forms. Burford and Bremner (1975) observed posi-
Validation
tive correlations between denitrification and min-
WASTEN has been extensively tested in both
eralizable C and also denitrification and water-
laboratory and field investigations. A listing of
soluble C. Stanford et al. (1975) found a positive
publications relating to WASTEN, its development
and its testing is provided in Appendix A.
correlation between denitrification and 0.01 M
WASTEN was recently used successfully and field-
CaCl2 extractable C. Because rhizosphere effects
verified independently by researchers in Czecho-
are generally thought to be caused by root excre-
slovakia to model the transport and transforma-
tions of C compounds, rhizosphere and rootsoil
tion of nitrogen from fertilizer applications (Benes
interactions can also influence denitrification.
et al. 1989).
For soil systems that cycle between aerated and
saturated conditions, C drives both the aerobic and
the anaerobic processes. At the onset of saturation,
dissolved O2 in the soil solution creates aerobic
DENITRIFICATION
conditions until the O2 is depleted by microbial
respiration. During respiration, O2 consumption
Introduction
A major component of WASTEN for wastewa-
by microbial reduction requires C as an electron
ter treatment is denitrification--the microbial re-
donor. As O2 is depleted and soil atmosphere con-
duction of nitrate ( NO3 ) and nitrite ( NO2 ) to gas-
ditions become increasingly reduced, C oxidation
eous nitrogen (N) products, either as molecular N
will continue to occur, but NO3 serves as the elec-
(N2) or as oxides of N (NO, N2O). Firestone (1982)
tron acceptor. Consequently the cycling and deni-
depicted the overall pathway as:
trification of N is intimately linked with C metabo-
lism.
NO3 → NO2 → NO → N2O → N2 .
Soil solution reaction
WASTEN uses first-order kinetics to describe the
The effect of soil solution pH has been shown
rate of denitrification. The reaction kinetics are also
to influence denitrification activity (Firestone
linked to soil aeration, expressed as and calculated
1982). Denitrification has been observed in soils
from soil moisture content. In this way, for the deni-
with a pH less than 5 (Gilliam and Gambrell 1978)
trification component of the model to be active,
and has even been observed in more acid soils of
both nitrate and aeration criteria must be met.
pH 3.54.0 (Klemmedtsson et al. 1978). It has been
Firestone (1982) reported that approximately 23
hypothesized that the effects of pH on denitrifica-
genera of bacteria can perform denitrification and
tion may be related to interactions of pH and trace
that almost all denitrifying bacteria are aerobic or-
metal activity, especially molybdenum. Molybde-
ganisms capable of anaerobic growth only in the
num is a necessary cofactor for nitrate reductase,
presence of nitrogen oxides. The important conse-
a key enzyme in denitrification. There are numer-
quence is that for land treatment of wastewater,
ous observations supporting denitrification over
the microbes responsible for denitrification are able
a wide pH range, and the most common influence
to survive in the alternating saturated and nonsatu-
rated conditions to which the soil profile is sub-
pH decreases (Firestone 1982).
jected. For denitrification to occur, several criteria
must be met: obviously an NO3 source must ex-
Temperature
ist, oxidation conditions must be reduced, and a
Within limits, denitrification appears to follow
carbon (C) source must be available.
the Arrhenius equation:
ln v = ( ∆H*/RT) + C
Carbon
Given necessary nitrate levels and anaerobic
where v = velocity
conditions, other factors influencing denitrification
∆H* = activation energy
include available C, pH and temperature. Of these,
R = gas constant
C is frequently the limiting factor. It is well estab-
T = temperature (K)
lished that C levels are important in controlling
C = constant.
denitrification, both as a source of cell material and
as electron donors. The form of C is also impor-
Although temperature affects the rate of chemi-
tant. In general, more-soluble C sources have a
cal and biological processes, significant denitrify-
more rapid and greater effect than less-soluble C
ing activity has been found in soils at low tem-
4
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