Selective Extraction Methods for Aluminium, Iron and Organic Carbon from Montane Volcanic Ash Soils

Abstract

Montane volcanic ash soils contain disproportionate amounts of soil organic carbon and thereby play an often underestimated role in the global carbon cycle. Given the central role of Al and Fe in stabilizing organic matter in volcanic ash soils, we assessed various extraction methods of Al, Fe, and C fractions from montane volcanic ash soils in northern Ecuador, aiming at elucidating the role of Al and Fe in stabilizing soil organic matter (SOM). We found extractions with cold sodium hydroxide, ammonium oxalate/oxalic acid, sodium pyrophosphate, and sodium tetraborate to be particularly useful. Combination of these methods yielded information about the role of the mineral phase in stabilizing organic matter and the differences in type and degree of complexation of organic matter with Al and Fe in the various horizons and soil profiles. Sodium tetraborate extraction proved the only soft extraction method that yielded simultaneous information about the Al, Fe, and C fractions extracted. It also appeared to differentiate between SOM fractions of different stability. The fractions of copper chloride- and potassium chloride-extractable Al were useful in assessing the total reactive and toxic Al fractions, respectively. The classical subdivision of organic matter into humic acids, fulvic acids, and humin added little useful information. The use of fulvic acids as a proxy for mobile organic matter as done in several model-based approaches seems invalid in the soils studied.