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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 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,R0.25).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 R0.25 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.

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 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.

Much of the North China Plain( NCP) is dedicated to cultivation of winter wheat and summer maize. In order to achieve high crop yields,nitrogen fertilizer has been used in excess,constituting the major cause of water pollution in the area. The saying that"all rivers are dry and polluted"has come to characterize the NCP's water resource environment. The grey water footprint of a product( WFproc,grey) is a novel method for quantitatively estimating water resource pollution due to agricultural production. Using agricultural data accurate at the county scale,we evaluated the WFproc,greyof crops grown in the NCP from 1986 to 2010 and analyzed spatial and temporal variability. The WFproc,greyof winter wheat was found to be0. 55—2. 97 m3/ kg and that for summer maize was 0. 50—2. 02 m3/ kg; these values are significantly higher( 2—10times) than those for similar crops grown in the rest of the world,such as the U. S. and Germany. On comparing regions within the NCP,we found relatively low WFproc,greyvalues for crops grown in Hengshui,Baoding,and some regions of Hebei province,while it was high for crops grown in Bohai Bay. The total grey water volume of winter wheat and summer maize production over 25 years was estimated in the range of 2. 67 × 1010—5. 84 × 1010m3,with an average of 3. 90 × 1010m3;despite yearly fluctuations,a sustained upward trend was apparent. In light of these results,the need becomes apparent for a reduction in the amount of fertilizer used in agriculture,improvements in the efficiency of applied fertilizer,the development of new methods( such as formula fertilization by soil testing and controlled-release fertilizers),and the replacement of traditional fertilizer with organic fertilizer and biogas slurry. Water resource pollution can be further reduced by preventing fertilizer losses from surface runoff and leaching into groundwater,adopting water-saving irrigation techniques( e. g.,drip irrigation and micro irrigation with integrated water and fertilizer technology),reducing water and soil loss via straw mulching and film mulching,and increasing water storage and organic matter accumulation in subsoil.