Low-pyrolysis-temperature biochar promoted free-living N2-fixation in calcareous purple soil by affecting diazotrophic composition

•Low-pyrolysis-temperature biochar (CB300) increased total nitrogen in N-poor soil.•CB300 increased nifH gene abundance, nitrogenase activity and Azotobacter abundance.•Changes of electrical conductivity led to the difference in diazotrophic community.•The consumption of DOC improved nitrogenase activity, nifH abundance and Azotobacter.•Soil available molybdenum was positively associated with soil nitrogenase activity.