Objective  Effects of combined application of organic and chemical fertilizers on the bacterial community in soil of banana plantations were studied.

  Method   Various proportions of chemical fertilizer combined with an organic fertilizer, as well as all or reduced chemical fertilizer, were mixed in the soil from a banana plantation to determine the effect of the applications on the bacterial community in soil. The treatments included the uses of no fertilizer as control (T1), 25% chemical fertilizer (T2), 50% chemical fertilizer (T3), 100% chemical fertilizer (T4), and 50% reduced chemical fertilizer without organic fertilizer (T5). 16S rRNA genes of the bacteria were analyzed by Illumina MiSeq high-throughput sequencing. A bioinformatic analysis was performed to determine the structure, abundance, and diversity of the bacterial communities in soil under treatments.

  Result   The OTUs of the T1, T2, T3, T4, and T5 treatments were 30, 33, 31, 34, and 31, respectively. The proportion of beneficial bacteria in the soils varied by the treatments. The relative abundances of beneficial proteobacteria and acidobacteria were increased by the treatment of T3. The Shannon index of the bacteria in soil under different treatments ranked as T3＞T5＞T2＞T1＞T4. In comparison to T5, T3 improved and T2 reduced the soil bacterial diversity indicating a disadvantage of the reduced use of chemical fertilizer. However, the lowest diversity was observed under T4 which showed excessive chemical fertilization to be detrimental, nonetheless. The PCoA and heat map analyses revealed significant changes on the bacterial compositions, relative abundance, and beneficial bacteria in the soil by the treatments.

  Conclusion   Comparing to the use of chemical fertilizer alone, appropriate combination of organic and inorganic fertilizers effectively improved the bacterial diversity and composition in soil. A 50% reduction of chemical fertilizer usage coupled with organic fertilizer (T3) could not only conserve the chemical fertilizer, but also enhance the relative abundances of beneficial bacteria and bacterial diversity resulting in an improved soil fertility.