Low-Temperature Ashing (LTA) by oxygen plasma has been used for removing organic matter (OM) from the mineral matrix of soil aggregates with minimal disturbance and damage to the inorganic constituents. This technique allows a controlled oxidation at low temperature of organic substances of undisturbed soil aggregates. LTA removes OM, layer by layer, without altering or damaging the residual layers, like a peeling of onion skins. The aim of the work was to study the effect of progressive removal of the OM from undisturbed soil aggregates on adsorption of recPrP, a recombinant protein whose structure mimics the prion of TSE disease. Undisturbed natural aggregates (dry sieved at 0.5-1 mm size) collected from the surface horizon of a sandy and a loamy soil were used. Aliquots of these aggregates were submitted to LTA oxidation to partially reduce the natural C content and then treated with recPrP. Other aliquots of undisturbed aggregates were treated directly with recPrP. All aggregates were placed in Pasteur pipettes and treated by recPrP solution. The concentration of recPrP to be used for the treatments was estimated by preliminary adsorption isotherms of recPrP determined at 25°C on powered samples before and after LTA treatments. The percolate from aggregates were analysed for the presence of recPrP by colorimetric analysis (Lowry test). The aggregates-recPrP complexes were analysed by spectroscopic techniques such as FTIR-PAS (photoacoustic).

The adsorption of recPrP on undisturbed aggregates was greater in the loamy soil probably due to larger OM content than the sandy soil; maximal adsorption as high as 90% (loamy soil) and 50% (sandy soil) of the added amount at plateau were found. The reduction of about 30% of OM in the samples led to a reduction of recPrP adsorbed in both soils up to about 50% indicating the important role played by OM in the recPrP adsorption in soil aggregates.