The Impact Of Continuous Soybean Monoculture On Soil Communities In The Rhizosphere

Continuous crop monoculture results in a transformation from a conducive to suppressive soil, but we lack a comprehensive understanding of how soil microbiota develop in the process over time. The aim of this study was to investigate whether the dynamic processes in the composition and abundance of rhizosphere bacterial, fungal, nematode, as well as fusarium communities and Phytophthora sojae in the long-term soybean monoculture (up to13years) are linked to the transformation and which the main factor drive shifts in these soil microbial taxa. To determine abundance of bacterial, fungal, nematode, fusarium communities and P. sojae, we used quantitative PCR (qPCR), and to study the structure and diversity of bacterial, fungal and fusarium communities, we used a454-pyrosequencing technique. Furthermore, we used T-RFLP to test the compositions and abundances of nematode community in soils. In addition, to understand the ecological function of root exudates, we established a microcosm system to evaluate the effects of genstein and daidzin as well as their mixture on fusarium communities and P. sojae. The results are as following:(1) The abundances of fungal communities were significantly higher from1to3years than those from6to13years. The Shannon index and evenness of fungal communities are higher from6to13years than those from1to3years. Evenness is negative related to fungal abundance. The relative abundance of pathogenic fungi such as Boeremia and Clavariopsis were higher in the second and third cycle, whereas the relative abundance of beneficial fungi such as Bionectria, Melanospora, and Hypocrea were decreased. However, after6-year soybean monoculture, fungal communities were characterized by several beneficial species, while the abundances of pathogenic fungi were lower than year2and3. In addition, the special root exudates such as isoflavones, genstein and soil chemical factors, primarily pH, had a regulatory effect on the fungal communities. Bacterial abundance were rapidly reduced from1to3years and remained constant until the end of experiment. Bacterial community composition was affected by Olsen P, NO3-and pH. Our results suggest that the transformation from initial healthy to conducive soil may be attributed to not only high proportion of fungal pathogen taxa, but also a large fraction of fungal community and a small fraction of bacterial community. Then, the transformation from conducive to suppressive soil and the sustainment of soil suppressive may be due to the high proportion of beneficial taxa in soils.(2) No clearly temporal compositional trends in Fusarium community composition and abundance were observed from1to13year. Isoflavonoids concentrations in soils were significantly correlated to Fusarium community shifts. In addition, P. sojae abundance increased from1to3years, whereas, the abundance of P. sojae was negatively correlated to isoflavonoids concentrations. The strong response to isoflavonoids of these two rhizosphere fungi populations highlights that soil microbes depend more on plant physiology than soil chemical parameters. Fusarium communities from species-poor fields were found to be subsets of those in richer fields, indicating nestedness and a progressive ’loss’ from the species pool. Our results suggest that Fusarium community is influenced by the combination of deterministic and stochastic processes.that the Fusarium community present in soybean fields might not be the main factor in yield shift, but P. sojae population density might was of an importance.(3) The species richness of nematode communities was decreased with the increase of continuous monoculture. Acrobeloides was prevalent in the soil. The relative abundances of plant parasite nematode are higher from2to3years than those from6to13years. NMDS indicated that the nematode communities in rhizophere soil collected from the9,11, and13years closely clustered together, outside of the first three years. Furthermore, nematode community composition was affected by available P, soil organic matter (SOM), pH, bacterial and fungal abundance. The abundance of nematode community was increased from1to6years, and then it was decreased from9to13years. In addition, the abundance of nematode community was positively correlated to NH4and genisten and was negatively correlated to NO3-and bacterial abundance. Our results suggest that nematode community is influenced by soil microbiology and many soil factors, and that nematode community present in soybean fields might be an important factor in yield shift.(4) Exogenous daidzin and genistein as well as their mixture resulted in the changes of Fusarium community. The number of the intense bands assayed by DGGE that may represent numerically dominant fusarium species were increased from the third day to the seventh day and then decreased at the forteenth day. Cluster analyses indicated that samples from different treatements could be distinguished from each other. Daidzin, genistein and their mixture have strongly impacted on the abundance of Fusarium community in incubated the third day, but have only weakly affected on it with the increase of incubated days. However, these solutions have no effect on the abundance of P. sojae in incubated the third day, while the high concentrations of these solutions have an inhibitory effect on it in incubated the seventh day. In incubated the forteen days, only the highest concentrations of these solutions increased the abundance of P. sojae. The inhibitory or sitimulatory effects of isoflavones on these two pathogenic fungi depended on incubated time and their concentrations.In conclusion, we found that long-term monoculture soybean led to decreases of richness in soil bacterial, fungal and nematode communities and that soil biotic community sizes were decreased by monoculture soybean.