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Chemical and Physical Stability of Microbially Stabilized Aggregates¹

ABSTRACT

Chemical properties of microbially bound aggregates were determined by organic solvent extraction and by the rate of sonic dispersion of the aggregates. Seven fungi, six streptomycetes, and four bacteria, produced in situ chemically different binding agents which conferred different chemical and physical properties on soil aggregates. A. tenuis, S. atra, and two species of Penicillium apparently bound soil aggregates by the production of humic-like binding materials. Six streptomycetes and two Aspergillus species obtained their effectiveness by production of a combination of humic-like components and polysaccharides. Components of lignin-like character were involved in the stabilization of aggregates bound by two Aspergillus species. B. polymyxa, R. trifolii, and two species of Agrobacterium bound soil aggregates largely by NaIO₄-oxidizable polysaccharide gums. M. hiemalis was the only organism that bound aggregates by waxy or fatty components.

Microorganisms differ widely in their ability to affect mechanical stability of water-stable soil aggregates. The ability of each microorganism to bind mechanically stable aggregates varied between soil types. Aggregates bound by A. tenuis and S. atra, B. polymyxa, and A. radiobacter were more stable than those bound by other microorganisms, as evidenced by the longer exposure time required for sonic dispersion. The aggregates produced by S. purpurascens and S. coelicolor were less resistant to sonic dispersion. Aggregates of three soils bound by A. zonatus and M. hiemalis were destroyed easily by sonication. Microbially-bound Kewaunee clay aggregates were generally the most resistant to sonic dispersion.

¹ Contribution from the Dept. of Bacteriology and Soil Science, Univ. of Wis., Madison. Published with approval of the Director, Research Division, College of Agricultural and Life Sciences, Univ. of Wis., Madison under Hatch Project 722.

² Research Assistant, Professor of Bacteriology, Professor and Associate Professor of Soil Science, respectively.

Received for publication September 8, 1970. Accepted for publication October 8, 1970.