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Theory of BaPS
Denitrification is the reduction of NO3 under anaerobic conditions, with further
reduced N-compounds succeeding and resulting in molecular Di-nitrogen [N2]
(nitrate → nitrite → NO → N2O → N2).
8.1.1 Established measuring methods
Nitrification and denitrification are of major significance for the N-circulation in
soils (see also fig.1). Still, nitrification is the process to focus on, as it is the most
important stabilisation factor of the subsequent nitrate contribution and the only
quantitatively important process of nitrate production in soils. Nevertheless, until
now the 15N-pool dilution technique is the only method to determine gross
nitrification rates in soils. This method is complex and needs an intense
expenditure for tools and staff. The knowledge of the gross rates is absolutely
necessary to make statements about actual conversion processes. When
measuring the net rate no statement about the individual processes is possible..
8.1.2
N-pool dilution technique
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With this method nitrate labelled with 15N-tracers is introduced to the soil. Then,
the time dependent gradual dilution of the 15N-pool caused by the subsequent
conversion of nitrate by nitrification is detected (15N-pool dilution) [MOS 1993;
DAV 1992]. The use of 15N and its detection with a mass spectrometer makes this
method very expensive (equipment costs, purchase of the marked substances,
required replication of the tests, regenerating the samples). Besides, the following
problems have to be considered:
1.
The soil should be as homogenous as possible to achieve a symmetrical
distribution of the marked nitrate. This would disturb the natural soil
structure.
2.
Adding 15N tracers can cause a stimulation of the microbial decay processes.
To avoid this problem it is recommended to terminate the 15N-tests within
two days.
8.1.3 Determination of the net rate
Due to the high expenditure most investigations relating to N-cycles in soils
actually do not determine the relevant gross nitrification rate, but simply the net
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