Stabilisation of HF soluble and HCl resistant organic matter in sloping tropical soils under slash and burn agriculture

Soil organic matter (SOM) stabilisation on tropical sloping land can be influenced by slash burning and erosion processes. However, information on the behaviour of potentially stable components of organic matter on burned sites is scarce. This study is focused on organic matter soluble after demineralisation of the samples with hydrofluoric acid (HF soluble) and organic matter remaining after hydrolysis with hydrochloric acid (HCl resistant), which are both part of the potentially stabile OM fraction in tropical soils. The objective was to examine their contribution to carbon stabilisation along a toposequence of soil profiles sampled in a small cultivated catchment in Northern Laos with slash burning as common agricultural practice. Bulk soil as well as residues after acid hydrolysis using HCl have been analysed for chemical composition and mean residence time of SOM. The tropical soils showed similar amounts of HF soluble and HCl resistant OC, representing between 20 and 60% of the total OC of the soils. OC stabilisation at the sites is chiefly determined by mineral interactions as shown by the good correlations of the clay and the HF soluble OC contents with 14C activity. Analysis of the chemical structure by solid state 13C nuclear magnetic resonance spectroscopy (13C CPMAS NMR) showed that aromatic carbon contributes substantially to HCl resistant organic matter. The 14C activity of the hydrolysis residue was lower compared to the untreated soil, reaching a 14C age of around 1000 years B.P. in B horizons. This is consistent with the observation that chemical recalcitrant black carbon, produced by slash burning, may remain on site for a long time, once it is incorporated into the mineral soil.