The role and diversity of arbuscular mycorrhizal fungi in Acer saccharum-dominated forest ecosystems under natural and nitrogen-amended conditions

Human activities have altered global nitrogen (N) deposition and fixation, through fossil fuel combustion, the use of fertilizers, and planting N-fixing legumes. Mycorrhizal fungi make up a large part of the microbial biomass in terrestrial ecosystems and play a pivotal role in plant carbon and nutrient balance, supplying nutrients, including nitrogen, to host plants in exchange for carbon. I assessed changes in the functioning of arbuscular mycorrhizal fungi (AMF) in an ecosystem exposed to elevated levels of nitrogen using four established long term research sites in northern hardwood forests dominated by sugar maple (Acer saccharum). Since 1994, 30 kg N ha-1 yr-1 has been applied in six equal increments of NaNO3 during the growing season. Intra- and extraradical AMF biomass and AMF mycelium production were decreased by N-amendment. The different methods applied to measure AMF biomass (root staining, PLFA (phospholipid fatty acid) analysis, and hyphal in-growth bags) all showed comparable results. Furthermore, total microbial biomass decreased and a change in microbial community composition was found under N-amendment. The microbial composition change was dominated by a decrease in fungal to bacterial biomass ratios. We also studied the AMF community composition by performing molecular analyses on maple roots. AMF community composition was significantly affected by N-amendment. Over 80% of all the AMF clones present in the roots were represented by seven dominant OTU's (Operational Taxonomic Units). Some of the OTU's declined in response to N-amendment, some increased and a few were unaffected by N-amendment. Last we studied AMF hyphal respiration using hyphal in-growth bags. Hyphal in-growth bag CO2 flux was not significantly decreased by N-amendment, but a trend was seen at two sites, and an average decrease of 7% was found. However, hyphal in-growth bags CO2 flux was positively related to hyphal biomass, which suggests that AMF hyphal CO2 flux is mainly controlled by the biomass of AMF mycelium. The observed decrease in AMF abundance correlated with a decrease in AMF CO2 flux suggests reduced carbon (C) allocation to these fungi or a direct soil N-mediated decline. Our observed decrease in AMF and microbial biomass together with changes in intraradical AMF and soil microbial community composition with N-amendment has the potential to substantially change both nutrient and carbon cycling within northern hardwood forests.