Mechanistic insights into enhanced nitrogen retention and microbial nitrogen cycling in soil–plant systems mediated by hydrogen peroxide-modified biochar

• H 2 O 2 oxidation enhanced biochar surface area and oxygen-containing functional groups. • Oxidized biochars reduced NH 4 + and NO 3 – leaching compared to raw biochars. • Oxidized biochars increased 15 N recovery and decreased N losses from soil. • Urease activity and N transformation were effectively regulated by oxidized biochars. • Predicted abundance of N-cycling functional genes increased with oxidized biochars. Minimizing nitrogen (N) loss from fertilized soils remains a critical environmental challenge. While modified biochars can improve N management, concerns about cost, pollution trade-offs, and secondary environmental risks underscore the need for cleaner alternatives. This study evaluated hydrogen peroxide-modified biochars from poultry manure and rice straw for their effects on N retention, plant uptake, and microbial dynamics. A column and pot experiments were conducted, with 15 N-labeled urea applied in the pot experiment. Treatments included oxidized rice straw biochar at 1 % (ORBH) and 0.5 % (ORBL), its raw form (RB); oxidized poultry manure biochar at 1 % (OPBH) and 0.5 % (OPBL), its raw form (PB); fertilizer-only (F); and a control. The ORBH and OPBH significantly reduced ammonium leaching by 32 % and 29 % relative to F, and by 20 % and 8 % than their raw forms. Nitrate leaching decreased by 59 % (ORBH) and 61 % (OPBH) versus F, and by 51 % and 22 % compared to RB and PB, respectively. Enhanced N retention was attributed to improved surface properties of oxidized biochar that increased N sorption. Oxidized biochars also decreased urease activity and slowed N transformation, promoting better immobilization-mineralization turnover. Fertilizer recovery was significantly higher in ORBH (75 %) and OPBH (60 %) than in F (43 %). Predicted gene abundances of key N-cycling microbes were higher under ORBH and OPBH, potentially reflecting a slow-release N effect that enhanced plant uptake. These findings demonstrate that oxidized biochars can serve as effective soil amendments to enhance fertilizer N efficiency and reduce N losses.