Compositions, Aspects of Structures, and Clay Associations of Soil Humic Components.
Michael H.B. Hayes, Univ of Limerick, Chemical and Environmental Sciences, Foundation Building, Limerick, Ireland, André J. Simpson, Dept of Physical and Environmental Sciences, Univ of Toronto, Scarborough College, 1265 Military Trail, Toronto, ON M1C 1A4, Canada, C. Edward Clapp, USDA ARS & Dept of Soil, Water & Climate, Univ of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108, and James Burdon, Univ of Birmingham, School of Chemistry, Edgbaston, Birmingham, England.
A sequential, exhaustive extraction procedure was used to isolate and fractionate organic components from soils. Extractions involved aqueous base at pH 7 and 10.6 followed by 0.1M NaOH (under N2), then 0.1M NaOH + 6M urea, and then in some instances with 0.5M NaOH. The base + urea, was washed out and the clay fraction isolated, freeze dried, and extracted (exhaustively) with DMSO + 6% H2SO4 (v/v), and finally the residual clays were removed by successive washings with 10% HF. The urea, DMSO, and residuals from HF treatment are humins in the classical definitions. The urea extracts were similar to the aqueous extracts in base, but the residual organics were very different (B, Fig. 1) as were those in DMSO extracts and the residues from the HF treatment (C and D, Fig.1). The latter three can be regarded as true humin materials. Each aqueous isolate was processed by resin technologies and, for example, up to 12 compositionally different humic acids (HAs) and fulvic acid (FAs) fractions were isolated from each soil. In general the differences were relatively small between the components isolated at pH 12.6 and in base + urea, indicating that urea, a breaker of hydrogen bonds, released materials hydrogen bonded in the humin matrix, or released from steric constraints within that matrix. Diffusion ordered (NMR) Spectroscopy (DOSY) clearly showed that, for the aqueous extracts, the macromolecular concept for HAs and FAs no longer holds. Instead it is clear that the so-called FAs, and especially the HAs are associations of molecules of relatively low to intermediate (up to 2000 Daltons) molecular weight values, and on the basis of chemical shift assignments the associations consist predominantly of carbohydrate and peptide residues and of altered lignin components, and with some long chain fatty acids, hydrocarbons, and waxes (which may be binding agents). Fig. 1 shows the contrast between HAs isolated at pH 12.6 (A) in the extraction sequence, the humin associated with clay after base + urea extraction (B) of a grey-brown podzol, and the humin isolated from a Mollisol in DMSO + H2SO4 (C) after base + urea extraction, and after treatment with HF (D). In all cases the humin is low in lignin-derived aromaticity, rich in hydrocarbon, and in carbohydrate/peptide materials. The hydrophobic hydrocarbon residues (including long chain acids/esters) and waxes bind strongly to clays, as do mucopolysaccharides of glomalin type structures (indicated by carbohydrate and peptide residues in the humin fractions). Figure 1. CPMAS 13C NMR spectra of a humic acid isolated at pH 12.6 (A), a humin/clay association (B), humin isolated from a Mollisol in DMSO + acid (C), and humin released by dissolving clay in HF.