Pristine old-growth forest experiences decrease in mycorrhizal abundance following inorganic N fertilization.
Lucas, Richard*,1, Vann, David2, Casper, Brenda1, 1 Biology Department, Philadelphia, PA, USA2 Department of Earth and Environmental Science, Philadelphia, PA, USA
ABSTRACT- Temperate forest growth is thought to be limited by nitrogen (N) availability. Traditional thought has centered around inorganic N availability but it is becoming clear that trees can gain access to pools of organic N through associations with mycorrhizal fungi. The importance of the organic N pool remains unknown. Pristine, old-growth forests that maintain high productivities despite having extremely low N mineralization rates and negligible anthropogenic N inputs, such as those in the Parque Nacional de Chiloé in southern Chile, provide an excellent place to test the hypothesis that mycorrhizal abundance would be decreased in areas receiving inorganic N fertilization. In the presence of increased inorganic N, we hypothesized that trees would experience a release from the need to allocate photosynthate to mycorrhizal fungi and thus mycorrhizal abundance would decrease. This study used a subsurface microscopic camera system which was fashioned from a QX3® toy digital microscope and a TV walkman© to make in situ measures of mycorrhizal abundance. Six paired plots were selected in a mixed evergreen angiosperm-gymnosperm forest. Two minirhizotron tubes were installed in each plot and roots and hyphae were allowed to colonize for one year before image capture and N fertilization began. Fifty kg ha-1 of NH4H2CO3 was added semi-annually to one plot of each pair over three years while the other plot served as a control. We calculated hyphal lengths in each image using WinRHIZO© Pro 2003b. Fertilized plots had 9.1 mm (1.7 se) of hyphae which was significantly lower (p = 0.0156) than the 14.7 mm (1.5 se) of hyphae that were found in unfertilized plots. Our results support our hypothesis and are consistent with the idea that plants pay fungi for access to N when it is otherwise unavailable.
Key words: mycorrhizae, inorganic nitrogen, hyphal length, minirhizotron
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