Abstract

Soils developed from volcanic ash deposits vary extremely in physical and chemical soil properties and are known to degrade easily through erosion or soil compaction, which limits intensive utilization. Our objective was to test if aggregate stability, AS, as a key factor for erodibility, can be assessed from surface parameters, namely particle wettability in terms of the solid-water contact angle, CA, and the specific surface charge, SSC. To relate wettability to the chemical composition of particle interfaces, the amount of polar and nonpolar carbon (C) species within the particle interface layer was assessed by X-ray photoelectron spectroscopy, XPS. Samples were taken from four locations in southern Chile (40 degrees south) along a 120 km transect from the central volcano range to the coastal mountain region. Sites were different in stage of soil development and land use. Aggregates (8-12 mm) were sampled on forest, meadow or arable plots from upper top- and subsoil. To increase the natural range of SSC and CA, soil pH was modified by treatment with HCl and NH3 gas, respectively. Results showed a general trend of increasing AS with increasing soil development, i.e. decreasing bulk density and increasing clay and soil organic carbon, SOC, content. A clear relation was also found between CA and AS with a wide range of AS (about 9 to > 90%) at CA < 35 degrees and always high AS for CA > 35 degrees. XPS analysis showed that CA was closely related to the amount of nonpolar C species. Interestingly, the relation between CA and AS was still consistent after pH variation (initial pH congruent to 6, modified to pH 2 to 9) which led to net changes in SSC from around 0 C g(-1) (pH 6) to values between +10 and -8 C g(-1), respectively. CA for negative SSC were still in the range from 0 degrees to > 90 degrees, similar to untreated soil, whereas positive SSC reduced the maximum CA to < 40 degrees. Increasing or decreasing SSC caused higher AS, while the least stable aggregates were found at point of zero charge, PZC. XPS analysis showed that a relative increase in interfacial C content increased AS (r(2) = 0.77) and CA (r(2) = 0.98) significantly. Generally, CA was revealed to be a significant parameter that allows rapid assessment of the present status and possible modifications of AS for a wide range of environmental conditions.