Aims Soil microbial and enzymatic activity have important roles in soil health, nutrient cycle and plant growth. Biochar is an excellent biomass for increasing soil water content, but some biochar ompounds, due to their high pH, are harmful to the soil. This study aimed to investigate the effect of biochar and sulfur-modified biochar on improving the soil water content and biochemical properties in a millet cropping system. Methods The field experiment was performed in a completely random design with three replications. Experimental treatments included control, 15 t ha− 1 sunflower biochar (B), and 15 t ha−1 sulfur-modified biochar (BS). Results Biochar (B and BS) improved the soil water content by 47% and 35%, respectively, compared to the control. B and BS treatments improved soil biological properties, including microbial biomass carbon, basal respiration, and the activity of catalase, dehydrogenase, urease, and alkaline phosphatase enzymes compared to the control. The application of B and BS treatments decreased soil EC and sodium adsorption ratios compared to the control. The highest (8.26) and lowest (7.83) values of soil pH were related to the B and BS treatments, respectively. Treatment of soil with B and BS enhanced soil dissolved organic carbon (90% and 33%), available nitrogen (74% and 48%), and available phosphorus (60% and 96%) compared to the control. The application of B and BS treatments enhanced plant nutrients (nitrogen, phosphorus, and potassium) and the leaf relative water content by 40% and 14%, respectively. Biochar (B and BS) improved millet yield by 58% and 115%, respectively, compared to the control. Conclusion Sulfur increased the efficiency of biochar in amending saline and calcareous soil and enhanced millet yield by improving the chemical properties (pH and CEC) of biochar.

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