COMBINING THE N-15 AND ACETYLENE INHIBITION TECHNIQUES TO EXAMINE THEEFFECT OF ACETYLENE ON DENITRIFICATION

Using acetylene to inhibit the reduction of N2O to N-2 is the simplest and most commonly used technique to quantify denitrification, The ace tylene treatment may not always inhibit N2O-reductase completely when soils are wet, low in NO3- or with a high C-to-NO3- ratio. Acetylene b locks nitrification as well as N2O-reductase, so if nitrification is t he source of NO2- or NO3- for denitrification, then the denitrificatio n rate would be underestimated by the C2H2 inhibition technique. Acety lene can also be metabolised in soil and so enhance denitrification. W e show how the N-15 method for measuring denitrification can be combin ed with the C2H2 inhibition technique to check the effect of acetylene on N2O-reductase, respiration and nitrification. No method exists, ho wever, for the analysis of N-15 in the N-2 and N2O of headspace sample s which also contain 10% v/v C2H2. We describe a technique to remove C 2H2 from headspace samples with alkaline KMnO4 prior to analysis by is otope-ratio mass spectrometry. The C2H2 removal procedure had no effec t on the determination of N2O concentration, or on the determination o f the N-15 content of N2O and N-2. Then using data for fluxes of N2O, N-2, (N2O + N-2), CO2 and the size and N-15 content of the soil NO2- p ool for a soil incubation experiment, we show how the effects of C2H2 can be quantitatively assessed. The data also allowed the amount of de nitrification, as estimated by the C2H2 inhibition technique, to be co mpared with that measured using the N-15 technique. For the soil condi tions during this incubation, 10% v/v C2H2 completely blocked N2O-redu ctase, slightly inhibited respiration and completely blocked nitrifica tion. The average total flux of N gases as measured by N-15 (4.3 nmol N g(-1) h(-1)) was not significantly different (P > 0.05) from the: av erage flux measured by the C2H2 inhibition method (3.9 nmol N g(-1) h( -1)). Including C2H2 as a treatment in the N-15 method provides a dire ct way of confirming that some of the assumptions in the C2H2 inhibiti on technique are valid. (C) 1997 Elsevier Science Ltd.