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Evaluation of Microbial Activity in Soil Aggregate Stabilization and Degradation by the Use of Artificial Aggregates¹

ABSTRACT

A method for evaluating the mechanisms and factors involved in soil aggregate stabilization by microorganisms is introduced and data are presented to illustrate its use. This method involves the incubation of artificial soil aggregates of uniform and controlled physical and chemical properties, so that the effect of one or more variables of soil composition or environment on aggregate stability can be investigated.

The stabilities of differently-amended artificial aggregates of two diverse soil materials, incubated for up to 20 weeks, were determined by wet-sieving the dried aggregates. The two soil materials, one obtained from the Ap horizon of a Kewaunee silty clay loam (Gray-Brown Podzolic) and the other from the Al horizon of a Waupun silt loam (Brunizem), differed markedly in their contents of clay, nitrogen and organic carbon.

Microbial gum-amended Kewaunee aggregates possessed a higher initial stability and were more resistant to degradation with continued incubation than corresponding Waupun aggregates. Added nitrogen was associated with accelerated degradation of Kewaunee but not Waupun gum-amended aggregates. Incubated sucrose-amended Kewaunee aggregates showed a slower increase to maximum stability but retained stability longer than sucrose + nitrogen-amended Kewaunee and sucrose- and sucrose + nitrogen-amended Waupun aggregates. Nonamended and nitrogen-amended aggregates of both soils were completely unstable throughout the incubation period. The stability of moist aggregates was a function of wetting rate and moisture status prior to sieving; aggregates moistened slowly to saturation were 100% stable to wet-sieving.

Periodate treatment of the aggregates indicated that, with the exception of incubated sucrose-amended Waupun aggregates, "periodate-oxidizable" polysaccharides were important in aggregate stabilization.

Four days of incubation under highly aerobic or anaerobic conditions caused a rapid increase in the stability of sucrose-amended Waupun aggregates. Continued aerobic incubation resulted in a rapid decrease in aggregate stability; aerobically incubated aggregates were totally unstable after 12 days. In contrast, continued anaerobic incubation was associated with an increase in stability; 100% aggregate stability was reached after 15 days and was still present after 4 weeks. Anaerobically-stabilized aggregates lost their stability completely after 9 days of aerobic incubation.

¹ Contribution from the Departments of Bacteriology and Soils, University of Wisconsin, Madison. Published with the approval of the Director, Wisconsin Agr. Exp. Sta. under Project Heading No. 722. Presented before Div. III. Soil Science Society of America, Aug. 22, 1962, at Cornell University, Ithaca, N. Y.

² Research Assistant, Professor of Bacteriology, Assistant Professor of Soils, and Professor of Soils, respectively.

Received for publication October 5, 1962. Accepted for publication December 11, 1962.