The influence of nitrogen source on the nutrition of arbuscular mycorrhizal plants

Arbuscular mycorrhizal fungi (AMF) form a symbiosis with 70% of land plants and may contribute significantly to plant phosphorus (P) and nitrogen (N) acquisition in exchange for photosynthetic carbon. AMF mainly acquire N as simple, inorganic molecules such as ammonium and nitrate, but whether AMF display a preference for either source, or can increase plant N content is uncertain. Furthermore, soil nutrient availability, especially that of N, may determine the extent to which a fungus engages in nutrient-for-C trade with plants; further experimental validation of this suggestion is warranted. Experiments ranging from Petri plate microcosms to field trials were carried out to address these questions.
Plant N uptake via AMF was traced using stable isotope 15N, having been added to hyphal-only compartments which allow access to AMF, while excluding plant roots. AMF species Glomus aggregatum and Rhizophagus irregularis were used individually to inoculate plants allowing comparisons between AMF isolates, while N uptake preferences were tested by providing 15N as ammonium, nitrate or ammonium nitrate.
R. irregularis contribution to plant nutrient uptake was sufficient to increase host biomass in chapter 2, but limited evidence of AMF preference for nitrate or ammonium-N was seen in any experiment. N acquisition by AMF was highest in a Petri microcosm experiment when supplied as ammonium nitrate.
Functional diversity among AMF species was observed in terms of plant growth and nutrient uptake, and carbon acquisition from the plant: G. aggregatum was less beneficial to plant partners than R. irregularis. Experimental field-trial data suggests that soil nutrient levels influence both the community structure of AMF and the extent to which they engage in N-for-C trade with plant partners. These findings highlight the complexities and potential significance of the AM route for N uptake both in simplified experimental systems and full-scale commercial crop field trial plants.