Soil structure plays important roles in soil ecosystem functioning as it controls water, gas and nutrient fluxes in soil. Aggregate stability is used as an indicator of soil structure. Current approaches to investigate the soil aggregation dynamic 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 previous studies, most experiments were carried out with pot or other single compartment devices, this made it difficult to clarify different effects of mycorhiza symbiosis on soil aggregation. In present study,the split root device with four compartments was used to quantitatively compare the change of soil aggregates in mycorrhizosphere, rhizoshere, hyphosphere and bulk soil. The results showed that there were significant positive correlation between hyphal length density, easy extractable glomalin (EEG) and mean weight diameter( MWD), geometric mean diameter(GMD)and the amount of larger macroaggregates (>250 μm up to several millimeters diameter, R_0.25). The correlation coefficient between water stable aggregates and hyphal length density in mycorrhizosphere soil and hyphosphere soil was 0.777, 0.671 respectively. The GMD and MWD of R_0.25 in hyphal compartment were higher than those in root compartment but were lower than those in mycorrhizal compartment, which means mycorrhizal hyphae had greater effect than root did, but less effects than mycorrhizae did, on formation and stabilization of soil aggregates,. The different ability of different AM fungi to improve soil structure revealed the function diversity of AM fungi. More AM fungal species and soil types will be considered in following studies.