Characterization of humic acids from termite mounds and surrounding soils, Kenya

Abstract

Humic acids (HAs) extracted from termite mounds constructed by two major species of Macrotermes ( M. michaelseni and M. herus) and from surrounding soils in five ecological regions of Kenya were characterized by 13C NMR, IR spectroscopy, elemental analyses and optical measurements. Termite mounds had higher pHs, more clay but less sand and lower C N ratios than adjacent soils. Except for one site, HAs from termite mounds contained less C and more O than did HAs extracted from adjacent soils. E 4 E 6 ratios in all instances were lower for HAs from surrounding soils than for HAs from termite mounds, indicating that the molecular weights of the HAs for soils were higher than those of HAs from mounds. 13C NMR spectra of HAs showed major resonances at 115.2, 128.6 and 136.6 ppm (due to aromatic C) and at 175 ppm (C in CO 2H groups). Oxygen-substituted C (including carbohydrates) decreased, whereas CO 2H groups increased from the outer crust to nursery to the royal chamber of the mound. The distribution of C in HAs from different parts of termite mounds resembled that of HAs from adjacent soils except that soil HAs were less aromatic. Site appeared to affect the composition of HAs: HAs extracted from both mounds and soils at sites 2–5 were more aliphatic than HAs isolated from the mound and soil at site 1. In general, HAs from soils adjacent to termite mounds contained more CO 2H groups. IR spectra did not show any significant differences between mound and adjacent soil HAs.