Humus composition of Amazonian Dark Earths in the Middle Amazon, Brazil

Abstract

The process of humus accumulation has been one of the most important aspects in the pedogenesis of Amazonian Dark Earths (ADE) and Terra Mulata (TM). ADE has been characterized by higher contents of P, Ca and humus than adjacent Yellow Latosol (YL) under anthropogenic effects, and TM has been characterized by slightly less dark topsoil than ADE. The objective of this study was to analyze the composition of humus in ADE using the Kumada's method. A total of 20 soil samples of the A horizons and the transitional AB and BA horizons from three soil profiles of ADE (12 samples), adjacent two YL (4 samples) and a TM (4 samples) were subjected to a humus composition analysis, in which successive extraction with NaOH and Na4P2O7 was conducted. The amounts of the humic acid fractions in NaOH and Na4P2O7 extracts from ADE were distinctly higher than those from the adjacent YL. The RF values, the absorbance at 600 nm per unit concentration of humic acids in 0.1 mol L−1 NaOH of ADE, were not necessarily the highest in the surface horizons, but the darkest humic acids often existed in the transitional AB or BA horizons. The humic acids extracted with NaOH from seven of 16 samples of ADE and TM were classified into Type A, the most humified group, while the humic acids extracted with Na4P2O7 were Type A in 11 samples. Moreover, the amounts of humic acid fractions and RF values of humic acids were positively correlated with the Al + Fe contents in humic acid fractions, especially those extracted with Na4P2O7. Thus, it is possible that humic substances in ADE are stabilized by forming complexes with Al and Fe.