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Harvard Forest Symposium Abstract 2008

  • Title: Abiotic Immobilization of Nitrate in Forest Soils: a Double Label Approach
  • Primary Author: Richard MacLean (University of New Hampshire - Main Campus)
  • Additional Authors: Bryan Dail (University of Maine); Serita Frey (University of New Hampshire - Main Campus); Scott Ollinger (University of New Hampshire - Main Campus)
  • Abstract:

    Mechanisms of soil nitrogen (N) retention remain a key uncertainty in the terrestrial N cycle. During recent work at the Harvard Forest Chronic N Experiment, researchers observed rapid immobilization of 15N added to soils as ammonia nitrate on a time scale of minutes. They hypothesized that abiotic immobilization of ammonia and nitrate could explain the rapid scale of the immobilization. The study of abiotic immobilization of nitrate in soil has been ongoing since the first half of the 20th century, mainly using ideal synthesized compounds and soil sterilization techniques. However, critics of these studies argue that while in vitro studies may indicate the possibility of an abiotic reaction, they cannot demonstrate its plausibility in soils. Critiques of soil sterilization include the inability to eliminate all microbial action, and leaving the drastic departure from natural conditions. Both of these techniques have used 15N to help determine the destination of added nitrate, but the isotopic label does not help differentiate between abiotic and biotic products. This study is an attempt to investigate abiotic immobilization of nitrate in soil samples through the use of double labeled nitrate (15N18O3-). The resolution to differentiate between biotic and abiotic products with this method depends on the biochemistry of microbial immobilization of nitrate; reduction of nitrate to nitrite, then ammonia and glutamine before incorporation into microbial biomass. Reduction of 15N18O3- before microbial utilization of the 15N implies that retention of both heavy isotopes in the solid soil component can only occur through abiotic reaction of 15N18Ox species. In biotic immobilization, the 18O is lost to the system in water. While nitrate has proven unreactive in soils, its reduced product, nitrite, is known to be readily reactive with various soil compounds. Nitrite can be introduced into the soil environment naturally by both ‘leakiness’ in nitrification and denitrification and may possibly be generated abiotically through methods such as the proposed Ferrous Wheel hypothesis. Samples of the O-horizon of Harvard Forest red pine soils were incubated at several short intervals (15 m, 1 h, 4 h), based on previous observations made at Harvard Forest, under both anoxic and oxygenated conditions. Following incubation, a KCl extraction of available N, a DI rinse of the KCl, freeze-drying and grinding of the samples, the isotopic enrichment was determined by EA/TCEA IRMS. Preliminary results show a significant enrichment in 15N and a small but significant enrichment in 18O, indicating that a portion of the nitrate has undergone abiotic immobilization in these soil samples. Full results are still pending sample analysis and will not be available at the time of the symposium.


    Richard W. MacLean1, Scott V. Ollinger1, Erik A. Hobbie1, Serita D. Frey2, D. Bryan Dail3

    1 Complex Systems Research Center, University of New Hampshire, Durham, NH.

    2 Department of Natural Resources, University of New Hampshire, Durham, NH.

    3 Department of Plant, Soil, and Environmental Sciences, University of Maine, Orono Maine.

  • Research Category: Soil Carbon and Nitrogen Dynamics