Biological and physical processes that mediate micro-aggregation of clays

The aggregation of clays after the addition of organic materials is described. Clays were incubated with or without added organic matter in the form of grass, straw, or charcoal and needed to be dried to a water potential of -1.5 MPa or less to aggregate. It was the fine fraction of the clay (< 0.5 mu m) that aggregated after organic additions, and this was apparent even in clays or clay mixtures that had a broad-particle size distribution. Grass was more effective than straw at aggregating the clay. When chloroform was added to the samples, there was little aggregation, suggesting that micro-aggregation after the addition of organic material to clay is mainly microbially mediated. The aggregation of clay in the presence of added substrate was temperature dependent, with an optimum between 20 and 30 degrees C, but in the absence of substrate aggregation increased with temperature without a maximum. This supports the hypothesis that biological mechanisms are playing an important role in aggregation.