Geoscience Reference
In-Depth Information
acidified samples (FA-AS). This is due to antisymmetrical vibration of -COO
-
groups (1,550-1,650 cm
-1
); to -CO stretching vibrations of esters, ketones, and
aldehydes; and to -CO absorption and -NH
2
deformation of amide I and II bands
(1,650-1,750 cm
-1
). In the acidified fulvic acid, the carboxyl group content
(1,700 cm
-1
) dominated the -CO stretching of esters (shoulder at 1,730 cm
-1
).
The differences between the two absorption peaks are more evident in the fraction
smaller than 3 kDa, which shows only the peak due to -COO
-
groups. Fulvic acid
acidification caused the peak corresponding to -OH deformation and -CO
stretching of alcohols and phenols to decrease (1,050-1,350 cm
-1
). These data
show that the main effect of soil acidification is reflected in the structural compo-
sition of fulvic acids and in the breakup of the original molecules, due probably to a
hydrolysis process. This suggests an irreversible increase in fulvic acid solubility.
Acid rain leaching leads also to changes in the composition of dissolved organic
carbon (DOC). Release of different components of DOC obtained from a red clay
soil (China), under leaching with simulated acid rain at pH of 2.5-5.6, was studied
by Liu et al. (
2009
) using three-dimensional excitation-emission matrix spec-
troscopy (EEMS). Humic and fulvic acids, as well as microbial byproduct mate-
rials, were released at the beginning of leaching with low pH solution; in contrast,
aromatic proteins were released with a greater time of leaching and a high pH.
Atmospheric deposition can affect DOC in soil by changing either the soil
acidity or the ionic strength of the soil solution. An increase in soil acidity as a result
of acid rain deposition may lead to a decrease in DOC. Potential anthropogenically
induced changes in the DOC status in the soil-subsurface domain can be deduced
by extrapolating the findings of large, multinational monitoring programs on
atmospherically deposited sulfur and sea salts on continental surface waters.
Monteith et al. (
2007
) used the results of 25 years of monitoring studies in Europe
and North America, on DOC content in surface waters located on glaciated, acid-
sensitive terrain, to define DOC trends resulting from changes in atmospheric
deposition chemistry. Monitoring data showed that between 1990 and 2004, some
of the sites measured a DOC increase; this finding was attributed to a gradual
decline in the sulfate content of atmospheric deposition. A simple model that
applied changes in deposition chemistry and catchment acid sensitivity were used
to describe a partially buffered ecosystem where changes in organic acidity and rise
in DOC were related to recovery from acidification. The effect on the ecosystem of
a decrease in DOC due to acid deposition is irreversible. Even with global efforts to
reduce sulfur emissions into the atmosphere, and a decrease in soil acidity, the soil
dissolved organic carbon will never return to its initial concentration.
18.3.2 Impact of Effluents
Municipal and industrial effluents may contain dissolved organic and inorganic toxic
chemicals, and suspended organic solids, which are introduced into the soil-sub-
surface system during land disposal of wastewater effluents and by the discharge of
