations stimulated total N2O production as well as denitrification in both treatments, whereas glucose additions lowered N2O production in both treatments. These experiments demonstrated that N2O production is strongly limited by available nitrogen (N) and that the addition of labile C is likely to reduce the amount of N2O produced by nitrification. Our results collectively suggest that CO2 enrichment of this N-limited ecosystem may reduce N2O flux during the growing season, when soil C inputs and plant-microbial competition for NH4+ are high. Alternatively, elevated CO2 may enhance N2O flux in the winter, when conditions are moist and cold and plants are less active. The potential indirect effects of CO2 enrichment (greater soil moisture and labile C inputs) could reduce N2O flux from nitrification in summer and enhance N2O flux from denitrification in winter, resulting in no net change in total ecosystem N2O flux at the soil-atmosphere interface.