Fjölrit RALA - 15.06.2004, Side 102

The effect of acid deposition on ion leaching and weathering rates of an
Andosol and a Cambisol
Rannveig Guicharnaud1 and G.l. Paton2
Agricultural Research Institute, Iceland', Aberdeen University, Scotland2
An evaluation of the respond of an Andosol and a Cambisol to acid deposition and weathering
rates was studied by using a controlled laboratory leaching experiment. Both soils where
derived from basic parent material, a Histic Andosol from Westem Iceland and a Cambisol
from North East Scotland. De-ionized water and water acidified with H2SO4 (pH 3) was
leached through reconstmcted soil columns to simulate 34 years of precipitation. Soil
solution leachates where collected weekly for chemical analyses. Measured cations were Ca,
Mg, Fe, Na, K, Mn and Si. Measured anions were S02“4í Cl" and NO3". Cations from soil
solution samples were used to calculate wethering rates of both soil types.
Acidic input increased cation and anion leaching in both soil types arid reduced pH levels.
The Andosol proved generally to have higher weathering rates, which were calculated
according to Zulla & Billet (1994), leaching potential, ion exchange and buffering capacity.
This was due to differences in parent material and mineral composition. The Andosol
developed from volcanic tephra, which had higher dissolution rates due to its amorphous
mineral stmctures. The Cambisol was developed from gabbro with more stable mineral
stmctures.
Towards the end of the experiment, after 26.6 equivalent years of acid deposition, the
Andosol pH values decreased rapidly. At lower pH values S04‘2 concentrations where higher
in the output solution than the input solution. This was due to the Andosol sampling
location, 300 m from a smelting factory. The Andosol was therefore receiving SO4"2 input
from the factory prior to the experiment, suggesting a build up of S04‘2 in the soil. After
intense acid leaching, pH values decreased due to a decline in the buffer capacity of the
Andosol resulting in increased S04"2 concentrations in leachates.
Andosol is a soil type known for its great capacity to retain pollution due to its
variable charge surfaces and high cation exchange capacity. This experiment shows however
that despite the Andosol great capacity to receive pollution, exchange sites can be fully
occupied as a consequence of intense leaching of pollutants. This would lead to a decrease in
the soil buffering capacity and leaching of pollutants to groundwater until a new equilibrium
is reached.
References
Zulla, Y., and M.F. Billet. 1994. Long-Term Changes in Chemical-Weathering Rates between
1949-50 and 1987 in Forest Soils from Northeast Scotland. EJSS. 45:327-335.
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