Structural variations of humic acids in two sola of Alberta Mollisols

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Humification is one of the dominant pedogenic processes responsible for the development of Mollisols but systematic studies of humic acids in the whole sola were rarely documented. To further understand this process, humic acids were extracted from sola of two Mollisols in central Alberta with 0.1 M Na 4P 2O 7 and analyzed in our laboratory. Humic acid yields decrease abruptly with soil depth in mineral horizons under grass-forest communities as compared to grass vegetation. Their carbon content increased with soil depth while the hydrogen content decreased. The 13C CP/MAS NMR results and X-ray diffraction analysis indicated that humic acids became more aromatic with soil depth. Formation of polynuclear aromatic rings was likely one of the mechanisms responsible for the higher aromaticity. Further study of the simple aromatic components in their CuO digest indicated that the contents of vanillyl, syringyl and cinnamyl phenols were reduced and the p-hydroxyl phenols became progressively more dominant as soil depth increased. We propose that high content of p-hydroxyl phenols could serve as an indicator of humic acids from mineral horizons in different soil environments. Our data show that the middle Ah horizon was the point where the nature of humic acids was transformed into more aromatic, possibly more polycondensed, and less lignin-like substances.