Rj. Stevens et al., MEASURING THE CONTRIBUTIONS OF NITRIFICATION AND DENITRIFICATION TO THE FLUX OF NITROUS-OXIDE FROM SOIL, Soil biology & biochemistry, 29(2), 1997, pp. 139-151
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.