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Measurement of δ15N and δ18O in Nitrate from Soil Water Samples

Iris Schmiedinger1, Sebastian Haas1, Michael Thoma1, Gebauer Gerhard1
1 BayCEER - Laboratory of Isotope Biogeochemistry, University of Bayreuth

P 3.12 in Ecosystems: Function and Services

Excessive amounts of nitrate in terrestrial ecosystems raise a couple of serious environmental hazards. Nitrate leaching from soils contributes to groundwater and therefore drinking water pollution. Surface runoff of nitrate causes eutrophication of aquatic ecosystems. And nitrate is the substrate for microbial denitrification ending up with the emission of the greenhouse gas nitrous oxide. The isotopic composition of nitrate can be used to identify and quantify the origin and fate of nitrate. In principle stable isotope signatures of nitrate contain two categories of information:

  1. A source/sink (tracer) information that can be used to quantify the mixing of certain nitrate sources (e.g. nitrate from atmospheric deposition and fertilizer derived nitrate)
  2. A process information that originates from characteristic isotope fractionations during (bio)chemical nitrate conversion reactions (e.g. microbial denitrification in soils)

Though the application of nitrate stable isotope natural abundance has already a fairly long history (e.g. Durka et al. 1994) a break through towards broad applications was decelerated by either rather laborious or costly sample preparation techniques. This limitation may be overcome in future thanks to a new nitrate preparation procedure suggested by Huber et al. (2011). This procedure is based on the different solubilities of inorganic salts in an acetone/hexane/water mixture and requires only small sample amounts. So far this procedure has been successfully tested for the preparation of nitrate from freshwater samples. However, in-depth going investigations on the fate and origin of nitrate require also the preparation and analysis of nitrate from more complex matrices, like soil water extracts.

We tested the nitrate preparation procedure proposed by Huber et al. (2011) for forest soil water samples gained through suction cup extraction from three different soil depths and modified the preparation technique in various points. The original soil water samples with a minimum nitrate amount of 10 µmol up to 30 µmol (determined by anion exchange HPLC and UV detection) were freeze dried. After the Huber et al. (2011) preparation procedure we dried the BaNO3 in 2 ml PP-centrifugation tubes again and measured δ15N with EA-IRMS and δ18O with TC-IRMS. Here we present first results of nitrate δ15N and δ18O values along soil profiles. To further reduce the required nitrate sample amount down to 1µmol were currently elaborate a simultaneous δ15N and δ18O analysis method by TC-IRMS as described by Gehre and Strauch (2003).


Durka W., Schulze E.-D., Gebauer G., Voerkelius S. (1994) Effects of forest decline on uptake and leaching of deposited nitrate determined from 15N and 18O measurements. Nature 372: 765-767.

Huber B., Bernasconi S.M., Luster J., Graf Pannatier E. (2011) A new isolation procedure of nitrate from freshwater for nitrogen and oxygen isotope analysis.

Gehre M., Strauch G. (2003) High-temperature elemental analysis and pyrolysis techniques for stable isotope analysis.

last modified 2012-09-28