Some effects of biological and physical processess on soil aggregate stability

The effects of biological and physical processes on the aggregate stability of some weakly structured arable and pasture soils were investigated. Preliminary investigations showed significant correlations between soil organic matter and both wet sieving and turbidimetric methods of aggregate stability; the latter method was chosen on grounds of convenience. Scanning electron microscope pictures showed the presence of both fungi and bacteria when soils were incubated. Growth of fungi, estimated by ergosterol measurement, correspond to temporary stability increases, which could be explained by retention of soil particles within the reticulum of fungal hyphae. The effect disappeared as the fungi were destroyed and replaced by bacteria and actinomycetes. Effects caused by fungi were examined separately, using vancomycin to inhibit bacterial growth, and bacterial effects by using cycloheximide to eliminate fungi. Bacterial growth had little direct effect in stabilising soil aggregates; periodate oxidation showed that polysaccharides produced by bacteria are mainly responsible. To examine the contribution of physical processes to increased stability in remoulded soils biological processes were eliminated by sterilisation. Thixotropic changes made a contribution to age hardening in remoulded aggregates similar in magnitude to biological processes. Thixotropic changes were reversible and accompanied by soil strength and metric water potential changes. Polysaccharides did not contribute to thixotropic aging processes. Remoulded soils were subjected to wetting/drying and freezing/thawing cycles. After 3 to 6 cycles the stability of both sterilised and unsteriliserd soils recovered to that of natural aggregates, suggesting a contribution by thixotropy. Repeated weathering cycles decreased the stability of unsterilised, and more so sterilised, field aggregates suggesting that in the former, bond reformation due to biological activity counteracted the destruction caused by wetting/drying and freezing/thawing