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 mi crostructure. A combination of LTA with FTIR photoacoustic spectroscopy PAS ; which enables the study of sample surfaces) and appropriate soil dispersi on tests was employed to assess the influence of OM on aggregate dispersion and to characterize the organic fractions involved in destabilizing or mai ntaining microstructure in a tropical soil that suffers severe crusting und er cultivation. For this purpose, the cultivated soil was also compared wit h uncultivated soil from an adjacent field. This study suggested that the observed decrease in dispersion after LTA tre atment of the aggregates of cultivated soil could be ascribed to the remova l of negatively charged, low molecular weight humic substances. These subst ances, 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 unculti vated soil, appeared to be caused by the removal of aliphatic hydrophobic c ompounds. These aliphatic compounds which were more abundant in the unculti vated soil, protect aggregates from the action of water (slaking and disper sion). The combination of LTA with soil dispersion test enabled to demonstr ate. in this study, the contrasting roles of soil OM. These roles were rela ted to the OM quality and its interaction with soil minerals. (C) 1999 Else vier Science B.V. All rights reserved.