• • Rice straw biochar (S) increased soil nitrate nitrogen by 148.74%–152.68% versus control, significantly enhancing nitrification; combined with Bacillus subtilis (SJ) raised average net nitrogen mineralization rate by 77.28%–99.38%.
• • Rapeseed straw biochar (Y) decreased nitrate nitrogen by 51.66%–57.61% and reduced net nitrogen mineralization rate by 82.07%–84.73% when combined with inoculant (YJ), indicating a suppressive effect on nitrification.
• • Biochar amendments S and Y increased microbial biomass nitrogen (MBN) by 2.02- and 2.20-fold over control, respectively; combined treatments (SJ, YJ) further boosted MBN by 103.26%–149.44% relative to single biochar.
• • YJ treatment (4% rapeseed straw biochar + 5 mg/kg Bacillus subtilis) is the optimal amendment for dryland soil, reducing inorganic nitrogen loss risk while enhancing microbial nitrogen activity, as evidenced by lower nitrate leaching potential and higher MBN.