Glomalin is an insoluble glycoprotein produced by hyphae of arbuscular mycorrhizal (AM) fungi. It is resistant to degradation and is found in large amounts in soil. Classical operationally defined extracts of soil organic matter include a large proteinaceous fraction. Therefore, clarification of glomalin as a separate fraction of extractable soil organic matter (SOM) is needed. Proof that glomalin accumulates over long periods of time has not been attempted. The overall hypothesis tested for this dissertation is that AM fungi are the source of an abundant, unique and important SOM component. The quantity of glomalin in soils was compared with particulate organic matter (POM), glomalin, humic acid (HA), and fulvic acid (FA) which were sequentially extracted from 5 8 undisturbed U.S. soils, aggregates and agriculturally managed soils that differed in tillage, crop rotation, and/or fertilizer amendment. Each fraction was extracted with the appropriate procedure: glomalin in pH 8.0 citrate at 121C, POM by floatation in NaCl solution, and HA and FA in NaOH with acidic separation. Organic matter fractions were evaluated for total and immunoreactive protein and/or gravimetric and C weights. Percentages of C, N and H were used to characterize each fraction. Glomalin structure was examined by proton nuclear magnetic resonance (1H NMR), removal of iron and separation of amino acid and carbohydrate groups. Glomalin accumulation in pot cultures was assessed at 14-week intervals in a 294-day experiment. Glomalin was unique in protein, C, H, and N contents compared with HA, FA and POM. Glomalin contributed ca. 20% of soil organic carbon. A recalcitrant glomalin pool was discovered that might have a functional role in water-stability of aggregates. 1H NMR spectra of glomalin were unique compared with HA spectra. Extracted glomalin had tightly bound iron, organic matter, amino acids and carbohydrates. Sustainable agricultural management practices reduced tillage, increased crop diversity, and reduced synthetic amendments increased aggregate stability of bulk soil and glomalin and POM concentrations. Glomalin production under controlled conditions was affected by irradiance. These results provide evidence that glomalin is a separate and unique fraction of SOM and is important in terrestrial carbon sequestration and sustainable agricultural practices.