Composition and reactivity of morphologically distinct charred materials left after slash-and-burn practices in agricultural tropical soils

The two BC fractions were higher in carbon than the plant material. On an ash-free basis the CF and FF sampled from the two different sites had remarkably similar elemental contents. Stable isotope ratios of carbon showed enrichment or depletion depending on the morphological fraction under C3 vegetation. The ratios tended to be depleted in 13C with regards to the plant material in both fractions for samples taken under C4 vegetation. The reactivity of BC towards dichromate oxidation and acid hydrolysis was lower for CF compared to FF. 13C CPMAS NMR spectroscopy showed that BC fractions were aromatic but could also show substantial contribution from alkyl and O-alkyl C. Analytical pyrolysis and CuO oxidation indicated that part of the lignin backbone was remaining in all BC fractions. We conclude that the differentiation of BC fractions according to morphological criteria was meaningful.