Submitted to: Humic Substances Seminar V
Publication Type: Abstract Only
Publication Acceptance Date: March 20, 2001
Publication Date: March 20, 2001
Citation: NICHOLS, K.A., WRIGHT, S.E., SCHMIDT, W.F., DZANTOR, E.K. GLOMALIN AS A CONSTITUENT OF HUMIC SUBSTANCES. HUMIC SUBSTANCES SEMINAR V. 2001.
Humic acid (HA), fulvic acid (FA) and humin are operationally defined by extraction method and solubility. Solubilized HA and FA are heterogeneous mixtures of biopolymers, polysaccharides, proteins, and metals that have been further purified by chemical fractionation techniques. A novel extraction method (citrate, pH 8.0, at 121C for 1 h) has been used on soil to remove a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi. This ubiquitous and recalcitrant glycoprotein, glomalin, accumulates in soils throughout the US and the world (typically 2-14 mg/g). Two approaches were taken to define glomalin as a constituent of humic substances: (1) citrate extraction followed by alkaline extraction, and (2) alkaline extraction followed by citrate extraction. When the first approach is used, glomalin is removed by citrate, and HA and FA are alkaline extractable from the residual soil. Likewise, the second approach yields citrate extractable glomalin from residual soil after HA and FA have been removed. If glomalin is not extracted first, it is present in small amounts in the HA fraction as a proteineaous contaminant. This indicates that a specific extraction procedure and sequence should be used to obtain relatively pure HA or FA. For glomalin, the extreme conditions used in the citrate extraction limit contaminants. Weight, C and N content, total protein analysis, and proton NMR spectra all show that glomalin, HA and FA are distinct constituents. Since glomalin may be further extracted from the soil after alkaline extraction and requires extreme conditions for solubilization, we propose that glomalin is part of 'insoluble' humin in soils.