Improving grain yield, water and nitrogen use efficiency of Nigella sativa with biological and chemical nitrogen under different irrigation regimes

•Nitroxin bio-fertilizer was used to replace the chemical nitrogen.•Nitrogen utilization and remobilization efficiency, and seed water use efficiency were measured in the experiment.•Combination of 40 kg N ha-1+Nitroxin bio-fertilizer in deficit irrigation regimes was associated with water use efficiency.•The combined application of chemical and biological fertilizers reduced the drought impacts on black cumin. Biofertilizers are an important source for completing the plant nutrition program, which has a significant effect on reducing the cost and adverse effects of chemical fertilizers and increasing plant resistance to environmental stresses. The aim of this study was to investigate the effect of Nitroxine and nitrogen biofertilizer on grain yield, nitrogen utilization and use efficiency, nitrogen remobilization and plant water use efficiency in Nigella sativa (black cumin) grown under different irrigation regimes. The experiment was conducted at Yasouj University Research Field in split plots in a randomized complete block design with three replications in 2016 and 2018. Factors studied in this experiment consisted of four irrigation regimes (based on phenological stages) including weekly irrigation from emergence to harvest (WI0) and withholding irrigation from blooming to maturity (WIb), from flowering to maturity (WIf), and from the start of grain formation to maturity (WIs) as a main factor and five levels of fertilizer including no application of fertilizers (N0), 80 kg N ha−1 (N80), 40 kg N ha−1 (N40), combination of 40 kg N ha-1+Nitroxin biofertilizer (N40 + B), and Nitroxin biofertilizer (B) as sub factors. Application of 80 kg N ha-1 in complete irrigation, and combination of 40 kg N ha-1+Nitroxin biofertilizers in other irrigation regimes were associated with greatest grain yield, N content in aerial organs and water use efficiency. In all irrigation regimes, combination of 40 kg N ha-1+Nitroxin biofertilizers resulted in the greatest N remobilization (N already has been stored in the vegetative organs, transferred to the developing grains) rate (12.46 kg ha-1). The greatest N utilization efficiency was obtained in non-fertilizer treatments (15.1 kg kg-1) and withholding irrigation from flowering stage (15.7 kg kg-1). Finally, the results showed that the application of nitroxin biofertilizer can reduce the use of urea chemical fertilizer and mitigate the effects of drought stress by N fixing and possibly producing growth hormones and decrease