Geoscience Reference
In-Depth Information
produced outside the system being studied and transported to it. For example, in lakes this
consists of organic matter supplied by rivers that is often a mixture of material derived from
soils and aquatic production upstream. Allochthonous inputs to an ocean basin can consist
of precipitation and exchange of water with neighboring seas. In contrast, autochthonous
FDOM is that which is released by organisms living within the system being studied. These
definitions at first appear relatively simple and robust; however, it is useful to clarify their
use in specific studies or systems, as they are often used interchangeably when referring
to fixed carbon originating from either from terrestrial or aquatic environments. But as can
be seen in the preceding example, aquatic systems are linked and it can be very difficult to
distinguish between aquatic organic matter produced upstream or within a system.
8.2.2 Terrestrial Organic Matter
In soils, microbes degrade and transform organic matter from living and decaying plants
and animals. Low molecular weight compounds either directly released or enzymatically
cleaved from larger polymeric structures such as cellulose and lignin and the remain-
ing material transformed to higher molecular weight compounds in a process commonly
referred to as humification (Stevenson,
1982
). During this process the chemical properties
are altered, among them the number of carboxyl functional groups and C/H ratio increases.
Fluorescence has for many years been used to characterize soil organic matter (Senesi
et al.,
1991
) and its degree of humification (Zsolnay et al.,
1999
). With increased humifi-
cation the emission spectrum shifts toward longer wavelengths (
Figure 8.2
). This shift can
be characterized using the humification index (HIX), which is the emission intensity from
435-480 divided by the emission from 300-345 nm (Zsolnay et al.,
1999
).
Soil-derived DOM is a dominant source of FDOM to many fresh and coastal waters.
Exceptions to this are hydrologically isolated systems surrounded by very little vegetation
such as the Antarctic Dry Valley lakes (McKnight et al.,
1991
) and high-altitude mountain
lakes (McKnight et al.,
1997
). The amount and quality of FDOM leached from soils can
be expected to vary as a function of climate, soil hydrology, and catchment characteris-
tics and slope, as seen for other DOM characteristics (Aitkenhead-Peterson et al.,
2003
;
Mullholland, 2003; Stedmon et al.,
2006
; Fellman et al.,
2009a
). The concentration of
FDOM in soil solutions and streams in natural catchments vary seasonally. For example,
in a temperate Danish stream the highest concentrations are found in summer and lowest
concentrations in winter (Stedmon and Markager,
2005a
). This trend follows what is also
observed for DOC in similar temperate systems (Tipping et al.,
1999
; Kalbitz et al.,
2000
)
and is explained by a combination of reduced water throughput and greater soil micro-
bial activity in the surface organic-rich layers (McDowell and Wood,
1984
; Guggenberger
et al.,
1998
; Kalbitz et al.,
2000
). In conjunction with this seasonal change in concentra-
tions clear seasonal changes are also seen in the fluorescence characteristics of the leached
FDOM. For example, for a small Danish forest stream the relative humic fluorescence at
412 nm and 504 nm varied considerably across season (components 3 and 2 respectively in
