An increased ratio of fungi to bacteria indicates greater potential for N2O production in a grazed grassland exposed to elevated CO2

Nitrous oxide (N2O) is an important greenhouse gas and emissions of N2O have been shown to increase under elevated CO2 (eCO2) resulting in a positive feedback on climate change. CO2-driven increases under grassland have often been associated with greater N2O 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 CO2. 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 N2O production using denitrification enzyme activity (DEA). We found greater gene abundance of fungi under eCO2 and reduced bacterial gene abundance. N2O DEA was dominated by fungi in both ambient and elevated CO2. Total potential N2O emissions were 49% higher under eCO2 entirely due to greater emissions from the fungal component. An increasing fungal contribution to N2O emissions presents a challenge to mitigation as, to date, mitigations have largely been targeted at bacteria.