Negative impacts of excessive nitrogen fertilization on the abundance and diversity of diazotrophs in black soil under maize monocropping

•Long-term ENF decreased the abundance of soil diazotrophs.•Long-term ENF decreased species richness and diversity of soil diazotrophs.•Soil pH and EC values were the main factors affecting soil diazotrophs.•The maize rhizosphere could enrich soil diazotrophs.•The diazotrophic bacterial variations under ENF might inhibit early maize growth.