According to Iranian water resource limitations it is mandatory to find water efficient strategies for crop production in accordance with reduction of chemical inputs. A split-factorial experiment was conducted in 2018 based on randomize complete block design with three replications to investigate the effects of biochar and nitroxin application on growth traits and yield of cowpea under different irrigation regimes. Experimental treatments were irrigation regimes (0, 90, 120 mm) evaporation from water pan class A which were considered as no stress, mild water stress and Sevier water stress, respectively. Irrigation treatments were arranged as main plot and three levels of biochar (0,4, 8 t.ha-1) and nitroxin (inoculated and no inoculated) considered as factorial subplots. Results showed that dry matter accumulation and relative crop growth rate exhibited a sigmoid pattern while leaf area and growth rate followed gaussian pattern. Based on model output DMTmax was 467 g.m-2 and RGRmax was 0/100 g, LAImax 3.65 and CGRmax was 12.6 g.m-2 which reach to 50% of final value in 55/9, 65.9, 40.5 and 31.7, respectively. Application 8 ton/ha biochar with nitroxxin resulted in 12% RWC and 10%, 27% improved nitrogen and phosphor status under severe stress condition. Irrigation at 120 mm evaporation caused 33% reduction of grain yield and nitroxin application at normal condition caused 19% increase in grain yield but at 120 mm evaporation it did not exhibit significant effect. Application of biochar at no stress condition led to 37% increase in grain yield and such changes reach to 29% at 120 mm evaporation from water pan class A. The highest grain yield (266 g.m-2) obtained from 8 t.ha-1 biochar at no stress condition and the maximum biological yield was obtained from 8 t.ha-1 biochar with nitroxin (809 g.m-2). Application of 8 t.ha-1 biochar incorporated with nitroxin could provide satisfactory yield for cowpea crop under water limited condition.
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