From Feedstock to Function: How Pyrolysis and Oxidation Shape Biochar Performance in Soil–Plant Interactions

Nutrient losses through leaching and low nutrient use efficiency are major challenges limiting crop productivity and causing environmental pollution. Biochar has been widely studied as a soil amendment to improve nutrient retention; however, the combined effects of pyrolysis temperature and post-production oxidation on soil nutrient dynamics and plant performance remain unclear. In this study, wheat straw and wood residue biochars were produced at two pyrolysis temperatures (350 and 450 °C) and subsequently modified by hydrogen peroxide (H₂O₂) oxidation to enhance surface functionality. A pot experiment with fava bean (<i>Vicia faba</i> L.) was conducted to evaluate the effects of pristine and oxidized biochars on soil chemical properties, nutrient leaching, and plant nutrient uptake. Results showed that pristine biochars increased soil pH from 6.82 (control) to 8.73-9.12 and EC from 2.15 to 3.06-4.71 dS m^-1, with wheat straw biochars having stronger alkalizing effects. In contrast, oxidized biochars decreased soil pH to 5.62-5.93 due to the introduction of oxygen-containing functional groups. All biochars reduced NO₃^--N, NH₄⁺-N, and PO₄^3--P leaching, with the most pronounced reductions observed in oxidized wheat straw biochar produced at 450 °C (O-BWS₄₅₀). Improved nutrient retention translated into higher plant nutrient uptake: fava bean plants grown in O-BWS₄₅₀-amended soil achieved the greatest N (6.71%) and P (3.89%) uptake, significantly higher than the control. These findings highlight the potential of oxidation-modified biochars, particularly wheat straw biochar produced at moderate pyrolysis temperature, to improve soil nutrient conservation and enhance crop nutrition simultaneously. Such modifications represent a promising approach for developing biochar-based soil amendments that promote sustainable nutrient management.