Measuring the contributions of nitrification and denitrification to the flux of nitrous oxide from soil

The flux of N 2O from soil can be due to nitrification or denitrification. 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 denitrification to the flux of N...

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Bibliographic Details
Published in:Soil biology & biochemistry 1997-02, Vol.29 (2), p.139-151
Main Authors: Stevens, R.J., Laughlin, R.J., Burns, L.C., Arah, J.R.M., Hood, R.C.
Format: Article
Language:English
Subjects:
Agronomy. Soil science and plant productions
Biological and medical sciences
Chemical, physicochemical, biochemical and biological properties
Fundamental and applied biological sciences. Psychology
Organic matter
Physics, chemistry, biochemistry and biology of agricultural and forest soils
Soil science
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Summary:The flux of N 2O from soil can be due to nitrification or denitrification. 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 denitrification to the flux of N 2O can be studied by differentially 15N-labelling the NO 3 − and NH 4 + pools in soils. By periodically measuring and comparing the enrichments of the N 2O, NH 4 + and NO 3 − pools, the relative importance of the two processes can be quantified. The conclusions are based on calculations which assume that the 15N atom fractions of the nitrification and denitrification pools remain uniform throughout the incubation. The initial uniformity of the denitrification pool was tested by adding a nitrification-inhibitor, C 2H 2, at time zero and examining the 15N-distribution of the accumulated N 2O at subsequent times. If the 15N distribution in the N 2O is random it originated from one source, but ifthe 15N distribution is non-random the N 2O originated from two or more sources. Two soil incubation experiments were conducted. In the first experiment fresh sieved soil was incubated over 10 days at 40, 50 and 60% moisture content with (NH 2) 2CO (70 μmol N g −1) and KNO 3 (14 μmol N g −1) differentially labelled at 10 atom% excess 15N. The headspace was sampled daily for N 2O before being refreshed with normal air. Every second day the sizes and enrichments of the NH 4 + and NO 3 − pools were determined by destructive sampling. In the second experiment the assumption that the method of addition of label created only one denitrifying pool was tested by blocking nitrification with C 2H 2 (10 kPa). Fresh soil was incubated for three incubation times (6, 12 and 24 h) with differentially-labelled NH 4NO 3 (1.46 μmol N g −1) enriched to 20 atom% excess 15N, with glucose (42 and 83 μmol C g −1) to promote denitrification. In the first experiment the enrichment of the N 2O did not match either the enrichment of the NH 4 + or NO 3 − pools, showing that N 2O was being produced by nitrification and denitrification. Quantification of the fractional contributions of nitrification and denitrification showed that denitrification was the dominant process in the first 2 days, but then nitrification became the dominant process for the rest of the incubation. More N 2O was produced at 50 and 60% moisture than at 40% moisture, but the relative contributions of the two processes were the same
ISSN:0038-0717
1879-3428
DOI:10.1016/S0038-0717(96)00303-3