Surface charge evolution of mineral-organic complexes during pedogenesis in Hawaiian basalt
Introduction
Soils derived from volcanic lava and tephra cover ca. 1% of the earth’s land surface and give rise to a wide range of soil types, depending upon genetic drivers (climate, biota, topography, and time). The primary mineralogy of basalt (plagioclase feldspar, olivine, pyroxenes, and ferromagnetic minerals) weathers rapidly under high throughputs of water, owing to high mass concentrations of volcanic glass, chain and nesosilicates, and small crystallite sizes relative to other igneous rock (Lowe, 1986; Gislason and Arnórsson, 1993; Gislason et al., 1996). Ejecta from Hawaiian volcanoes comprises a mixture of lava and tephra. In the presence of sufficient fresh water, incipient weathering of Hawaiian volcanic ash and lava promotes rapid dissolution of Si, Al, Fe, and nonhydrolyzing cations (e.g., Ca, Mg) in “early” weathering stages. Whereas large quantities of the latter are removed via leaching, the former are retained preferentially due to precipitation of poorly crystalline (PC) solid phases (e.g., microcrystalline gibbsite, allophane, imogolite, and ferrihydrite) from supersaturated solutions. Prevalence of these metastable solids, which is characteristic of “intermediate” weathering stages in humid tropical soils derived from volcanic ejecta (Shoji et al., 1993; Chadwick et al., 2003), may be accompanied by a smaller accumulation of structurally charged 2:1 layer-type silicates (Chorover et al., 1999a). With continued throughput of water, soil mineralogy is transformed to crystalline (C) minerals typical of “late” stage weathering, including halloysite or kaolinite, gibbsite, goethite, and hematite (Chadwick and Chorover, 2001).
Mineral evolution during pedogenesis should be accompanied by measurable changes in the surface chemistry of soil particles, which affects an array of biogeochemical processes including adsorption of nutrient or toxic ions, mineral weathering reactions, particle aggregation, and organic matter retention. PC secondary solids that are formed at intermediate weathering stages are known to have high specific surface area and reactive surface hydroxyl groups that should contribute significant variable charge per unit mass of bulk soil. Further, since leaching of soils by fresh water removes solid phase Si in preference to Al, Fe, and Ti (Sposito, 1989), the surface charge of soil mineral particles is expected to increase as the more strongly acidic silanol (SiOH) sites associated with primary minerals and metastable silicates are depleted relative to weakly acidic surface hydroxyl groups of Fe, Al, and Ti (hydr)oxides (FeOH, AlOH and TiOH).
Volcanic ejecta is weathered rapidly in humid tropical climates, and soils support high biomass forest ecosystems relatively soon after deposition (<300 yr at 2.5 m mean annual precipitation [MAP] and 289 K [16°C]). A significant fraction of fixed carbon (biomolecules and humic substances) is input to the soil solution as dissolved organic matter (DOM) or is incorporated into the soil solid phase (Neff et al., 2000). In forests, soil organic matter (SOM) is critically important to soil particle charge, since it contains a diverse mixture of molecules and associated functional groups varying in acidity (Perdue and Lytle, 1983; Aiken et al., 1985), which become intimately associated with soil minerals through interacting processes of mineral weathering and humification (Huang and Schnitzer, 1986).
Mineral-adsorbed SOM can modulate the surface chemistry of soil particles by masking the charge properties of underlying mineral colloids and reducing the variability in charge that is expected on the basis of heterogeneous mineral composition (Davis, 1982; Hunter and Liss, 1982; Beckett and Le, 1990; Chorover and Sposito, 1995b). However, the architecture of mineral-OM composites in volcanic ash soils is not confined to organic coatings on mineral surfaces. In soils comprising high concentrations of reactive Al and Fe, complexation with monomeric, polymeric, and colloidal hydroxy-metal species promotes coagulation and immobilization of SOM in the soil solid phase (Skjemstad, 1992; Jones and Bryan, 1998). These processes also contribute to the stabilization of SOM against decomposition (Torn et al., 1997) and to the extraordinary accumulation of organic C to nearly 40% of the total soil dry mass in some volcanic ash soils (Shoji et al., 1993; Wada, 1995; Chorover, 2002). In turn, SOM tends to decrease the rate at which minerals such as allophane and ferrihydrite transform to long-range ordered, crystalline solids, and thereby enhances the metastability of these PC phases. The change in surface charge of soil particles over the course of pedogenesis in basalt is certainly impacted by this coupling of mineral transformation and SOM retention, but there are no published studies that have provided an appropriate quantitative analysis of this process. The objectives of the present work were (i) to establish the relationships between parent material age, soil constituents, and particle surface charge in a well-characterized chronosequence of humid tropical soils derived from Hawaiian basalt (Vitousek et al., 1997; Hotchkiss et al., 2000); and (ii) to utilize the results to better understand the changing nature of mineral-organic complexes during pedogenesis.
Section snippets
Background
Surface charge of soil particles can develop as a result of proton or other ion complexation at the particle surface and from isomorphic ion substitution within the crystal structure. Net total particle surface-charge, σP, (in units of moles of charge per kilogram of solid [molc kg-1]) is the sum of four components (Sposito, 1998): where σ0 is the net permanent structural-charge density resulting from isomorphic substitutions; σH is the net proton surface-charge density
Bulk soil chemistry and mineralogy
Upon collection (before saturation with LiCl), surface soils exhibited lower pH than those collected from the subsurface (Table 1). They also had higher organic C contents (Table 1) and Li+ exchangeable acidity (Al and H+) (Fig. 1). There is a large decrease in the concentration of adsorbed nonhydrolyzing cations (K+, Na+, Ca2+, Mg2+) with increasing parent material age (referred to hereafter as “soil age”), whereas moles of exchangeable Al and H+ charge appear to be highest for the
Dissolution of organic C
Data extend over a larger pH range at high I relative to low I (Figs. 7 and 8). Underprediction of σH vs. Δq slope at high I (Table 3) suggests that, in addition to adsorption of H+ (low pH) or OH- (high pH), other side reactions are contributing to proton or hydroxide consumption. Given that (i) values of σH were corrected for mineral dissolution (Eqn. 6) and (ii) the greatest discrepancies are observed in the surface soils, we postulated that some H+ and OH- consumption occurred in
Conclusions
The use of proton titration to accurately measure surface charge properties of soils comprising PC mineral-organic complexes requires accounting for proton-consuming side (e.g., dissolution) reactions that do not contribute to charge development. Background ion adsorption was found to provide a more accurate measure of proton surface charge density, based on charge balance, under conditions where significant SOM dissolution occurs, provided that an independent measure of structural charge
Acknowledgments
We are grateful to Peter Vitousek for his leadership on the Hawaii Ecosystems Project, to Heraldo Farrington for exceptional management of field logistics and assistance in sampling, and to Patrick Hatcher for use of his NMR spectrometer. We also thank three anonymous reviewers and Associate Editor Nagy for constructive comments on an earlier version of this manuscript. Funding for this work was provided by USDA Program 25.0 in Soils and Soil Biology (# 97–35107–4360).
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