Black carbon contribution to soil organic matter composition in tropical sloping land under slash and burn agriculture

Tropical soils are generally depleted in organic carbon (OC) due to environmental conditions favouring decomposition and mineralisation of soil organic matter (SOM). In Northern Laos, sloping soils are subjected to slash and burn agriculture, which leads to production of black carbon (BC), a stable SOM fraction. BC may directly influence the quantity and quality of SOM sequestered in tropical soils. The aim of this study was to quantify BC content and evaluate its impact on the chemical and stable isotope composition of SOM along a catena composed of Dystrochrepts at the bottom of the slope, Alfisols (midslope) and Inceptisols at the top of the slope for different burning frequencies. Six soil profiles, situated on a slope ranging from a river bank to the summit of a hill, were sampled. The stable isotope compositions (¹³C and ¹⁵N) of samples from both organo-mineral A and mineral B and C horizons were determined. The chemical composition of SOM analysed by ¹³C CPMAS NMR spectroscopy and the contribution of BC determined by dichromate oxidation were compared to OC and iron oxide content as well as land management including the burning cycle.The highest C contents were recorded at midslope positions. At any position on the slope, δ¹³C and δ¹⁵N ratios showed an enrichment in ¹³C and ¹⁵N with increasing soil depth. The OC content of soil horizons was related to their aryl C content, which is the component most likely driven by BC inputs. The BC contributions analysed by dichromate oxidation ranged from 3% to 7% of total OC. A positive correlation was obtained between aryl C and the BC content of SOM. Comparison of BC content and stable isotope composition of SOM showed that BC influenced the δ¹³C and the δ¹⁵N stable isotope ratios of these soils. BC was not associated with the mineral phase. The highest BC contents were measured under intensive slash and burn practice in the vicinity of the boundary of Alfisols at the top of the slope, where erosion was severe. Therefore, BC, a SOM component strongly influencing OC sequestration of these soils, is susceptible to translocation down the slope.