UBC Theses and Dissertations

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UBC Theses and Dissertations

Interesting and unusual flux patterns of nitrogen and carbon in pinus contorta germinants colonized by the ectomycorrhizal fungus suillus tomentosus Smith, Joshua Michael


The most commonly known characteristics of the ectomycorrhizal symbiosis are translocation of soil nitrogen by fungi to plant roots, and the allocation of plant carbon to the fungi. Here, I report on observations that contradict these well-known ecophysiological pathways: movement of nitrogen from the plant toward the fungus, and the fungus behaving saprotrophically while in symbiosis. In both experiments for this thesis mycorrhizal Pinus contorta and Suillus tomentosus were used. In the first experiment, these organisms were in low-nitrogen microcosms where some nitrogen was accessible only by hyphae. After 73 days, the hyphal nutrient medium was replaced with water or fresh nitrogen solutions. Forty-eight hours after nutrient manipulations, shoot nitrogen concentrations had dropped by 70% in microcosms where nitrogen was added to the hyphal compartment. These seedlings contained only 55% of the nitrogen present in the seed. Loss of nitrogen did not occur if water was added to the hyphal compartment or if hyphal connections to the fresh nitrogen were severed prior to nitrogen addition. Because severing of hyphae prevented loss of nitrogen I speculate that seedling nitrogen was translocated to S. tomentosus. If a similar effect occurs in the field with germinants, the nitrogen would be especially beneficial to ectomycorrhizal fungi colonizing from spores after wildfire, a scenario typical for lodgepole pine. For the second experiment I used microcosms where the substrate was amended with soil organic carbon (SOC) that was at natural abundance or 13C-enriched. After 148 days of growth, both the CO₂ respired by a combination of plant plus fungus, and that respired by S. tomentosus alone, were significantly enriched in 13C over natural abundance levels. An isotope mixing model was used to determine that 35% of the carbon respired by the fungus came from the SOC. This indicates that S. tomentosus possesses the ability to behave saprotrophically while in symbiosis to supplement its carbon supply. Saprotrophy by ectomycorrhizal fungi is currently a debated topic in our field.

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