Bioactivity and chemical characteristics of humic acids from tropical soils sequence

Humic acids (HA) isolated from a sequence of six Brazilian tropical soils with different degrees of weathering were characterized to evaluate possible relationships among their structural features and bioactivities toward plant growth. The chemical properties of HA, including elemental, conformational, and spectroscopy characteristics, were related to the HA effects on the activity of the plasma membrane H+-ATPase, as a biochemical marker for HA-induced root growth processes. The HA isolated from highly weathered soils had a larger oxidized character than HA isolated from less weathered soils. Organic matter transformation in more weathered soil induced an increase in carboxylic acidity accompanied with a decrease in alkyl content, as well as fluorescence intensity and content of free radicals. These changes suggested an alteration of the originally condensed aromatic compounds (lignin units) into unsaturated carboxylic acids. The HA derived from less weathered soils stimulated the plasma membrane H+-ATPase activity and maize root growth about twofold more than HA isolated from more weathered soils. Hydrophobicity and acidic functional groups represented the HA characteristics mostly effective in the stimulation of plant bioactivity. These results revealed a still unexplored relationship between chemical and biological properties of soil organic matter as a function of the weathering degree of native soils.