Decline of arbuscular mycorrhizal fungi in a northern hardwood forest exposed to long-term increased nitrogen inputs.
van Diepen, Linda*,1, Lilleskov, Erik2, Pregitzer, Kurt1, 1 Michigan Technological University, Houghton, MI2 North Central Research Station, USDA Forest Service, Houghton, MI
ABSTRACT- Human activities have dramatically increased atmospheric nitrogen (N) deposition within large regions of the world. Within the Michigan Gradient Study there is a unique opportunity to explore the effects of long term (>10 years) increased N-fertilization in Acer saccharum dominated northern hardwood forests. Previous results have shown a change in carbon allocation across sites; specifically N-amended plots showed decreased soil respiration and increased aboveground tree growth. The decreased soil respiration is not explained by changes in root biomass or respiration, therefore current studies are investigating the response of saprotrophs and mycorrhizal fungi. Mycorrhizal fungi provide nutrients and water to plants in return for carbon. An N-mediated change in carbon allocation belowground, and changes in soil N availability could therefore affect the abundance and function of mycorrhizae. The abundance of arbuscular mycorrhizal (AM) fungi in sugar maple fine roots was measured by staining and by extracting phospholipid fatty acids and analyzing for the AM indicator fatty acid 16:15 in phospholipid (biomass indicator) and neutral lipid (lipid storage indicator) fractions. There was a decline in AM fungal biomass and lipid storage in response to N addition at multiple sites measured by fatty acid analysis as well as by percent total AM and vesicle colonization. Total AM colonization had a significant positive relationship with PLFA 16:15 and vesicle colonization with NLFA 16:15. These observed changes suggest either reduced carbon allocation to, or a soil N-mediated decline of these fungi.
Key words: Nitrogen deposition, Arbuscular mycorrhizae, Northern hardwood forest
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