Environmental Engineering Reference
In-Depth Information
carbon compounds including propane and a series of chlorinated aliphatic compounds of
great environmental concern (e.g., trichloroethylene). Nitrifiers were originally thought to
be strict aerobes, but it turns out that they are capable of growing facultatively under
anaerobic conditions as denitrifiers (discussed later). They were also thought to be highly
sensitive to low pH, and incapable of growth at pH less than 5, yet nitrification has been
found to be vigorous in acid soils (pH 4) in many locations. The relative importance of
these diverse physiologies in both the distribution and abundance of ammonia oxidizers is
an area of active research.
Denitrification
Pathway:
NO
3
!
NO
2
!
NO
!
N
2
O
!
N
2
Denitrification is a form of anaerobic respiration carried out by bacteria that use NO
3
2
,
NO
2
2
, nitric oxide (NO), or nitrous oxide (N
2
O) as electron acceptors producing progres-
sively reduced products, ending in N
2
. Most denitrifiers are facultative anaerobes; that is,
they normally respire oxygen, but in its absence use N-oxides. There are many heterotro-
phic denitrifiers; the ability is widely distributed among hundreds of genera. There are
also chemoautotrophic denitrifiers that use compounds such as NH
4
1
(nitrifier denitrifica-
tion) or sulfur compounds as electron donors, and the N-oxides as electron acceptors.
The ecology of denitrification is strongly controlled by the fact that it is an anaerobic pro-
cess. Therefore, we expect rates to be high in wetlands, in anaerobic sediments of aquatic
ecosystems, and in deep oceanic oxygen minimum zones. Denitrification is also strongly
regulated by the availability of NO
3
2
and is often transient in response to pulses of NO
3
2
input or production. For example, inputs of NO
3
2
to wetlands from surrounding agricultural
fields can create high rates of denitrification that will be sustained only as long as the supply
of NO
3
2
continues. Alternating periods of drying and wetting in wetland soils can lead to
increases of NO
3
2
production by nitrification during the dry periods, followed by denitrifica-
tion during the wet, anaerobic periods. In sediments, NO
3
2
production by nitrification can
be vigorous in an aerobic surface layer and can support high denitrification in anaerobic
sediment layers just below the surface (see later and
Figure 7.5
).
Denitrification is important in several applied contexts. Originally, denitrification was
seen as a threat to soil fertility because it directly removes N from the plant-available pool.
More recently, there has been interest in the role of denitrification in maintaining water
quality, as it can prevent the movement of N into aquatic ecosystems sensitive to N-induced
eutrophication. Efforts are active in many areas of the world to protect, restore, and/or cre-
ate ecosystems that facilitate denitrification of excess N from agriculture, sewage, and atmo-
spheric deposition. These range from riparian zones in agricultural watersheds, to highly
engineered systems to treat sewage wastewater, to small wood-chip bioreactors to directly
process N leaving agricultural fields in subsurface drainage pipes, to specialized anaerobic
tanks adjacent to individual sewage disposal systems (septic systems).
Denitrification also influences air quality, as NO is a precursor to ozone formation in
the troposphere (where ozone is a pollutant) and N
2
O is a greenhouse gas and contributes
