Integrated use of iron-modified biochar combined with a urease inhibitor reduces multiple gaseous reactive nitrogen emission in wheat-maize rotation

Excessive and inefficient N fertilization in intensive cropping systems is a major source of atmospheric reactive nitrogen (N r ) that contributes to air pollution and greenhouse gas accumulation. The North China Plain (NCP), a global hotspot of wheat–maize rotation, suffers high N r losses, yet simultaneous field assessments of integrated management strategies targeting multiple N r species (NH 3 , N 2 O, NO x , and HONO) remain scarce. This study quantified the emission dynamics of these four N r species over a one-year rotation and identified a practice that mitigates N r losses while without compromising crop yield and improving apparent N recovery efficiency (RE-N). A field experiment was conducted during 2024-2025 with six treatments: no N fertilizer (CK), conventional farmers’ practice (CU), optimized N rate (OU), urease inhibitor-amended urea (UI), UI plus biochar (BC), and UI plus iron-modified biochar (FeBC). Emissions of NH 3 , N 2 O, NO x , and HONO were monitored simultaneously using chamber-based methods. The maize season accounted for approximately 72% of total annual N r losses. UI primarily reduced NH 3 volatilization (by 40.6% compared vs. CU), whereas BC more effectively lowered N 2 O emissions (by 35.4% vs. CU). The integrated FeBC treatment achieved the strongest mitigation: annual cumulative emissions under FeBC were 36.7, 2.9, 0.5, and 0.3 kg N ha -1 for NH 3 , N 2 O, NO x , and HONO, respectively, corresponding to significant reductions ( P <0.05) of 57.6%, 46.2%, 47.0%, and 57.2% compared with CU. Grain yield under FeBC (19.3 t ha -1 yr -1 ) did not differ significantly from that under CU, while increasing RE-N by 64%. Our findings indicate that combining iron-modified biochar with a urease inhibitor offers a promising integrated strategy for mitigating multi-species N r emissions without compromising grain production. Longer-term observations are needed to confirm the stability and persistence of these effects under repeated cropping cycles.