Arbuscular mycorrhizal hyphae selectively suppress soil ammonia oxidizers – but probably not by production of biological nitrification inhibitors

Aims Arbuscular mycorrhizal (AM) fungi establish root symbioses that can improve acquisition of nutrients such as nitrogen (N) from soils, leading to improvements in host plant growth. Previously, we showed that applying AM fungal necromass to the rhizosphere could stimulate plant growth. Here we investigated the interactions between AM fungi (living or dead) and ammonia oxidizers (AO), a key group of soil prokaryotes involved in soil N cycling, to understand the effects of these interactions on plant N levels. We were interested in whether the necromass contained any significant biological nitrification inhibitory (BNI) activity that could account for the previously suggested antagonism between AM fungi and AO. Methods We set up a compartmented pot experiment with Andropogon gerardii as a host plant, which included rhizosphere (amended with living and/or dead AM fungal biomass) and a root-free zone. The latter contained a mesh bag with 15 N-labelled plant litter. Results Inoculation with living AM fungus enhanced plant biomass and nutrient acquisition, including increased transfer of 15 N from the mesh bags to the plants. Additionally, AO bacteria but not archaea were significantly suppressed in the mesh bags. In contrast, AM fungal necromass had minimal effect on plant biomass and nutrient acquisition. Surprisingly, it stimulated the growth of both AO bacteria and archaea in the rhizosphere of non-mycorrhizal plants. Conclusions Based on the above, we found no support for AM fungi suppressing the AO through an elusive BNI production. Rather, it seems that the main mechanism of AM fungal-AO interaction is substrate competition.