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Harvard Forest Symposium Abstract 2009

  • Title: Abiotic immobilization of nitrite in forest soils: a double label approach
  • Primary Author: Richard MacLean (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 different systems will react to anthropogenic N addition. One possible mechanism which may further complete our understanding of the N cycle is abiotic immobilization of nitrate (NO3-). Previous studies have demonstrated abiotic uptake of 15N from added 15NO3- in sterilized soils, a technique which has been criticized for sterilization’s alteration of the soil chemistry. The study sought to investigate the possibility of detecting abiotic immobilization without sterilizing the soil. Addition of double labeled nitrate (15N18O3-) allows differentiation of abiotic and biotic immobilization of NO3- in soil without sterilization. During biotic uptake of NO3- all O is cleaved to water, only the N is assimilated into the microbial biomass during uptake. In abiotic immobilization N is retained in the dry portion of the soil as an R–N, R-NO, or R–NO2 group. The original O associated with the nitrate is retained and allows the 18O to act as an indicator of abiotic immobilization. The 15N label was included should abiotic immobilization not be occurring in the soil, and to provide an estimate of total nitrate immobilized. Given the untested nature of this double label method and the cost of the 15N18O3-, soil samples from the Harvard Forest pine plantation (Pinus rubra) were used in laboratory incubations. Soil samples were treated with 0.070mg N/g soil and incubated for 15 min, 1 h, and 4 h, under ambient atmosphere or sealed and flushed with N2 for an anoxic treatment. The soils were then rinsed of extractable N by KCl and DI rinses, freeze dried, and analyzed in a TCEA mass spectrometer for O isotopes and an EA mass spectrometer for C and N isotopes. A multiple factor linear regression revealed a significant retention of 18O in the labeled treatments, demonstrating abiotic immobilization occurred in these soils. Of the experimental factors included in the regression, time incubated, an oxic/anoxic atmosphere, and soil subplot, only soil subplot and its interaction with atmosphere were significant factors for retention of added 18O. That time was not a significant factor in this experiment agreed with previous observations of rapid 15NO3- immobilization at the Harvard Forest using 15NO3-. The experiment has demonstrated abiotic immobilization without the use of sterilization and the related chemical alterations of the soil, and suggests that this method would be viable for in situ studies.

  • Research Category: Soil Carbon and Nitrogen Dynamics