Contrasting mechanisms of biochar–nitrogen interactions under inorganic and organic fertilizers: Integrated evidence from incubation, volatilization, leaching, and apparent nitrogen partitioning

Biochar is widely promoted as a strategy to mitigate nitrogen (N) losses from soils, yet its effectiveness varies strongly with N source (inorganic or organic) and biochar properties. To clarify these contrasting responses, we conducted an integrated assessment combining a 98-day soil incubation experiment, a 10-day ammonia (NH 3 ) volatilization experiment, a 60-day column leaching experiment, and a scenario-based apparent soil N partitioning framework. When co-applied with urea, biochar suppressed nitrification and reduced soil nitrate-N (NO 3 − -N) accumulation consistent with enhanced microbial immobilization and/or ammonium retention. However, relative to the urea control, NH 3 volatilization increased by 32–50% with biochar, and leaching losses also increased: ammonium-N (NH 4 + -N) leaching increased by 43–58% at high biochar application rates (20 t ha −1 ) while NO 3 − -N leaching increased 15–39%. In contrast, biochar combined with anaerobic digestion effluent (ADE) significantly enhanced nitrification and mitigated N losses. Compared with the ADE control, biochar reduced NH 3 volatilization by 15–33% across all biochar treatments and NO 3 − -N leaching decreased by 8–10% at the high application rates. However, NH 4 + -N leaching remained low across all treatments (0.11–0.60 mg N column −1 ). These reductions in N loss were associated with patterns consistent with interactions between ADE-derived organic components and biochar surfaces that may have contributed to reduced NO 3 − -N mobility, together with a more balanced microbial community structure. Overall, our results demonstrated that biochar was not a universal mitigation tool. While it exacerbated gaseous and leaching losses when combined with fast-release urea, it substantially enhanced N stabilization when paired with ADE. Fertilizer-specific biochar management, using low application rates with urea and high-temperature biochar at higher rates with ADE, provided a practical pathway to improve N use efficiency while mitigating environmental N losses.