Characterisation of Humic Substances Extracted from Soil Treated with Charcoal (Biochar)

The high fertility and resilience of Terras Pretas de Indios are attributed to the high contents of polycondensed aromatic structures with carboxylic functionality originating from the natural alterations of carbonised biomass added to the soil by the pre-Columbian indigenous populations. There are several proposals to reproduce the positive characteristics of these special soils, including the use of charred biomass (biochar); however, the time necessary to modify this material in the peculiar organic matter of Terras Pretas de Indios is still undetermined. The 13C NMR analysis of the humic acids extracted at pH 10.6 from soils treated with fines of charcoal indicates that after 3 years a small portion of the added charcoal is already partially oxidised to a compound similar to the ones found in Terras Pretas de Indios; however, the large fraction still looks like unaltered charcoal. Additionally, the proposed method of selective extraction of the characteristic humic fraction from Terras Pretas de Indios, combined with multivariate data analysis tools, can be used as a method to evaluate the proposals to reproduce these special soils.