More than 85% of terrestrial plant species associate with arbuscular mycorrhizal fungi ( AMF) . Most plant individuals stimulate nutrient availability through their fungal partner. Highly mycorrhizal-dependent plant species derive more benefits from AMF in comparison with plant species with a relatively low mycorrhizal dependency. Until recently, the effects of AMF on the competition between mycorrhizal and non-mycorrhizal plants ( with high phosphate foraging ability) remained largely unclear, particularly in an environment with low phosphate bioavailability.Mycorrhizal effects on plant interactions are most likely due to alterations in the soil nutrient availability. AMF colonization could increase nutrient ( particularly phosphate) availability for host plants and consequently increase plant biomass. The larger plants are then expected to be successful competitors.In the present study, maize ( Zea mays L. ) and rape ( Brassica napus L. ) plants were cultivated in three different interaction status, namely monoculture, intercropping and nylon net split. All the plants were inoculated with Glomus intraradices, Glomus mosseae or remained uninoculated. The strength of competition is measured as the reduction in biomass caused by the presence of a competitor. The results showed that maize growth and mineral nutrition was suppressed, while the mycorrhizal dependency increased to some extent, in intercropping compared with monoculture system. In intercropping system inoculation with G. intraradices and G. mosseae significantly decreased phosphorus concentration and content of rape roots compared with non-inoculation treatments, while in contrast mycorrhizal association improved phosphorus nutrition of http: / / www. ecologica. cn maize plants. The experimental results clearly showed the beneficial effects of mycorrhizal associations on AMF host plants in competition with non-host plants. All these revealed that AMF may play an important role in structuring plant community and influencing plant biodiversity in ecosystems.

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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 滋m) , and larger macroaggregates (>250 滋m 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 http: / / www. ecologica. cn density, easy extractable glomalin (EEG) and mean weight diameter(MWD) , geometric mean diameter(GMD) and the amount of larger macroaggregates (>250 滋m 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.