Fjölrit RALA - 15.06.2004, Qupperneq 106

Soil solution composition and weathering rates of a Histic Andosol, Iceland.
B. Sigfusson1, S.R. Gislason1 and G.l. Paton2
1) Science Institute, University of Iceland, 2)University of Aberdeen, School of Biological Sciences, UK
Chemistry of soil solutions from a Histic Andosol profile in Westem Iceland was studied in
the field, in repacked laboratory microcosms and undisturbed outdoor mesocosms. The
experimental setup allowed simulation of 50 equivalent years of precipitation in microcosms
and 4.2 equivalent years in mesocosms.
Amalds (2004) reported that pedogenesis of Icelandic Andosols was govemed by the
deposition rates of aeolian-andic materials and drainage. Those factors where ftxed in the
current experimental setup, that is there was no aeolian-andic depostion and the soils drained
freely in microcosms and mesocosms.
Concentrations of main ionic components in the soil solution correlated with depth (water in
contact with soil) rather than to pH values.
Soil solution chemistry was controlled by three dominant processes:
a) Dissolution of volcanic tephra, mostly basaltic glass.
b) Photosynthetic binding of carbon by surface vegetation and subsequent leaching of
dissolved organic and inorganic carbon into the soil profile.
c) Incongruent reaction between allophane and imogolite.
Dissolution of volcanic tephra (in this case primarely basaltic glass) and tumover of carbon
near the surface were the dominant processes in near surface horizons. However, incongment
reaction between allophane and imogolite controlled the composition of soil solution deeper
than 50 cm in conjunction with dissolution of volcanic tephra. Aluminium released into soil
solution from volcanic tephra was primarely complexed into organic complexes according to
simulations in PHREEQC (Parkhurst & Appelo, 1999) and was therefore unavailable to form
allophane or imogolite in surface horizons. Free aluminium was up to 30 % of total
aluminium in horizons below 80 cm and was therefore available to form allophane rather than
to precipate and from humus complexes.
Fluxes of silicon, aluminium and base cations in soil horizons generally increased
downwards as expected (table 1). Weathering rates of horizons in microcosms were on the
same order of magnitude as weathering rates from river catchment studies in the vicinity of
the research area (Gislason et al., 1996; Moulton et al., 2001; Stefansson and Gislason, 2001).
Gislason et al. (1996) reported there was no correlation between vegetation cover and overall
chemical denudation rates in river catchments in SW-Iceland. The influence of vegetation
cover on weathering rates was clearly observed in current research and those by Moulton et
al. (2001). The soil thickness could also be influential on the total denudation rates of river
catchments as concentration of dissolved elements will increase downwards the soil profile
and rivers draining areas with thin soil covers will therefore have a lower solute
concentrations and consequently lower denudation rates.
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