Geoscience Reference
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is generally abundant in soil-derived humic substances high in molecular weight. Spencer
et al. (2007b) examined the freeze-thaw cycles on river DOM fluorescence and found vari-
able responses to the process. Both increases and decreases to peak positions (e.g., red or
blue shifting) as well as increases and decreases in peak intensities were observed both for
protein and for humic fluorescence peaks. No clear pattern emerged in terms of the more
than 30 freshwater samples studied and, perhaps most importantly, no EEM fluorescent
features of the DOM were able to predict responses to freeze-thaw cycles.
Similar to ice exclusion (leading to organic matter concentrating in the resultant solu-
tion), DOM drying (dehydration) can induce substantial changes to fluorescence, a process
that becomes important in pore water and soil solutions in arid climates. Zsolnay et al.
(1999) studied soil DOM and found that drying temperature or rate (air vs. oven) was
important in determining desiccation effects to DOM fluorescence. Air drying produced a
red shift indicating a slight enhancement of humic fluorescence, probably resulting from
polymerization and condensation reactions. Oven drying (105°C), noted as artificial yet
analogous to soil desiccation, resulted in a blue shift toward protein-type fluorescence.
Zsolnay et al. (
1999
) attributed these observations to biomass lysis, but noted that changes
to physical pore structure in soils may affect soil DOM leaching. Otero et al. (
2007
) made
similar observations on pore water DOM fluorescence as modified by freezing sediments.
Freeze-thaw cycles that modify DOM fluorescence appear coupled to dehydration
and rehydration cycles, though this effect has not been investigated widely. Hudson et al.
(
2009
) offer some of the first controlled studies on DOM fluorescence due to cyclical dehy-
dration and rehydration. After one cycle at neutral pH, the 13 freshwater samples from
central England survey exhibited significant decreases in DOM fluorescence in the Trp
fluorescence region (34% and 40% loss for freeze-thaw and dehydration-rehydration,
respectively) that were substantially greater than humic fluorescence loss (7% and 18%).
Comparing one to five cycles of these processes, the authors note that the two distinctive
peaks found for Trp fluorescence (T1 at ex/em of 280/350 nm and T2 at 215-220/340 nm,
respectively) behaved differently - with T2 apparently being most susceptible to dehy-
dration. Similarly for the humic peaks, the C peak was apparently more labile than the A
peak. Dehydration caused a greater decrease in DOM fluorescence than did freezing. This
suggests some destruction of fluorophores but it is unclear whether collisional or static
quenching is induced by dehydration processes.
7.5 Effect of pH
A change in solution pH is expected to cause variable and complex effects on DOM fluor-
escence. Due to hydroxyl and carboxyl group protonation at low pH and their ionization
at high pH, DOM fluorophore electronic state is altered by variation in pH, resulting in an
enhancement of fluorescence when pH increases (
Figure 7.3A
). Changes to the structure
and bonding environments by lowering or raising pH or to extremes were observed to have
an effect on DOM fluorescence which was most likely due to protonation and ionization
charge effects (cf. Hosse and Wilkinson,
2001
). Ghosh and Shnitzer (
1980
) observed an
