Geoscience Reference
In-Depth Information
the dominant soil horizon flow path of the rainwater with recent labile surface organic mat-
ter being leached during storms compared to that during low-flow conditions. It is clear the
quantity and characteristics of FDOM exported from catchments is largely controlled by
a combination of soil microbial activity and the overall contact time that precipitation has
in soils.
Lignin is a biopolymer unique to terrestrial plants (Sarkanen and Ludwig,
1971
) and
as a result useful for tracing the flux of organic matter from terrestrial ecosystems to riv-
ers and eventually the open ocean (Ertel et al.,
1986
; Opsahl and Benner,
1997
). Current
techniques for quantifying and characterizing lignin content of DOM involve solid phase
extraction and subsequent chemical oxidation to a suite of lignin phenol derivatives, which
are detected using gas chromatography and mass spectrometry (e.g., Louchouarn et al.,
2000
). The approach is labor intensive and far from suitable for either routine or intensive
sampling programs. As a result correlations between the lignin phenol content and charac-
teristics and DOM ultraviolet (UV)-visible spectroscopic properties are being investigated
(Del Vecchio and Blough,
2004
; Boyle et al.,
2009
; Hernes et al.,
2009
; Spencer et al.,
2009
). Hernes et al. (
2009
) found that the region of fluorescence that gave best predict-
ability of lignin concentrations and characteristics was the region with excitation below
300 nm and emission between 300 and 350 nm. These findings contradict previous under-
standing and current models that link the lignin fraction with humic material with longer
wavelength fluorescence (Ertel et al.,
1986
; Lochmuller and Saavedra,
1986
; Del Vecchio
and Blough,
2004
). For instance, strong similarities were observed between the spectral
dependence of fluorescence quantum yields, emission peak maxima, and fluorescence life-
times among extracted lignin and FDOM, suggesting that lignin and FDOM exhibit com-
mon photophysical and structural properties (Del Vecchio and Blough,
2004
; Boyle et al.,
2009
). However, in support of the Hernes et al. (
2009
) finding, the fluorescence maxima
of two breakdown products of lignin, vanillic and syringic acid, are at 326 and 338 nm
respectively (
Figure 8.3
). Similarly, the results of Maie et al. (
2007
) also support this.
They found that tryptophan-like fluorescence peak from DOM from the Florida Coastal
Everglades could be chromatographically split using size-exclusion chromatography into
two fractions; one correlated with organic nitrogen content (i.e., proteins) and another cor-
related to humic fluorescence signals.
Developments in data analysis now allow us to separate the fluorescence signal using
advanced data analysis techniques, into underlying independent signals (Stedmon et al.,
2003
). This greatly simplifies distinguishing between different sources and processes act-
ing on FDOM (e.g., Stedmon and Markager,
2005b
), although much work is still required
on understanding the actual chemical origins of these fluorescence signals, albeit specific
fluorophores versus signals arising from complex interactions. In
Figure 8.4
the fluores-
cence characteristics of components identified in some early studies are compared to those
of organic fluorophores that can be expected to be found in aquatic environments. Ferulic
acid and coumaric acid are two oxidation products from lignin. Soil fungi are known to
produce enzymes (phenol oxidases) that degrade lignin to a range of polyhydroxy carboxy-
lic acids. The similarity of the fluorescence spectra of these fractions with the standards
