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Harvard Forest Symposium Abstract 2011

  • Title: Saprotrophic fungi as a mechanism for nitrogen transport in a temperate hardwood forest
  • Primary Author: Katharine Burnham (University of New Hampshire - Main Campus)
  • Additional Authors: Serita Frey (University of New Hampshire - Main Campus)
  • Abstract:

    The Harvard Forest Chronic Nitrogen Amendment plots were established in 1988 and as part of an ongoing effort to understand how temperate hardwood ecosystems respond to chronic, long-term N deposition, the plots have been annually fertilized with 0 kg N/ha (ambient N deposition, control), 50 kg N/ha (low N), or 150 kg N/ha (high N). The purpose of this study was to determine if decomposer fungi have the ability to utilize and vertically translocate nitrogen from the soil up into decomposing litter and to determine the rate of movement. Secondary goals were to determine to what extent chronic, long-term N fertilization has affected this process.



    Litterbags containing oak, beech, and maple litter were placed in the hardwood Chronic Nitrogen Amendment plots in May 2010 and destructively sampled after 4 (off-plot control only) and 6 months (off-plot control, low N + S, high N). Litterbags were constructed from two different mesh sizes such that soil-litter hyphal connectivity was facilitated (all large mesh, 0.3 mm pore size) or restricted (large mesh on top and small mesh, 1 μm pore size, on the bottom).



    Soil was enriched with 15N to track fungal N translocation from the soil up into decomposing litter to quantify the rate of movement with mass balance. A solution containing 99.8 at. % 15N-NH4SO4 was horizontally injected into the soil under litterbags. Litterbags incubated in-situ for 7 days and were then destructively sampled and brought back to the lab for analysis. In a preliminary study (after 4 months of decomposition, off-plot control only), 15N-NH4SO4 was injected into the organic horizon at 1 cm below the soil-litter interface or into mineral soil 1 cm below the organic-mineral interface at rates to enrich the inorganic 15N concentration to 10 at. %, 50 at. %, and 100 at. %. Enrichment of 15N was expected in the decaying litter of fungal inclusion bags if fungi were actively transporting N throughout the incubation period; no enrichment was observed in any treatment and litter in fungal inclusion bags had a similar δ15N as litter in fungal exclusion bags. It was assumed that this might have been the result of a prolonged dry period throughout the summer months or the 15N-NH4SO4 solution was rapidly abiotically fixed and unavailable to biota. After 6 months of decomposition, organic soil in control, low N + S, and high N plots was enriched to 10 at. % inorganic 15N; again, litter enrichment of fungal inclusion treatments was minimal and δ15N were no different from fungal exclusion treatments. No difference was observed between N treatments. Another attempt to measure fungal N translocation is scheduled for May 2011.

  • Research Category: Soil Carbon and Nitrogen Dynamics