A novel dual-isotope labelling method for distinguishing between soil sources of N(2)O

We present a novel (18)O-(15)N-enrichment method for the distinction between nitrous oxide (N(2)O) from nitrification, nitrifier denitrification and denitrification based on a method with single- and double-(15)N-labelled ammonium nitrate. We added a new treatment with (18)O-labelled water to quantify N(2)O from nitrifier denitrification. The theory behind this is that ammonia oxidisers use oxygen (O(2)) from soil air for the oxidation of ammonia (NH(3)) but use H(2)O for the oxidation of the resulting hydroxylamine (NH(2)OH) to nitrite (NO(2)(-)). Thus, N(2)O from nitrification would therefore be expected to reflect the (18)O signature of soil O(2), whereas the (18)O signature of N(2)O from nitrifier denitrification would reflect that of both soil O(2) and H(2)O. It was assumed that (a) there would be no preferential removal of (18)O or (16)O during nitrifier denitrification or denitrification, (b) the (18)O signature of the applied (18)O-labelled water would remain constant over the experimental period, and (c) any O exchange between H(2)(18)O and NO(3)(-) would be negligible under the chosen experimental conditions. These assumptions were tested and validated for a silt loam soil at 50% water-filled pore space (WFPS) following application of 400 mg N kg(-1) dry soil. We compared the results of our new method with those of a conventional inhibition method using 0.02% v/v acetylene (C(2)H(2)) and 80% v/v O(2) in helium. Both the (18)O-(15)N-enrichment and inhibitor methods identified nitrifter denitrification to be a major source of N(2)O, accounting for 44 and 40%, respectively, of N(2)O production over 24 h. However, compared to our (18)O-(15)N-method, the inhibitor method overestimated the contribution from nitrification at the expense of denitrification, probably due to incomplete inhibition of nitrifier denitrification and denitrification by large concentrations Of 02 and a negative effect of C(2)H(2) on denitrification. We consider our new (18)O-(15)N-enrichment method to be more reliable than the use of inhibitors; it enables the distinction between more soil sources of N(2)O than was previously possible and has provided the first direct evidence of the significance of nitrifier denitrification as a source of N(2)O in fertilised arable soil. Copyright (c) 2005 John Wiley & Sons, Ltd.