Rhizospheric Bacterial And Ammonia-oxidizer Communities Under Continuous Monoculture Of Soybean Crop

Cultivation of soybean (Glycine max) in Northeast China is characterized by high proportion of continuous monoculture which results in reduced crop production. Among all biotic and abiotic factors, changes in soil microbial communities induced by roots activities, especially root exudation might play an important role in it. The aim of present study was to investigate response of bacteria、ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) to root exudation in an experimental field of soybean monoculture continuously up to13years. Furthermore, a laboratory incubation experiment was performed to determine the effects of root exudates on the compositions and abundances of bacteria、AOA and AOB. In addition, to evaluate effects of root exudates as carbon sources on microbial communities, we used stable isotope probing under field and pot conditions and pulse labeled of soybean plants with13CO2. The main results were as follows:(1) The abundance and community composition of bacteria and microbial biomass in soybean changed significantly with years of mono-cropping. The abundance of bacterial community increased from1to4years, and then it decreased from5to9years, then increased again from9to13years, except the11th year. Microbial biomass was correlated with both of genistein and daidzein. The change of community composition of bacteria was no clearly temporal compositional trends and affected by available P, total N, NH4+, microbial biomass, daidzein and genistein.(2) The link between plant and rhizosphere microorganisms by13CO2labeling was applied to study the effect of root exudation of soybean as carbon sources on rhizosphere microbial communities. Terminal-restriction fragment length polymorphism (T-RFLP) analyses revealed158bp and438bp T-RF predominated in the heavy13C-labled bacterial rRNA, clearly showed that these bacteria were the populations most actively assimilating the carbon provided by the pulse labeling of the soybean plants in the greenhouse. Bacterial T-RFLP fingerprints from DNA taken from different density gradient fractions after pulse labeling showed similar fingerprints with increasing buoyant density, indicating that DNA-SIP failed to tracking carbon flow through different compartments of the rhizosphere microbiota.(3) The community composition analyzed by T-RFLP fingerprints changed in the rhizosphere and non-rhizosphere with years of mono-cropping up to13years, including ammonia-oxidizing bacterial (AOB) and ammonia-oxidizing archaeal (AOA). NMDS indicated that the AOB community of the first year was far from2to13years. Furthermore, AOB community composition was affected by genistein and total N. AOA community composition was affected by genistein and soil organic matter (SOM). The abundance of AOB was not significantly changed with years of mono-cropping, but the abundance of AOA was decreased from1to4years, and then it was increased from5to13years.(4) During soil incubations, we tested response of bacteria, AOA and AOB to exogenous daidzein and genistein as well as their mixture. Our results demonstrated that the compositions of AOA and AOB were not responsive to the isoflavones, but the abundance of AOA and AOB were strongly accelerated at14th by daidzin, genistein and their mixture. In addition, community composition of bacterial was influenced by the mixture of daidzin and genistein at3rd. Daidzin, genistein and their mixture have strongly impacted on the abundance of bacterial community. Daidzein, low concentrations of genistein, mixture of daidzein and genistein had an inhibitory effect on it in incubated3rf,7th. High concentrations of genistein had no effect on the abundance of bacterial community at7th,14th.In conclusion, the abundance and community composition of bacteria changed significantly with years of mono-cropping, the community composition of AOA and AOB were affected by soybean mono-cropping, but had little effort on the abundance of AOB, the abundance of AOA was decreased from1to4years, and then it was increased from5to13years. The compositions of AOA and AOB were not responsive to the isoflavones, but the abundance of AOA and AOB were strongly accelerated at14th by daidzein, genistein and their mixture. Daidzein, genistein and their mixture had impacted on the abundance and community composition of bacterial. The approach of RNA-SIP was more effective than DNA-SIP in the study of interaction between plant-microbes.