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Harvard Forest Symposium Abstract 2007

• Title: Abiotic immobilization of nitrate in forest soils: a double label approach
• Primary Author: Rich 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:

  Anthropogenic sources of reactive nitrogen (N) have become an important source of N to natural systems in the industrialized world. A thorough understanding of how N cycles through ecosystems is required to predict how a given system will react to anthropogenic N addition, yet there is still uncertainty left to be resolved. One possible mechanism, which may further complete understanding of the N cycle is abiotic immobilization. Recently, observations were made during the chronic N addition experiment at Harvard Forest. In the experiment, added 15N was rapidly immobilized, and abiotic immobilization has been suggested as the driver. The possibility of abiotic immobilization has been studied since the first half of the 20th century, mainly using ideal compounds and soil sterilization. However, critics of these studies have argued that while in vitro studies may indicate the possibility of an abiotic reaction, they cannot demonstrate its plausibility in soils and soil sterilization methods have been criticized, because they are not effective enough to eliminate biotic interactions within an experimental treatment.
  
  
  
  This study is an attempt to demonstrate abiotic immobilization of nitrate in soil samples through the use of double labeled nitrate (15N18O3-). The resolution of this method depends on the biochemistry of microbial immobilization of nitrate; reduction of nitrate to nitrite, then ammonia and glutamine before utilization. Reduction of 15N18O3- before microbial utilization of the 15N implies that retention of both heavy isotopes in the soil can only occur through abiotic reaction of 15N18Ox species. While nitrate is not very reactive with the soil, nitrite is. The product of nitrate reduction, nitrite can be introduced into the soil environment by both nitrification and denitrification and may possibly be generated abiotically through methods such as the proposed Ferrous Wheel hypothesis. Samples of the O-horizon of red pine Harvard Forest 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, KCl extraction of available N, and freeze drying, isotopic enrichment was determined by EA Mass Spectroscopy. Hypothetically, abiotic immobilization should leave N and O isotope ratios consistent with binding of the double label as nitro and nitroso groups, while biotic immobilization would cleave the labeled oxygen to produce water, and leave samples depleted in 18O. Success using this double label technique will open the door to quantification of the abiotically immobilized pool of N in both the lab and the field.
  
  
  
  Authors:
  
  
  
  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.
  
  

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