Plant genotype and nitrogen fertilization effects on abundance and diversity of diazotrophic bacteria associated with maize (Zea mays L.)

TLDR: The hypothesis that plant genotype and fertilization conditions should be taken into account when searching for N-fixer inocula is supported.

Abstract: This research has investigated the effect of two genotypes of maize (Zea mays L.) on the number and diversity of diazotrophic bacteria under different N-fertilization rates. Cultivars NK940 and PAU871 were grown in soil:sand with 0, 40, and 80 kg N ha−1 added as NH4NO3 under controlled conditions for 60 days. The number of diazotrophic bacteria of rhizosphere (S), disinfected roots (R), and stems (T) was determined by most probable number (MPN) method. Diazotrophic bacteria were isolated by N-free media, confirmed by amplification of nifH gene fragments, characterized by amplified 16S rDNA restriction analysis (ARDRA) and (GTG)5 fingerprinting, and identified by partial 16S ribosomal DNA (rDNA) sequence analysis. The diversity of the diazotrophic community from S and R was determined by Terminal Restriction Fragment Length Polymorphism (T-RFLP) analysis considering the nifH gene. Maize genotype had a marked effect on the number and diversity of endophytic communities, the NK940 community being more abundant and diverse than that of PAU871. Additionally, N-fertilization increased the number and diversity of diazotrophs in endophytic communities but not in rhizosphere samples. One hundred and six diazotrophs were isolated from S, R, and T samples. Pseudomonas and Enterobacter were the dominant and ubiquitous genera isolated and detected by culture-independent method. T-RFLP showed that the N-fixing populations of the rhizosphere of both cultivars were more diverse than those of inside roots. Principal component analysis (PCA) separated the samples by cultivar and demonstrated a more marked effect of N-fertilization on the NK940 diazotrophic community than on PAU871. These results support the hypothesis that plant genotype and fertilization conditions should be taken into account when searching for N-fixer inocula.