Can a mixed stand of N2-fixing and non-fixing plants restrict N2O emissions with increasing CO2 concentration?

Initial effects of elevated atmospheric CO2 concentration on N2O fluxes and biomass production of timothy/red clover were studied in the laboratory. The experimental design consisted of two levels of atmospheric CO2 (ca. 360 and 720 μmol CO2 mol−1) and two N fertilisation levels (5 and 10 g N m−2). There was a total of 36 mesocosms comprising sandy loam soil, which were equally distributed in four thermo-controlled greenhouses. In two of the greenhouses, the CO2 concentration was kept at ambient concentration and in the other two at doubled concentration. Forage was harvested and the plants fertilised three times during the basic experiment, followed by harvest, a fertilisation with the double amount of nitrogen and rise of water level. Under elevated CO2, harvestable and total aboveground dry biomass production of a mixed Trifolium/Phleum stand was increased at both N treatments compared to ambient CO2. The N2O flux rates under ambient CO2 were significantly higher at both N treatments during the early growth of mixed Phleum/Trifolium mesocosms compared to the N2O flux rate under elevated CO2. However, when the conditions were favourable for denitrification at the end of the experiment, i.e. N availability and soil moisture were high enough, the elevated CO2 concentration enhanced the N2O efflux.