A three-year field experiment with maize was carried out on fluvisol. The aim of the present study was to evaluate the impact of biochar on maize yield at different levels of water stress and its effect on the amount of soil microorganisms.The following variants were studied: control with no biochar addition, and variants with biochar. Biochar was applied in 2 t/ha in 2016, produced by pyrolysis of rice straw, then in 3 t/ha in 2017 and 10 t/ha in 2018, both obtained by pyrolysis of oak bark. These variants were divided into three more depending on the irrigation regime: i.e. non-irrigated, irrigation at dT>0 °C, and irrigation at dT<-1 °C. Under non-irrigated conditions, the effect of biochar on maize yield was not pronounced over three years. Under irrigation conditions with water added according to crop water stress, higher yields of maize were obtained in the third year when biochar application to the soil was highest (10 t/ha). Also, yield was higher when faster (at lower water stress dT<-1 °C) irrigation rates were applied. Soil moisture changed slowly in the biochar variants and the best values of the temperature differences were obtained for irrigation at dT<-1 °C in 2018.These results showed that the influence of biochar on soil microbiota was significantly greater, and the most stimulating effect of biochar was obtained for bacterial populations.

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