Optimizing nitrogen use in rapeseed systems: A global meta-analysis of yield gains and environmental trade-offs

Rapeseed ( Brassica napus L. ) is often grown under high nitrogen (N) fertilization, increasing the risk of N losses and associated environmental impacts. Using a systematic literature search of Web of Science and Google Scholar (1995-2024) and predefined inclusion criteria, we synthesized 106 field studies to quantify rapeseed yield, nitrogen-use-efficiency, and environmental responses to N rate. Across studies, yield responded nonlinearly to N, with rapid gains at low-moderate rates and diminishing returns thereafter; a linear-plus-plateau (LLP) fit indicated yield stabilization around ∼200-225 kg N ha -1 (mean yield ≈2.22 Mg ha -1 ). Region-specific dose-response functions showed distinct yield-maximizing tendencies: Asia (∼150), Australia (∼125), Europe (∼160-170), and an earlier saturation signal in North America. Both NUE and AEN declined consistently as N rate increased, with greater dispersion and a steeper efficiency erosion in developing-country datasets than in developed-country datasets. Environmental context significantly modified the yield-N relationship: mean annual precipitation (MAP), soil pH, and soil organic matter (SOM) shifted response curvature and implied N optima, while yields were highest under intermediate precipitation (500-1000 mm), near-neutral pH (≈6.5-7.5), and higher SOM. Environmental indicators increased disproportionately at higher N supply: N 2 O and CH 4 followed significant nonlinear trends, and soil NO 3 - -N and NH 4 + -N pools rose markedly under fertilized treatments. Integrating diminishing yield gains with accelerating environmental costs supports a balanced N management window of ∼100-150 kg N ha -1 for many systems, with rates above ∼150-200 kg N ha -1 requiring clear, site-specific justification and mitigation (e.g., split/variable-rate application, enhanced-efficiency fertilizers, and SOM-building soil management). This framework offers actionable pathways to reconcile agricultural productivity with environmental sustainability. • Rapeseed yield responds nonlinearly to N, with diminishing returns beyond ∼150-200 kg N ha -1 , and stabilization around ∼200-225 kg N ha -1 (mean ≈2.22 Mg ha -1 ). • A pragmatic N window of ∼100-150 kg N ha -1 maintains yield with lower surplus risk. • NUE and AEN decline as N rate increases, reducing returns per unit N. • N 2 O increases curvilinearly; CH 4 exhibits a significant U-shaped response, and soil mineral N pools (NO 3 - –N, NH 4 + –N) accumulate with fertilization. • Regional N optima differ: Asia ∼150; Europe ∼160-170; Australia ∼125; North America ∼50 kg N ha -1 .