Current methodology to differentiate between N₂O production from nitrification, nitrifier denitrification and denitrification is based on the inhibitors acetylene and oxygen. As several problems are associated with this approach ¹, we developed a novel ¹⁵N, ¹⁸O enrichment approach for this purpose. This new enrichment technique was based on a method with single- and double-¹⁵N-labelled ammonium nitrate ². A new treatment with ¹⁸O-labelled H₂O was introduced. This exploits the fact that ammonia oxidisers use O₂ from soil air for the oxidation of NH₃, but H₂O for the oxidation of the resulting hydroxylamine to nitrite ^{3, 4}. Thus, N₂O from nitrification should reflect the ¹⁸O signature of soil O₂, while the O in N₂O from nitrifier denitrification can come either from soil O₂ or H₂O, depending on which O is split off in the reduction of NO₂^- to N₂O. It was assumed that a) no preferential removal of ¹⁸O or ¹⁶O takes place during nitrifier denitrification or denitrification and b) no O-exchange between H₂¹⁸O and NO₃^- takes place. These assumptions were tested and could be validated for the soil investigated. The results of the new method were compared with those of a conventional inhibition method using low concentrations of acetylene and high concentrations of oxygen ⁵. Compared to the isotope methodology, the inhibitor approach overestimated the contribution from nitrification at the expense of denitrification. Interesting shifts in N₂O-producing processes with changing WFPS and fertilizer type are discussed.

References

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3. Schmidt H-L, Voerkelius S. Isotopes in Nature, 5th working meeting: Leipzig, 1989.

4. Voerkelius S. Technical University: Munich, 1990.

5. Webster EA, Hopkins DW. Biology and Fertility of Soils 1996; 22: 331.