Effect of Organic Matter Removal by Low-Temperature Ashing on Dispersion of Undisturbed Aggregates from a Tropical Crusting Soil.

Low-temperature ashing (LTA) can be used to remove organic matter (OM) from undisturbed soil aggregates with minimal disturbance and damage to soil microstructure. A combination of LTA with FTIR photoacoustic spectroscopy PAS; which enables the study of sample surfaces) and appropriate soil dispersion tests was employed to assess the influence of OM on aggregate dispersion and to characterize the organic fractions involved in destabilizing or maintaining microstructure in a tropical soil that suffers severe crusting under cultivation. For this purpose, the cultivated soil was also compared with uncultivated soil from an adjacent field. This study suggested that the observed decrease in dispersion after LTA treatment of the aggregates of cultivated soil could be ascribed to the removal of negatively charged, low molecular weight humic substances. These substances, formed by the degradation of OM due to cultivation, may destabilize the microstructure of the soil under specific physico-chemical conditions. Conversely, the increase in dispersion, after LTA treatment, of the uncultivated soil, appeared to be caused by the removal of aliphatic hydrophobic compounds. These aliphatic compounds which were more abundant in the uncultivated soil, protect aggregates from the action of water (slaking and dispersion). The combination of LTA with soil dispersion test enabled to demonstrate. in this study, the contrasting roles of soil OM. These roles were related to the OM quality and its interaction with soil minerals. (C) 1999 Elsevier Science B.V. All rights reserved.