Controlled removal of organic matter from undisturbed aggregates by Low-Temperature Ashing for studies on soil structure stability

Recent studies for various different applications have proposed Low-Temperature Ashing (LTA) by oxygen plasma for the removal of organic matter (OM) from soil samples with minimal or no damage to the mineral constituents. In the work the LTA procedure was applied to remove OM from undisturbed aggregates to investigate: i) the dynamics of the plasma oxidation processes; and ii) the effect of the partial removal of OM on the structural stability of aggregates. Undisturbed (3-4 mm) and ground aggregates from different type of soils - an Inceptisol under forest (Italy), an Alfisol under savannah (Zimbabwe) and a Vertisol under pasture (Italy) - were submitted to LTA treatment with different time exposures. Total C content was measured at each step of the experiment on the bulk sample and on the surface and inside of aggregates. The effect of the oxidation of OM on soil structure was studied using a test of the stability of aggregates to an osmotic stress applied with minimal mechanical disturbance. The oxidation of OM was shown to proceed from the surface to the inside of the aggregates, as evidenced by the different C contents find in the same. The removal of OM increased progressively until a plateau was reached in C content. The duration of treatment required to reach the plateau and the C content at the plateau differed with soil type and aggregate size. In general the oxidation of OM seems to be controlled by its physical accessibility. The results of aggregate stability tests of the various samples during the course of the treatments, have led to a better understanding of the complex contribution of OM to the dynamics of soil structural stability. LTA is therefore an useful tool for investigating the role of OM on the formation and maintenance of soil structure.