『Abstract
It is important to study the rate determining processes of chemical
weathering and soil formation in volcanic islands since a significant
part of the carbon fixed by chemical weathering of silicates on
earth id fixed at the surface of volcanic islands. These soils
are fertile and much of the river suspended matter delivered to
the ocean stems from these islands. This study determines the
factors that drive the pedogenesis of a Histic Andosol in western
Iceland. soil solutions were extracted from the profile in the
field, from undisturbed ex situ mesocosms and from repacked laboratory
microcosms. Concentrations of measured and calculated inorganic
species in the field and experimental soil solutions were used
for thermodynamic and kinetic interpretation, and to calculate
the weathering rates.
The main primary rock constituent of the 205 cm thick soil profile
was basaltic glass, allophane content ranged from 2 to 22% and
the soil carbon content ranged from 11 to 42%. Mean soil solution
pH value ranged from 4 to 6 with the lowest value at 80 cm depth
and highest between 150 to 205 cm. The high solute concentrations
in soil solutions in the beginning of the microcosm weathering
experiment declined faster for anions than cations. Under field
conditions inorganic anions were supplied by marine and anthropogenic
rather than pedogenic sources and hence these anions were subsequently
leached out during the experimental duration when there was a
limited input of anions through experimental precipitation. The
factor, which defined the rate at which each ion was depleted
from the exchange complex of the soil, decreased down the soil
profile. The release sequence at 50 cm depth was Cl>Na>SO4>F>Si. The Si and base cations experimental weathering
rate when normalized to geographical surface area are similar
to or lower than those measured from river catchments in Southwestern
Iceland. The dissolved Al flux was much higher from the soil when
compared to the river catchments
Field and experimental soil solutions were all highly undersaturated
with respect to basaltic glass. Field and mesocosm samples were
supersaturated with respect to secondary allophane and imogolite,
while samples from the microcosms were often undersaturated with
respect to allophane and imogolite. Predicted dissolution rate
was dictated by the soil solution aH+3/aAl3+
activity ratio but slowed down by up to 20% and 30% by decreasing
undersaturation in field and mesocosms respectively. Predicted
dissolution rates according to the aH+3/aAl3+
activity ratio increased up to factor of 7, 30 and 37 by speciating
Al3+ with oxalate in field, mesocosms and microcosms
respectively. Speciation with oxalate, which represents the maximum
effect of the dissolved organic carbon (DOC) on dissolution rates,
generally had more effect near the surface than at deep levels
in the soil profile. This study shows that at fixed temperature,
reactive surface area, and composition of the volcanic glass in
the soil, the chemical weathering rates of Andosols are dictated
by: 1) aeolian deposition rates and drainage, which affect the
saturation state and the aH+3/aAl3+ activity ratio, 2) the
production of organic anions within the soil, and 3) external
supply of anions capable of complexing Al3+.
Keywords: Mesocosms; Microcosms; Exchange complex; Saturation
state; Dissolution rate; Basaltic glass; Allophane; Imogolite』
1. Introduction
2. Materials and methods
2.1. Study area
2.2. Collection and analysis of field soil solution
2.3. Collection of soil for ex situ intact mesocosms, repacked
microcosm and general characterisation
2.4. Construction of repacked microcosms
2.5. Collection and analysis of experimental gas and aqueous
solutions
2.6. Saturation state of components in the soil
2.7. Fluxes and weathering rates
2.8. Statistical analysis
3. Results
3.1. Soil characteristics
3.2. Soil solution characteristics
3.2.1. Silicon
3.2.2. Aluminium
3.2.3. Iron
3.2.4. Calcium
3.2.5. Magnesium
3.2.6. Sodium
3.2.7. Potassium
3.2.8. Chloride
3.2.9. Sulphate
3.2.10 Fluoride
3.2.11 Nitrate
3.3. Ion release from the exchange complex in microcosm experiments
3.4. Fluxes through microcosm horizons
3.5. Weathering rates of microcosms
4. Discussion
4.1. Soil characteristics
4.2. Soil solution characteristics
4.3. Ion release from the exchange complex
4.4. Controls on pedogenesis in the soil
4.5. Fluxes in microcosms
4.6. Weathering rates
5. conclusions
Acknowledgements
References