Environmental Engineering Reference
In-Depth Information
3 Origin of DOM in Natural Waters
DOM is generally originated from three major sources in natural waters: (i) DOM
derived from terrestrial soils, termed allochthonous DOM; (ii) DOM derived from
in situ production in natural surface waters, termed autochthonous DOM, and (iii)
DOM derived from human activities (e.g. industrial synthesis), termed anthropo-
genic DOM.
3.1 Origin of Allochthonous DOM in Soil Ecosystems
DOM including fulvic and humic acids (humic substances) originates from the
decomposition of vascular plant material, root exudates and animal residues in ter-
restrial soil. Origin of allochthonous DOM from vascular plant materials or partic-
ulate detrital pools is significantly varied in different regions (tropical, temperate
and boreal), which is regulated by the occurrence of three key factors or functions
(Mostofa et al.
2009a
; Wetzel
1983
,
1990
,
1992
; Malcolm
1985
; Dai et al.
1996
;
Nakane et al.
1997
; Wershaw
1999
; Jaramillo and Dilcher
2000
; Kalbitz et al.
2000
; Trumbore
2000
; Uchida et al.
1998
,
2000
; Moore et al.
2008
; Braakhekke
et al.
2011
; Spence et al.
2011
; Tu et al.
2011
): (i) Physical functions that include
temperature and moisture; (ii) Chemical functions that include nutrient avail-
ability, amount of available free oxygen and redox activity, and (iii) Microbial
processes that include microfloral succession patterns and availability of microor-
ganisms (aerobic or anaerobic).
It is suggested that microorganisms can alter sugars, starch, proteins, cellulose
and other carbon compounds bound to organic matter of plant or animal origin
during their metabolic processes. These processes can transform the aromatic and
lipid plant components into amphiphilic molecules including humic substances,
i.e., molecules that consist of separate hydrophobic (non-polar) and hydrophilic
(polar) parts (Wershaw
1999
). The non-polar parts of the molecules are composed
of relatively unaltered segments of plant polymers, while the polar parts include
carboxylic acid groups (Wershaw
1999
). Aerobic microorganisms can decompose
organic matter at a faster rate than anaerobic ones, depending on the availability
of free oxygen. Compositional changes of DOM occur with soil depth, leading to
a decrease of aromatic compounds and carbohydrates whilst alkyl, methoxy and
carbonyl moieties increase with depth (Dai et al.
1996
). The increase in alkyl and
carboxylic C with depth are the result of biodegradation of forest litter and oxi-
dation of lignin side chains, respectively (Zech et al.
1985
; Kogel-Knabner et al.
1988
; Kogel-Knabner
1992
).
The origin of allochthonous DOM from microbial processes can be judged
from significant variations in respired organic carbon in different soil environ-
ments. The mean age of soil respired organic carbon determined using
14
C tracer
is lowest (1 year) in tropical forest soils (eastern Amazonia, Brazil), relatively
