Stabilisation of soil against wind erosion by six saprotrophic fungi

Soils with biological crusts that consist of fungal hyphae, bacteria and other small organisms usually resist erosion. However, the processes by which soil organisms stabilise air-dry aggregates against wind erosion are not well understood. We used saprotrophic fungi to examine some of these processes in a sandy clay loam (Hypercalcic Calcarosol). Soil aggregates, wetted with distilled water or glucose solution, were inoculated with one of six fungi, and incubated in darkness at 24 °C for 7 d in petri dishes under sterile conditions. Abrasion resistance (taken as resistance to wind erosion), tensile strength of soil, hot-water extractable carbohydrates (HWEC), dispersion index, pH, and hyphal length density (HLD) were each measured across all treatments. In all treatments, stability (abrasion resistance) and tensile strength, were positively related to HLD. Such relationships have not been reported elsewhere. All fungi enlarged the aggregates of the soil by cross-linkage and entanglement of particles, but with different processes, or different intensity of the processes, between species (for the same amount of substrate). The skins seen in scanning electron micrographs of stabilised soil were probably extracellular polysaccharides also produced by the fungi. We propose that the ductile failure of disks of soil, particularly those inoculated with Mucor sp., under tensile stress was due to movement of enmeshed particles, whereas the brittle failure of disks of soil inoculated with the other fungal species was due to metabolites or dispersed clay on the surface of the hyphae which limited deformation.