『Abstract
　Changes in surface charge of soil particles that accompany mineral transformations during soil formation were measured for a humid tropical chronosequence in Hawaiian basalt ranging i lava flow age from 0.3 to 4100 kiloyears (ky). Parent mineralogy is dominated by glass, olivine, pyroxene, and feldspar, whereas poorly crystalline (PC) weathering products (allophane, microcrystalline gibbsite, ferrihydrite) accumulate in early to intermediate weathering stages (through 400 ky), and crystalline secondary minerals (kaolinite, gibbsite, goethite) are dominant i the oldest (1400 and 4100 ky) soils. Detailed characterization of the solid phase was accomplished with chemical extractions, X-ray diffraction analysis, and molecular spectroscopy (FTIR and ¹³C MAS NMR) Simultaneous proton titration and background ion adsorption measurements were made on LiCl saturated soils over a range in pH (2-9) and ionic strength (0.001 and 0.01 M LiCl). Dependence of variable surface charge on solution composition reflects the changing nature of mineral-organic interactions over the course of pedogenesis. Points of zero net proton charge (PZNPC) ranged from 3.4 to 6.2 and 2.0 to 5.8 at 0.001 and 0.01 M ionic strength (I ), respectively. Intermediate-aged soils containing the highest mass concentration of humified soil organic matter (SOM) and its complexes with PC minerals gave rise to the steepest charging curves (largest pH dependence) and highest PZNPC values. Surface charge properties of these soils most closely reflected their weakly acidic Al and Fe hydroxide constituents, which is consistent with metal hydroxide saturation of organic functional groups, rather than organic coating of mineral surfaces. Charging curves were less steep and PZNPC values were lower for the older soils, consistent with SOM coating of more crystalline goethite, kaolinite, and gibbsite surfaces in a soil system less impacted by labile Al and Fe.』

1. Introduction
2. Experimental methods
　2.1. Background
　2.2. Chronosequence field sites and sample collection
　2.3. Preparation and characterization of homoionic soils
　2.4. Surface charge components
　2.5. Data analysis
3. Results
　3.1. Bulk soil chemistry and mineralogy
　3.2. Spectroscopic analyses of the soil solid phase
　　3.2.1. DRIFT spectra of LiCl saturated whole soils
　　3.2.2. NMR spectra of soil humic acids (HAs)
　3.3. Proton titration experiments
　　3.3.1. Adsorption of Al
　　3.3.2. Background ion adsorption
　　3.3.3. Balance of surface charge
4. Discussion
　4.1.Dissolution of organic C
　4.2. Charging of soils across the LSAG
　　4.2.1. Structural charge
　　4.2.2. Slopes of the charging curves
　　4.2.3. Role of SOM structure
　　4.2.4. Role of poorly crystalline minerals
　4.3. Points of zero charge indicate changing surfaces in mineral-organic complexes
5. Conclusions
Acknowledgments
References