Arbuscular mycorrhizal fungi reduce nitrous oxide emissions from N₂ O hotspots

Nitrous oxide (N₂ O) is a potent, globally important, greenhouse gas, predominantly released from agricultural soils during nitrogen (N) cycling. Arbuscular mycorrhizal fungi (AMF) form a mutualistic symbiosis with two-thirds of land plants, providing phosphorus and/or N in exchange for carbon. As AMF acquire N, it was hypothesized that AMF hyphae may reduce N₂ O production. AMF hyphae were either allowed (AMF) or prevented (nonAMF) access to a compartment containing an organic matter and soil patch in two independent microcosm experiments. Compartment and patch N₂ O production was measured both before and after addition of ammonium and nitrate. In both experiments, N₂ O production decreased when AMF hyphae were present before inorganic N addition. In the presence of AMF hyphae, N₂ O production remained low following ammonium application, but increased in the nonAMF controls. By contrast, negligible N₂ O was produced following nitrate application to either AMF treatment. Thus, the main N₂ O source in this system appeared to be via nitrification, and the production of N₂ O was reduced in the presence of AMF hyphae. It is hypothesized that AMF hyphae may be outcompeting slow-growing nitrifiers for ammonium. This has significant global implications for our understanding of soil N cycling pathways and N₂ O production.