Properties of black soil humic acids from high altitude rocky complexes in Brazil

Forty two samples of black soils were collected from superficial horizons of the High Altitude Rocky Complex (HARC) that is situated on top of the most prominent mountain ranges of eastern Brazil. Our objective was to characterize the humic acids extracted from these pedoenvironments and evaluate the effect of natural paleo-fires on thermogravimetric properties, elemental composition, visible and infrared absorption of humic acids (HA). The high thermostability, low H:C ratio, and abundance in carboxylic groups, suggest that the HARC black soil HA present larger biostability and reactivity. The molecular characteristics indicated that HARC black soil HAs have a great contribution of transformed charred vegetable residues on its origin. This persistence and reactivity of black soil HA explain the capacity of these soils to maintain their cation exchange capacity, since the soil clay fraction is composed predominantly by low activity minerals.