Pedogenesis and weathering rates of a Histic Andosol in Iceland: Field and experimental soil solution study

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 is 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 > SO 4 > 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 a H + 3/ a Al 3+ 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 a H + 3/ a Al 3+ activity ratio increased up to factor of 7, 30 and 37 by speciating Al 3+ 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 a H + 3/ a Al 3+ activity ratio, 2) the production of organic anions within the soil, and 3) external supply of anions capable of complexing Al 3+.