Nitrous oxide (N 2 O) is an important greenhouse gas and emissions of N 2 O have been shown to increase under elevated CO 2 (eCO 2 ) resulting in a positive feedback on climate change. CO 2 -driven increases under grassland have often been associated with greater N 2 O emitted during denitrification. We examined the soils from a Free Air Carbon Dioxide Enrichment (FACE) experiment on grassland on the west coast of the North Island of New Zealand that had received long-term exposure to elevated CO 2 . Importantly, the grassland was grazed thus representing much of the world's grassland situation and providing data for a land use that has not been well studied. We conducted soil incubations where a fungicide and bactericide were used to isolate the contribution of bacteria and fungi to potential N 2 O production using denitrification enzyme activity (DEA). We found greater gene abundance of fungi under eCO 2 and reduced bacterial gene abundance. N 2 O DEA was dominated by fungi in both ambient and elevated CO 2 . Total potential N 2 O emissions were 49% higher under eCO 2 entirely due to greater emissions from the fungal component. An increasing fungal contribution to N 2 O emissions presents a challenge to mitigation as, to date, mitigations have largely been targeted at bacteria.