Arbuscular mycorrhizal fungi (AMF) form symbioses with plant roots and aid in nutrient absorption. While the importance of AMF in phosphorus acquisition is well established, recent studies have uncovered a potentially important role of AMF in soil nitrogen (N) acquisition as well. It’s unclear how AMF N absorption in the soil shapes other members of the N cycling community such as ammonia oxidizing bacteria and archaea (AOB and AOA) which convert ammonia into nitrate.
In this study, we tested the effects of AMF on i) soil and plant N and ii) AOA and AOB abundance within the rhizosphere and bulk soil in an indoor pot experiment. Within these we planted maize mutants (AMF-) unable to initiate AMF symbiosis and wild type maize (AMF+) that can. We planted these genotypes in soils obtained from a long-term cropping system experiment that compares a conventional corn-soy rotation receiving inorganic N-fertilizer and a diversified corn-soy-oats/alfalfa-alfalfa rotation with manure amendments, referred to as conventional and diversified treatments respectively. We hypothesized that AMF would reduce the abundance of AOA and AOB by reducing the availability of ammonium in a cropping-system dependent manner.
AMF+ plant N contents were greater (P<0.0001) than AMF- plant N contents independent of soil treatment. Bulk soil ammonium levels were higher (P<0.0001) in soil planted with AMF+ than AMF- genotypes while nitrate was lower (P<0.0001) in soils planted with the AMF+ than AMF-. In the presence of AMF- plants nitrate levels were greater (P<0.01) in conventional compared to diversified soil. In contrast, in the presence of AMF+ plants nitrate was low regardless of the soil treatment. Despite differences in soil inorganic N pool sizes, AOA and AOB abundances were unaffected by AMF. While rhizosphere AOA abundance was greater (P<0.0001) than in the bulk soil of both systems, AOB abundance was greater (P<0.0001) in the conventional soil only.
These data indicate that AMF did not lower ammonium availability but rather may have stimulated mineralization. Because soil nitrate levels were higher in the AMF+ than in the AMF- pots, AMF may have instead aided the plant in nitrate uptake. These data coincide with the lack of AMF effect on ammonia oxidizer abundances because nitrate removal would not compete with ammonia oxidizers as ammonium removal would. Overall, AMF influences were not significant enough to alter ammonia oxidizer abundance within these N-rich soils.