Arbuscular mycorrhizal fungi reduce soil NO emissions by altering root traits and soil denitrifier community composition.

Arbuscular mycorrhizal fungi (AMF) increase the ability of plants to obtain nitrogen (N) from the soil, and thus can affect emissions of nitrous oxide (NO), a long-lived potent greenhouse gas. However, the mechanisms underlying the effects of AMF on NO emissions are still poorly understood, particularly in agroecosystems with different forms of N fertilizer inputs. Utilizing a mesocosm experiment in field, we examined the effects of AMF on NO emissions via their influence on maize root traits and denitrifying microorganisms under ammonia and nitrate fertilizer input using N isotope tracer. Here we show that the presence of AMF alone or both maize roots and AMF increased maize biomass and their N uptake, root length, root surface area, and root volume, but led to a reduction in NO emissions under both N input forms. Random forest model showed that root length and surface area were the most important predictors of NO emissions. Additionally, the presence of AMF reduced the (nirK + nirS)/nosZ ratio by increasing the relative abundance of nirS-Bradyrhizobium and Rubrivivax with ammonia input, but reducing nosZ-Azospirillum, Cupriavidus and Rhodopseudomonas under both fertilizer input. Further, NO emissions were significantly and positively correlated with the nosZ-type Azospirillum, Cupriavidus and Rhodopseudomonas, but negatively correlated with the nirS-type Bradyrhizobium and Rubrivivax. These results indicate that AMF reduce NO emissions by increasing root length to explore N nutrients and altering the community composition of denitrifiers, suggesting that effective management of N fertilizer forms interacting with the rhizosphere microbiome may help mitigate NO emissions under future N input scenarios.