Effects of intercropping and Rhizobial inoculation on the ammonia-oxidizing microorganisms in rhizospheres of maize and faba bean plants

• Treatments affected abundances of ammonia oxidizers and total prokaryotes.
• Treatments affected community composition of AOA at the pod-bearing stage.
• Abundances of bacteria > archaea and AOA > AOB were observed in all treatments.
• Combination of intercropping and rhizobial inoculation presented stronger effects.

To clarify whether intercropping and the Rhizobium leguminosarum inoculation affected the nitrification and subsequent nitrate leaching in soil, quantitative and qualitative analyses of total bacteria (TB), total archaea (TA), ammonia-oxidizing bacteria (AOB) and archaea (AOA) in the rhizospheres of maize and faba bean were performed. The greater abundances of TB than TA and of AOA than AOB were observed in all the samples, with ratios of TB:TA 21.1 and 8.0; and AOA:AOB 24.5 and 2.3, at the anthesis and pod-bearing stages, respectively. In faba bean rhizosphere, the intercropping and/or inoculation of rhizobia decreased the abundances of TB, TA, AOB and AOA at the anthesis stage; and inoculation of rhizobia alone decreased AOA at the pod-bearing stage. In maize rhizosphere, intercropping alone only enhanced the AOA at the pod bearing stage; while the combination of intercropping and rhizobial inoculation decreased the TA at the anthesis stage, and enhanced TB, AOB and AOA at the pod-bearing stage. The diversity of AOB and AOA were pronounced by intercropping and rhizobial inoculation treatments: at the anthesis stage for AOB and at the pod-bearing stage for AOA. However, Nitrosospira always remained as the predominant AOB in all the treatments. Conclusively, intercropping and rhizobial inoculation brought various shifts in microbial abundance and community compositions in rhizosphere depending on the plants and growth stages, which may decrease the nitrification in rhizosphere. Furthermore, the combination of intercropping and rhizobial inoculation presented stronger effects in most cases than the separated treatments.