Arbuscular mycorrhizal fungi reduce nitrous oxide emissions from N(2)O hotspots

Nitrous oxide (N(2)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 N(2)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 N(2)O production was measured both before and after addition of ammonium and nitrate. In both experiments, N(2)O production decreased when AMF hyphae were present before inorganic N addition. In the presence of AMF hyphae, N(2)O production remained low following ammonium application, but increased in the nonAMF controls. By contrast, negligible N(2)O was produced following nitrate application to either AMF treatment. Thus, the main N(2)O source in this system appeared to be via nitrification, and the production of N(2)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 N(2)O production.