Organic matter stabilization in soil aggregates and rock fragments as revealed by low-temperature ashing (LTA) oxidation

Low-temperature ashing (LTA), which is able to remove the organic material from mineral grains or aggregates without any disturbance to their physical structure, was used to oxidize the organic matter present into soil aggregates and sandstone- and siltstone-derived rock fragments. The three fractions were characterized for their mineralogy and pores distribution, and treated with LTA apparatus for 24 and 96 h. The losses of C and N were evaluated and the chemical modifications produced by LTA in the extractable organic matter and humin were investigated by chemical and spectroscopic methods. During the LTA treatment, the simplest organic molecules were lost, and the remaining material acquired a higher degree of oxidation, making this heterogeneous mix of fresh, partially and well-humified substances more homogeneous. In the aggregates and in the sandstone rock fragments, the composition of extractable material changed and was enriched in the aromatic component, due also to the contribution of fragments deriving from humin, as the oxidation produced compounds enriched in carboxyl groups and characterized by a higher solubility. This could mean the occurrence of a continuum between humin and extractable organic matter mediated by oxidation processes. In the siltstone rock fragments extractable organic matter and humin underwent lesser severe transformation than those in the sandstone rock fragments and aggregates; the mineralized C and N derived almost entirely from the extractable fraction, whereas humin appeared to be virtually indifferent to the LTA treatment. In fact, although the amount of C lost during the treatment was similar for the three fractions, the siltstone rock fragments lost only 11% of initial humin-C pool, against 30% of the aggregates and 60% of sandstone rock fragments. As the three fractions showed similar mineralogy, the obtained results indicated that the soil fraction richest in micropores exerts a better protection on the organic matter.