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Harvard Forest Symposium Abstract 2010

  • Title: Fungal Translocation as a Mechanism for Nitrogen Transport in a Temperate Forest
  • Primary Author: Katharine Burnham (University of New Hampshire - Main Campus)
  • Additional Authors: Serita Frey (University of New Hampshire - Main Campus)
  • Abstract:

    Fungi have the ability to translocate nutrients from sources to sinks through their mycelia, enabling nutrient-limiting processes, such as decomposition, to occur throughout the heterogeneous soil environment. While some studies have observed the horizontal translocation of nitrogen via saprotrophic fungi, there have been few studies of vertical nitrogen movement from the soil up into the leaf litter layer. Litter decomposition studies carried out over the past century have repeatedly observed an increase in nitrogen during the initial stages of the decomposition process; however, the increased nitrogen has yet been attributed to a single or major source. As saprotrophic fungi are key constituents in the terrestrial decomposition process, I hypothesize that the increase in N of decomposing litter is largely attributed to saprotrophic fungi translocating soil N up into litter. This directed movement of N decreases the litter carbon to nitrogen ratio, enabling further stages of decomposition where nutrients are converted to usable forms. Additionally, understanding the dynamics of nitrogen cycling in northern hardwood forests is becoming increasingly important as revegetation of abandoned agricultural areas is enabling these forests to serve as C and possibly N sinks. Moreover, because atmospheric N deposition throughout the United States continues to increase, it is imperative to understand how soil microbes and forest ecosystems respond and the degree to which nutrient cycles have been altered. I hypothesize that long-term, chronic N deposition will negatively impact saprotrophic fungi, as they evolved in N-limiting environments and higher N availability may reduce the necessity of saprotrophic N recycling.

    The objectives of this study are to (1) measure the rate at which saprotrophic fungi translocate total N from organic and mineral soil to surface litter and (2) determine if and to what extent chronic N fertilization has affected fungal N movement. Secondary objectives are to (1) measure the rate of leaf litter decomposition during one year and (2) compare active fungal biomass between different N fertilization regimes.

    This experiment will take place in the Chronic Nitrogen Amendment Study, located at the Harvard Forest Long Term Ecological Research site in Petersham, Ma. The Chronic N plots were established in 1988 in a mixed-hardwood stand and have been annually fertilized at rates of 0 kg N ha-1 (control; ambient N deposition is ~10 kg N ha-1), 50 kg N ha-1 (low N), and 150 kg N ha-1 (high N). Objectives will be met by the installation of small litter baskets, constructed from plastic sandwich containers and mesh, and will contain approximately 6 g of dried, mixed-species leaf litter. Baskets will be constructed so that leaf litter is either level with the top of the O-horizon or raised ~1.5 cm above the O-horizon to prevent fungal nutrient translocation. Soil directly beneath the litter baskets will be horizontally labeled with trace amounts of inorganic isotopic 15N in the organic or mineral horizons to enable direct quantification of N moved from the soil up into the litter. The baskets will be placed throughout each treatment plot and destructively sampled at 3, 6, or 12 months, taken back to the lab, and the litter will be analyzed for fungal biomass, total N and 15N, and inorganic N and 15N. Concurrently with basket sampling, soil directly beneath baskets will be sampled and analyzed for total N and 15N and inorganic N and 15N. Quantification of N pools in litter and soil will facilitate mass-balance and N movement calculations.

  • Research Category: Large Experiments and Permanent Plot Studies, Soil Carbon and Nitrogen Dynamics