Environmental Engineering Reference
In-Depth Information
1 mol N 2 reduced (equivalent to 1.5 mol C 2 H 4 per mol NH 3 ). More commonly,
a conversion factor of 4 mol C 2 H 2 reduced to 1 mol N 2 reduced is used (Boddey
1987 ). Hardy et al. ( 1973 ) suggested that a conversion factor should be experimen-
tally determined for each system.
7.5.4 Nitrogen Mineralization and Nitrification
7.5.4.1 Overview
Nitrification is an aerobic process and N mineralization is more rapid under aerobic
conditions than under anaerobic conditions. Field assays for estimating net rates
for N transformations were developed for upland conditions. However, they are
appropriate for some types of wetlands if care is taken so that the incubation period
does not correspond to large fluctuations in water table depth. The results will be
most reliable if soil moisture content is relatively constant during the incubation
period. They are not appropriate for situations of continuous soil saturation as the
rates of these transformations will be too low to be accurately measured.
Precise estimates of N mineralization or nitrification in the field are difficult to
obtain and beyond the scope of most field exercises. Nitrate can be removed from
the soil by plant assimilation, leaching, and dissimilatory nitrate reduction includ-
ing denitrification. Plant uptake can be eliminated by using some type of soil
containment device that keeps roots away from the sampled soil. Denitrification
and other dissimilatory nitrate reduction effects can be minimized by avoiding
saturated soil. Leaching effects can be minimized by preventing percolation during
the incubation period. However, N mineralization and immobilization occur simulta-
neously. Therefore, most reported figures for Nmineralization or immobilization reflect
net gains or losses to a soil N pool. For example, net N mineralization
(NH 4 + -N +
¼
(NO 3 -N) t+1
(NO 3 -N) t (Hartetal. 1994 ). Gross rates of these processes can only be determined
through isotopic techniques.
NO 3 -N) t+1
(NH 4 + -N + NO 3 -N) t , and net nitrification
¼
7.5.4.2 Buried Bag Method
The buried bag method is a simple technique for quantifying net N mineralization
and nitrification in the field. It is most appropriate for assessing surface soils; for the
assessment of subsurface soils the reader is referred to the closed-top, solid cylinder
method (Hart et al. 1994 ). Intact soil cores are taken with a coring device (PVC or
metal tubes), placed in polyethylene bags, sealed, returned to the original hole, and
incubated in the field for 1-2 months. The plastic bags used in this procedure are
permeable to gases but not to liquids (Gordon et al. 1987 ). One advantage to this
method in comparison to lab incubation assays is that the soil samples are subjected
to on-site temperature regimes. However, since the bags are impermeable to water,
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