Surface chemical composition of soil aggregates as a controlling factor of water repellency of forest soils
2006
Kobayashi, M.(Forestry and Forest Products Research Inst., Tsukuba, Ibaraki (Japan)) | Matsui, H.
Nondestructive surface chemical analysis using X-ray photoelectron spectroscopy (XPS) was conducted to clarify the effect of outer-surface chemical composition of soil aggregates on the water repellency of forest soils. The ratios of carbon to oxygen (C/O) and nitrogen to oxygen (N/O) were higher on the outer surface than in the interior of soil aggregates, indicating that soil organic matter tends to accumulate on the outer surface of aggregates. The surface C/O ratio determined by XPS was not necessarily proportional to the total carbon content (TC) measured by the thermal combustion method. TC did not sufficiently explain the degree of soil water repellency expressed by the ethanol percentage (EP) (R2=0.34), whereas the surface C/O ratio by XPS sufficiently explained water repellency (R2=0.74). The shapes of C 1 s spectra differed between strongly water-repellent soils under Japanese cypress and weakly water-repellent soils under deciduous broad-leaved trees. Soils with strong water repellency had C 1 s spectra rich in components of small chemical shift (small binding energy). These components correspond to carbon of nonpolar functional groups (C-C or C=O, i.e., not bound to atoms with high electronegativity, such as oxygen or nitrogen. The ratio of carbon with a small chemical shift to oxygen on the outer surface of soil aggregates could be regarded as a strong controlling factor of soil water repellency (R2=0.86).
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