Arbuscular mycorrhizae selectively suppress soil ammonia oxidizers – but not via constitutive production of biological nitrification inhibitors

Abstract Aims : Arbuscular mycorrhizal (AM) fungi forming root symbioses can improve acquisition of nutrients including nitrogen (N) from soils, which could lead to host plant growth improvements. We previously showed that application of AM fungal necromass to the rhizosphere could stimulate plant growth by as yet unknown mechanism. Here we tested how the effects on plant N could be explained by interactions between AM fungi, living or dead, and one important group of soil prokaryotes in soil N cycling, the ammonia oxidizers (AO) carrying out the first step of nitrification. We were interested in whether the necromass contained any significant biological nitrification inhibitory (BNI) activity that could explain previously suggested antagonism between the AM fungi and the AO. Methods : To this end we set up a compartmented pot experiment with Andropogon gerardii as a host plant, which included rhizosphere (amended with living 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 fungi generally enhanced plant biomass and nutrient acquisition, including the 15 N transfer from mesh bags to the plants, while significantly suppressing AO bacterial but not archaeal populations in the mesh bags. In contrast, the amendment with AM fungal necromass had minimal effect on plant biomass and nutrient acquisition, but actually stimulated both AO bacteria and archaea in the rhizosphere of non-mycorrhizal plants. Conclusions : Thus we reject the possibility that AM fungi suppress the AO via an elusive BNI production but rather the substrate (ammonium) competition.