Mechanisms Involved in Soil Aggregate Stabilization by Fungi and Bacteria

Microbial mechanisms of soil aggregate stabilization were investigated as a function of microbiological and water‐stability changes of incubated artificial aggregates, 2 to 4.76 mm. in size composed of soil particles < 0.5 mm., prepared from Waupun silt loam. Aggregate water‐stability was determined by wet‐sieving and expressed as the percentage of soil material bound into water‐stable aggregates 0.5 to 1 mm., 1 to 2 mm. and > 2 mm. in size. Microbial development was investigated by dilution plate and microscopic methods. Aggregate stabilization was a function more of microbial synthesis of soil‐binding substances than of numbers of microorganisms. Extensive production of mycelium and spores in soil aggregates by a fungus known to be capable of aggregate stabilization, did not, necessarily, result in an increase in the stability of the aggregates. Stabilization of aggregates by Mucor silvaticus involved the binding of soil particles into > 2 mm. water‐stable aggregates at all stages of incubation. Aggregates stabilized under anaerobic conditions exhibited a high percentage contribution of 0.5‐ to 1‐mm. and 1‐ to 2‐mm. aggregates to total stability in the early stages of incubation. A relatively high proportion of the total water‐stability of sucrose‐amended aggregates possessing the indigenous mixed microflora was attributable to stable 0.5‐ to 2‐mm. aggregates in the early stages of aerobic incubation, an indication that bacteria were more important than fungi in the initial stabilization of these aggregates.