Abundance and composition of the refractory organic fraction of an ancient, tropical soil (Pointe Noire, Congo)

A deep sample (300–310 cm), collected in a ferralic tropical soil near Pointe Noire (Congo), was submitted to extensive extractions and drastic hydrolyses to isolate the refractory (non-hydrolysable) organic fraction. Previous studies showed that organic matter at depth in this soil is entirely composed of stable carbon with a very low turnover rate (14C age of ca. 8300 years) and originated from the forest to which the present vegetation (savannah) succeeded. The refractory fraction only accounted for ca. 5% of the total carbon of the soil. A conspicuous uncoupling is therefore observed, in this soil, between stable and refractory carbon. The morphological and chemical features of the refractory fraction of the Pointe Noire sample showed (i) the lack of both ligneous materials and aliphatic resistant materials such as cutans and suberans, (ii) abundant contribution of melanoidins with a high degree of condensation and cross-linking and (iii) the presence of two types of particles of black carbon. Finally, the results obtained on this ancient soil were compared to the compositions previously observed for the refractory fractions isolated from temperate (forest and crop) soils which exhibit mean 14C ages of ca. one century.