Change of microflora in the rhizoplane and rhizosphere of different disease resistance soybean cultivars Ⅰ.Change of microflora in the rhizoplane and rhizosphere of soybean under normal rotation cropping condition

Previous research found many factors might cause the obstacle,such as the change of physical and chemical properties in the soil,plant allelopathy and soil-borne disease,among which the soybean cyst nematode(SCN) and the soybean root rot(SRR) were the most serious and popular soil-borne diseases.The microflora in the rhizoplane and rhizosphere were affected by the root activity,especially,the root exudates.Differences in component and amount of the root exudates between resistant cultivar and susceptible cultivar hadn't been found.In order to investigate the microflora in the rhizoplane and rhizosphere soil of different disease resistant cultivar,we chose three soybean cultivars,the resistant cultivar(Kangxian 2,K2) to SCN,the resistant cultivar(Suinong 10,S10) to SRR,the susceptible cultivar(Hefeng 25,H25) to both SCN and SRR.A root box container with three compartment was covered by nylon net(30μm) which could limit the plant root going over.the seeds sowed in the middle compartment.Sampling was taken at the trifoliate stage,beginning of flowering stage,early filling stage and harvest maturity stage,respectively.The microflora in the rhizoplane,rhizosphere and bulk soil of the three cultivars was investigated during the growth stage with plate culture method.The biomass of bacteria and fungi in rhizosphere and bulk soil was detected by direct fluorescence microscopic counting.The results showed that the species of microorganisms in bulk were the richest,followed by rhizosphere soil and the microorganism species in rhizoplane were most simplex.The numbers of colony forming units(CFU) of bacteria in rhizoplane and rhizosphere soil increased gradually during the trifoliate stage and the early filling stage,reached a peak at the early filling stage,and then decreased dramatically at the mature stage.The numbers of CFU bacteria in the rhizoplane and rhizosphere soil for different soybean cultivars varied significantly.The root nodule weight of resistant cultivar was significantly higher than that of susceptible one.A more pathogenic microbe(Fusarium sp.and cyst of Heterodera glycines) in the rhizoplane and rhizosphere soil of susceptible cultivar at the mature stage was observed.It indicated that soybean root exudates had certain selective acceleration or inhibition role on the microorganisms in the rhizoplane and rhizosphere soil.The component and amount of root exudates varied between different soybean cultivars and even different growth stage of same soybean cultivar,resulting in the formation of special microflora in the rhizoplane and rhizosphere soil.