Objectives  Biological nitrogen fixation (BNF) plays an important role in reducing the input of chemical N fertilizer, which is of great significance to alleviate soil acidification. N-fixing microorganism (diazotroph) widely exists in all sites of plants. Understanding of the abundance, diversity and composition of diazotrophic communities in different parts of maize will provide data support and theoretical basis for the potentials of BNF.

Methods  Two maize cultivars with different Al-tolerance levels, Xianyu 335 (relatively Al-tolerant cultivar) and Mo17 (Al-sensitive cultivar), were grown in an acidic soil. After 30 days, maize shoot, root, and biomass data were collected. At the same time, nutrient content in the plants and rhizosphere soils as well as the physicochemical properties of rhizosphere soils were determined. Based on nifH gene, diazotroph abundance was determined by quantitative RT-PCR and community composition was assayed using high-throughput sequencing.

Results  Compared with Mo17, Xianyu 335 had higher biomass of shoot and roots, but lower plant N and P concentration, rhizosphere soil pH, NH₄⁺-N and readily available K contents. The results of quantitative RT-PCR showed that the difference of nifH gene abundance among different parts of maize was greater than that between maize cultivars. The nifH gene abundance in roots was significantly higher than those in rhizosphere soils and leaves, and the abundance in the roots of Xianyu 335 was higher than that in Mo17. High throughput sequencing analysis showed that Proteobacteria and Cyanobacteria were the dominant phyla, Bradyrhizobium was the dominant genus in both rhizosphere and root samples, and Leptolyngbya was the dominant genus in leaves. The relative abundance of most of the dominant genera showed significant differences among different parts of maize. Sampling sites significantly affected the diversity index of diazotrophic community (Shannon index and OTU number), but there was no significant difference among cultivars in the same site. The diversity index of rhizosphere soils was significantly higher than that of roots, and both were higher than that of leaves. NMDS and PERMANOVA analysis showed that the effect of sampling sites on the diazotrophic community structure was greater than that of cultivars. The differences of community composition among rhizosphere, root and leaf samples reached the statistical significance, while only rhizosphere samples showed significant difference among cultivars.

Conclusions  The difference in abundance, diversity and composition of diazotrophic communities among different parts of maize were more noticeable than those among maize cultivars. Diazotroph abundance in roots was the highest, suggesting the greater potential of BNF. The composition of diazotrophic community in leaves was significantly different from that in rhizosphere soil and roots, and leaves had the lowest diazotroph abundance and diversity, which might be closely related to the harsh environmental conditions.