MEASURING THE CONTRIBUTIONS OF NITRIFICATION AND DENITRIFICATION TO THE FLUX OF NITROUS-OXIDE FROM SOIL

The flux of N2O from soil can be due to nitrification or denitrificati on. Since aerobic and anaerobic microsites can develop within the same soil aggregate, nitrification and denitrification could be occurring at the same time. The contribution of nitrification and denitrificatio n to the flux of N2O can be studied by differentially N-15-labelling t he NO3- and NH4+ pools in soils. By periodically measuring and compari ng the enrichments of the N2O, NH4+ and NO3- pools, the relative impor tance of the two processes can be quantified. The conclusions are base d on calculations which assume that the N-15 atom fractions of the nit rification and denitrification pools remain uniform throughout the inc ubation. The initial uniformity of the denitrification pool was tested by adding a nitrification-inhibitor, C2H2, at time zero and examining the N-15-distribution of the accumulated N2O at subsequent times. If the N-15 distribution in the N2O is random it originated from one sour ce, but if the N-15 distribution is non-random the N2O originated from two or more sources. Two soil incubation experiments were conducted. In the first experiment fresh sieved soil was incubated over 10 days a t 40, 50 and 60% moisture content with (NH2)(2)CO (70 mu mol N g(-1)) and KNO3 (14 mu mol N g(-1)) differentially labelled at 10 atom% exces s N-15. The headspace was sampled daily for N2O before being refreshed with normal air. Every second day the sizes and enrichments of the NH 4+ and NO3- pools were determined by destructive sampling; In the seco nd experiment the assumption that the method of addition of label crea ted only one denitrifying pool was tested by blocking nitrification wi th C2H2 (10 kPa). Fresh soil was incubated for three incubation times (6, 12 and 24 h) with differentially-labelled NH4NO3 (1.46 mu mol N g( -1)) enriched to 20 atom% excess N-15, with glucose (42 and 83 mu mol C g(-1)) to promote denitrification. In the first experiment the enric hment of the N2O did not match either the enrichment of the NH4+ or NO 3- pools, showing that N2O was being produced by nitrification and den itrification. Quantification of the fractional contributions of nitrif ication and denitrification showed that denitrification was the domina nt process in the first 2 days, but then nitrification became the domi nant process for the rest of the incubation. More N2O was produced at 50 and 60% moisture than at 40% moisture, but the relative contributio ns of the two processes were the same at all moisture contents. Nitrif ication was responsible for 70% of the N2O flux. In the second experim ent examination of the isotopic composition of the N2O showed that the N-15 atoms were randomly distributed throughout the molecules. The N2 O therefore originated from one denitrifying pool, confirming that our method of addition of label initially created one NO3- pool for denit rification. There seems to be no feasible way at present to test the u niformity of the nitrification pool. (C) 1997 Elsevier Science Ltd.