Soil structure plays important roles in soil ecosystem functioning as it controls fluxes of water, gases and nutrients. Aggregate stability is used as an indictor of soil structure. Current approaches to investigate the formation and stabilization of aggregates in soils have been influenced considerably by the hierarchical model of the aggregation process in which primary particles and clay microstructure are bound into microaggregates (20 250 μm), and larger macroaggregates (>250 μm up to several millimeters diameter) are formed by the binding together of microaggregates and smaller macroaggregates. Many physical, chemical and biological factors (and their interactions) contribute to soil aggregation, yet among the biological aspects, mycorrhizas are recognized as being of special importance. Arbuscular mycorrhizal (AM) fungi form mutualistic symbiosis with more than 80% of the higher plant species. The contribution of AM to soil structure varied at different hierarchical levels: plant community, individual root, and the soil mycelium.
In our studies to investigate the impacts of AM fungi on soil aggregation dynamics in the neutral purple soil, split root device was used and the soil were passed through sieves and then amounts of different size of aggregate were measured to obtain fractal features of soils. Path analysis was also used to evaluate the indirect or direct influencing factors. The results showed that inoculation with both G. intraradices and G. mosseae tends to increase the organic matter content and total glomalin related soil protein content in mycorrhizal soil. Mean weight diameter (MWD) and geometric mean diameter (GMD) with wet-sieving methods in mycorrhizal compartment were significantly higher than that of root compartment which resulted in the lower fractal features of mycorrhizal soil. All these results account for the important role of AM fungi in improving soil structure. Path analysis showed that hyphal density had the most important direct effect. The different ability of AM fungus to improve soil structure revealed the function diversity of AM fungi. More AM fungal species and soil types will be considered in our following studies.