| This study employed Terminal Restriction Fragment Length Polymorphisms (T-RFLP) to analyze community diversity of rhizosphere bacteria and arbuscular mycorrhizal fungi (AMF) of representative plants on Loess Plateau. Results showed impacts of plant species and rhizosphere environment on the microbial community structure, as well as interactions between rhizosphere microorganisms. Experiment was then conducted to study impacts of AMF on soil microbial communities, enzyme activity, nutrient levels and water-stable aggregates in the rhizosphere of Robinia pseudoacacia. In addition, pot experiment and stable isotope technique were used to study impacts of fertilization levels and AMF species on R. pseudoacacia growth and nutrient uptake. These demonstrated the functional mechanism of AMF on plant nutrient transmission. The major conclusions are as following:1. The community diversity analysis of rhizosphere bacteria and AMF in Liudaogou watershed in Shenmu County, Zhifanggou watershed in Ansai County and nursery in Yangling indicated that there was a great difference between rhizosphere bacteria according to plants species. The abundance and Shannon diversity index of bacteria were highest in the rhizosphere of Amorpha fruticosa in Yangling nursery (139and4.40, respectively), and lowest in the rhizosphere of A. fruticosa and Ulmus pumila in Zhifanggou watershed, significantly different from other plants (p<0.05). Rhizosphere bacterial community diversity and host plant species were not strictly specificity. The AMF community diversity varied in three regions and occurred at lower levels than bacteria. The abundance and Shannon diversity index of AMF were highest in the rhizosphere of Salix babylonica in Zhifanggou watershed (87and3.05, respectively), and lowest in the rhizosphere of Hippophae rhamnoides in Liudaogou watershed, significantly different from other plants (p<0.05). Despite high specificity of AMF community diversity of the same type of plants, there was high similarity amongst individual plants. Rhizosphere bacteria and AMF community diversity index were positively correlated (p<0.05). Bacterial and AMF community diversity in the rhizosphere of R. pseudoacacia was significantly higher than other plants in three regions (p<0.01). Redundancy analysis showed that rhizosphere soil organic matter and pH had great impact on bacterial community diversity and that available phosphorus had great impact on AMF community diversity.2. The study of impacts of fertilization levels and AMF species on the biomass and nutrition uptake of R. pseudoacacia indicated that single inoculation of Glomus mosseae (Gm) and G. constrictum (Gc) and mixed inoculation of (Gm+Gc) increased biomass of R. pseudoacacia. Different fertilization levels for R. pseudoacacia affected the mycorrhiza infection rate, and defined amount of nitrogen (N) fertilizer could increase mycorrhiza infection rate. The mycorrhiza infection rate was highest (74.5%)0.2～0.3g·kg-1N fertilizer was applied. Excessive N fertilizer appeared to decrease the mycorrhiza rate. By comparison, phosphorus (P) and potassium (K) had less affect on the mycorrhiza infection rate. AMF inoculation under different nutrient levels promoted the uptake of N, P and K by R. pseudoacacia. Among these, the uptake of P was most seriously affected. Inoculation of R. pseudoacacia increased the amount of absorbed P by73.0%when0.3g·kg-1P fertilizer was applied. There were differences between different type of AMF. Defined amount of calcium (Ca) fertilizer promoted the uptake of N and P by R. pseudoacacia. When0.1～0.2g·L-1Ca fertilizer was applied, AMF elevated total N and total P adsorbed by R. pseudoacacia to the peak,(7.85g·kg-1and6.62g·kg-1, respectively). However, applicatoin of Ca fertilizer restrained the uptake of K. Combined four compartment box and stable isotope technique, hyphal links were discovered between AMF. The hypha transfer rate of two kinds of AMF was exceeded25%. This indicated that nutrition could be absorbed and transmitted to the host plant by AMF hyphal links, which exerted positive effects on plant N nutrition. The uptake rate of ammonium sulphate-15N by R. pseudoacacia was higher than that of urea-15N. Compared to urea-15N, the ammonium sulphate-15N abundance in root, stem and leaves of R. pseudoacacia in single inoculation of Gc treatment was improved by98.36%,111.11%and54.76%, respectively.3. The study of AMF impact on soil microbial communities, enzyme activity and soil nutrients in R. pseudoacacia indicated that single inoculation of Gc and Gm, as well as mixed inoculation Gc and Gm all increased the abundance of bacteria, fungi and actinomycetes in the rhizosphere of R. pseudoacacia(p<0.05). Inoculation of AMF had greatest influence on the abundance of bacteria, and single inoculation of Gc lead to a3.6times increase in bacterial abundance compare to CK. In addition, single inoculation of Gc lead to a1.5times increase in the abundance of actinomycetes, whereas the influence of AMF inoculation on the abundance of fungi was minimum. Inoculation of AMF significantly improved the activity of soil alkaline phosphatase, sucrase and catalase(p<0.05), with impacts of single AMF inoculation was greater than mixed inoculation, Compare to CK, single inoculation of Gm increased enzyme activity by2.9,2.0and0.5times, respectively. In addition, mixed inoculation of Gc and Gm significantly improved urease activity (p<0.05), which increased by15.45%compare to CK. Inoculation of AMF decreased rhizosphere soil pH, and the effect of single inoculation of Gc was statistically significant (p<0.05). Inoculation of AMF had little influence on soil nutrient. Correlation coefficient test of various factors in the rhizosphere of plant showed mycorrhizal colonization rate and plant dry weight were positively related(p<0.05), indicating inoculation of AMF strongly influenced plant growth.4. The study of AMF impact on rhizospheric soil water-stable aggregate using three compartment box indicated that inoculation of AMF significantly increased rhizosphere soil polysaccharides and glomalin-related protein content(p<0.05), and the effect of single inoculation of Gm which increased by0.5and2.2times compare to CK, was superior to single inoculation of Gc. Inoculation of AMF substantially increased water-stable macro-aggregate content of particle size>0.25mm but had little influence on that of particle size<0.25mm. In soil of hypha room, hyphal density of single inoculation of Gm and Gc were9.5and7.2m·cm-3, respectively. Water-stable macro-aggregate content of particle size>0.5mm increased with the distance from plant roots, but micro-aggregate content of particle size<0.5mm did not change significantly. Results of correlation test showed that AMF had significant effects on water-stable macro-aggregate content of particle size>0.5mm, and aggregate content and hypha density were significantly correlated (p<0.05), with infection rates and glomalin content were significant positively correlated (p<0.01). In addition, water-stable macro-aggregate content of particle size>0.5mm and root dry weight and root phosphorus were significantly correlated (p<0.05). This showed the formation and development of soil aggregates were bound up with plant root growth. Water-stable aggregate content of particle size in0.25-0.5mm were to some extent correlated with various environmental factors, but the correlation was not significant.
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