Stimulated N₂O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO₂

Long-term exposure of native vegetation to elevated atmospheric CO₂ concentrations is expected to increase C inputs to the soil and, in ecosystems with seasonally dry periods, to increase soil moisture. We tested the hypothesis that these indirect effects of elevated CO₂ (600 μl l^-1 vs 350 μl l^-1) would improve conditions for microbial activity and stimulate emissions of nitrous oxide (N₂O), a very potent and long-lived greenhouse gas. After two growing seasons, the mean N₂O efflux from monoliths of calcareous grassland maintained at elevated CO₂ was twice as high as that measured from monoliths maintained at current ambient CO₂ (70 ± 9 vs 37 ± 4 μg N₂O m^-2 h^-1 in October, 27 ± 5 vs 13 ± 3 μg N₂O m^-2 h^-1 in November after aboveground harvest). The higher N₂O emission rates at elevated CO₂ were associated with increases in soil moisture, soil heterotrophic respiration, and plant biomass production, but appear to be mainly attributable to higher soil moisture. Our results suggest that rising atmospheric CO₂ may contribute more to the total greenhouse effect than is currently estimated because of its plant-mediated effects on soil processes which may ultimately lead to increased N₂O emissions from native grasslands.