Environmental Engineering Reference
In-Depth Information
defined as Q
red
=
, where Q
red
is the sum of the reduced components
and Q
ox
is the sum of the oxidized components. Obviously, high values of RI
represented DOM components of reduced origin while low values represented
oxidized DOM components. In addition to these indices, EEMs have also been
used to identify fluorescence peaks in specific regions indicating humic-like and-or
protein-like fluorescence (Coble et al.
1990
; Cory and McKnight
2005
). The humic-
like fluorescence is typically assumed to represent DOM from vascular plants, with
high molecular weight and aromatic in nature (Coble et al.
1998
). In contrast,
the protein-like fluorescence is assumed to represent DOM of low molecular
weight, representative of amino acids like tryptophan and tyrosine (Yamashita
and Tanoue
2003
), composed of DOM that is more bioavailable (Fellman
et al.
2009
), and DOM that may be of microbial origin (Hood et al.
2009
).
ð
Q
red
þ
Q
ox
Þ
7.5 Nitrogen
7.5.1 Overview
Inorganic forms of N prevalent in wetland biogeochemical cycles include
dinitrogen gas (N
2
), ammonium (NH
4
+
), ammonia (NH
3
), nitrate (NO
3
), nitrite
(NO
2
), nitric oxide (NO), and nitrous oxide (N
2
O). Wetlands receive N inputs via
atmospheric deposition, transport in the water column, and biological dinitrogen
fixation. Dinitrogen fixation is the conversion of N
2
to NH
3
which is then rapidly
converted to organic forms. Dinitrogen fixation occurs in algae, free living bacteria,
and bacteria in symbiosis with macrophytes such as legumes. Nitrogen minerali-
zation is the conversion of organic forms of N to inorganic forms (primarily NO
3
and NH
4
+
) during SOM decomposition. The reverse process is referred to as
immobilization. Nitrification is the conversion of NH
4
+
to NO
3
. Inorganic N is
removed from wetlands via leaching losses, lateral transport in the water column,
and gaseous losses to the atmosphere via denitrification and NH
3
volatilization.
Denitrification is the microbial reduction of NO
3
to gaseous products (primarily
N
2
O and N
2
) which are returned to the atmosphere. Denitrification represents a
significant path of N loss from wetlands and it is considered to be one of the more
important wetland functions as it contributes to water quality by removing nitrates.
Under high pH conditions (pH
8) NH
4
+
>
is converted to NH
3
which may be
volatilized.
Many of the N techniques presented below contain a laboratory component to
determine the N content of soil, water, or plant tissue. These chemical assays are
beyond the scope of this chapter. The reader is referred to the following references:
Bremner (
1996
), for total N; Mulvaney (
1996
), for inorganic N; and Stevenson
(
1996
), for organic N.
