The Impact of Mixed Application of Modified Straw Biochar and Organic Fertilizer on Soil Water Retention and Fertility Maintenance in the Loess Hilly Dry Area

[Objective] This study aimed to investigate the effects of different modified corn straw biochars on the water retention and fertilizer conservation capacity of arid loess soil and their effects on crop growth. [Methods] The corn straw biochar produced by pyrolysis at 500 ℃ for 2 h under a nitrogen atmosphere was modified by immersion in 1 mol/L H3PO4, NaOH, and NH4Cl reagents, respectively, to obtain different modified corn straw biochars. The elemental composition, and physical and chemical properties such as pore structure, of the biochar were systematically characterized. Pot experiments were used to examine soil water retention, nutrient conservation, and plant growth using soil treated with different ratios of fertilizer and biochar. [Results] In terms of the physicochemical properties of biochar, the oxygen content in the modified biochar increased by 4.73%~9.18%, while the ash content decreased by 12.69%~20.70%. The pH significantly decreased, and the increase in O/C ratio indicated enhanced hydrophilicity of the biochar. The specific surface area of the modified biochar increased by 3.26~5.19 times, and the total pore volume increased by 3~6 times. In terms of soil water retention and nutrient conservation properties, under the treatments of mixed application of biochar and fertilizer, soil bulk density decreased with the increase of the biochar-to-fertilizer ratio, and soil porosity increased by 2.42%~4.07%. The field water-holding capacity increased with the biochar-to-fertilizer ratio, with the highest value of 37.38% observed under the NaOH-modified biochar-to-fertilizer ratio of 2∶1. When the NH4Cl-modified biochar-to-fertilizer ratio was 2∶1, soil organic matter and total potassium contents reached the highest values, being 8.23 and 15.17 g/kg, respectively. The highest total nitrogen and total phosphorus contents were 0.37 and 0.95 g/kg, respectively, when the H3PO4-modified biochar-to-fertilizer ratios were 1∶1 and 2∶1. Mixed application of modified biochar and organic fertilizer could increase available phosphorus and rapidly available potassium that can be directly absorbed and utilized by plants. The growth promotion effect on corn plants was most significant when the NH4Cl-modified biochar-to-fertilizer ratio was 1∶2, and it was noted that an excessively high biochar-to-fertilizer ratio was not suitable. [Conclusion] In the arid regions of the loess hilly area, considering various factors, the application of NH4Cl-modified biochar with a fertilizer ratio of 1∶1 and mixed with organic fertilizer can enhance the soil’s water retention and nutrient conservation properties.