Microbial Communities In The Rhizoplane And Rhizosphere As Affected By Soybean Cultivar And Cropping System

Microbial communities in the rhizoplane, rhizosphere and bulk soil were investigated with three soybean cultivars, including a resistant cultivar ( Kangxian 2, K2) to the soybean cyst nematode (SCN) ,a resistant cultivar (Suinong 10, S10) to the soybean root rot (SRR) . and a susceptible cultivar (Hefeng 25, H25) to both SCN and SRR under the controlled conditions by plate counting, direct counting, and a molecule-based method, 16S rDNA-PCR followed by denaturing gradient gel electrophoresis ( DGGE) .The total amount of colony forming units ( CFU ) of bacteria in the rhizoplane and rhizosphere increased gradually with time from trifoliate stage to early filling stage, but decreased dramatically at mature stage after its reaching a peak at the early filling stage. Same trends were observed in the variation of bacterial biomass in the rhizosphere of soybean with time by the acridine orange direct counting method (AODC) .These trends above were similar to the changing pattern of root exudates within the growth stages reported previously. The weight of root nodules of S10 and K2 was significantly higher than that of H25. Compared with the resistant cultivars (S10 and K2) , higher amount of pathogenic microbe ( Fusarium sp. and cyst of Heterodera glycines ) in the rhizoplane and rhizosphere of H25 was found at the mature stage. The DGGE patterns showed that the rhizoplane bacteria community among different soybean cultivars and different growth stages varied greatly. Moreover, the variation among different cultivars was bigger than that among growth stages. These results indicated that the component and amount of root exudates varied among different soybean cultivars, and even different growth stages of the same cultivar; exudates from soybean roots had some selective inhibitory or stimulative effects on microorganisms in the rhizoplane and rhizosphere. This leads to the formation of special microflora in the rhizoplane and rhizosphere.The effects of root exudates from two cultivars (S10, SRR resistant cultivar, and H25, SRR susceptible cultivar) on the variation of microflora in the rhizoplane, rhizosphere and bulk soil were studied by using the soil after planted soybean last year eliminated the residual roots and leaves. The results showed the weight of root nodule of H25 was lower than that of S10 significantly. The bacteria populations in the rhizoplane and rhizosphere of both the resistant cultivar and susceptible cultivar were decreased gradually, but the amount of fungi and fusarium was accumulated with time during the growth stage. Compared with bulk soil, the species of fungi in the rhizoplane and rhizosphere of the two cultivars became more single. These results indicated soybean root exudates had further selective inhibitory or stimulative effects on the microorganisms in the rhizoplane and rhizosphere under continuous monocropping soybean condition.Microbial communities in the rhizoplane, rhizosphere and bulk soil under different rotation andcontinuous monocropping systems in two long-term field plots (plot 1 and plot 2) were also tested byplate counting and 16S rDNA-DGGE methods. The treatments consisted of normal rotation soybean(NRS) , alternation of soybean with other crops (ATS) , continuous monocropping soybean (CS)for 2~9 years. Our data showed that the plant growth and seed commodity quality of NRS were betterthan those of ATS and CS in the two field plots. Plant growth became worse with the years of continuous monocropping within five years. However, it became better after six years of CS. The sup-oppressiveness of cysts of SCN was found after five years of continuous monoculture of soybean. In plot 1, the numbers of Fusarium in the rhizoplane or rhizosphere of both CS and ATS were higher than that of NRS at the seedling stage. However, the weight of root nodules of NRS was significantly higher than that of CS. At the flowering stage in plot 2, the rhizoplane bacteria diversity of NRS was the richest, and CS the lowest, with that of ATS between them. Moreover, the diversity declined after short-ter...