Soil aggregates formed in vitro by saprotrophic Trichocomaceae have transient water-stability

The contribution of fungi to aggregation of soil is thought to be mediated via hyphal enmeshment of particles and increased adhesion between particles. The role of saprotrophic fungi in soil aggregation remains uncertain. This study explored the role of saprotrophic Trichocomaceae in water-stable aggregation of soil. Eighty-five isolates derived from agricultural and undisturbed Australian soils were identified by morphological and molecular (ITS1–5.8S rRNA–ITS2) methods. The formation and stabilisation of soil aggregates by each isolate either through increased adhesion between particles, mediated by extracellular materials, or enmeshment during hyphal growth, and the effects of nutrition on aggregation were examined. The creation of water-stable aggregates by Trichocomaceous fungi was uncommon and largely transient, whether measured by adhesive materials alone or growth of the fungus in soil. Water-stable aggregation by adhesion was increased significantly by only one isolate. The adhesive properties of extracellular materials produced by this isolate were affected by the source of carbon. When grown in soil only six isolates increased water-stable aggregation by more than 12% after three weeks. When grown on compost (complex carbon) selected isolates were capable of forming water-stable aggregates although these aggregates were generally not persistent. Mean weight diameter peaked after three weeks and declined thereafter with increased period of fungal growth. The potential to create water-stable aggregates by adhesion or soil enmeshment was isolate specific: the potential varied within species and was unrelated to extensive hyphal growth in culture. Saprotrophic Trichocomaceae are capable of aggregate formation by physical enmeshment; however the aggregates formed are not stabilised beyond the short growth period of the fungi. Any increase in water-stable aggregation by Trichocomaceae as a group is likely to be transient.

► 85 soil saprotrophic Trichocomaceae identified using ITS1 and ITS4 primers.
► All isolates tested for ability to create water-stable soil aggregates.
► 6 isolates increased aggregation by hyphal enmeshment; 1 increased by adhesion alone.
► Potential to aggregate was isolate specific and influenced by source of nutrition.
► Trichocomaceae have limited role in the formation of water-stable aggregates in soil.