Optimal plant density and N fertilization to achieve higher seed yield and lower N surplus for winter oilseed rape (Brassica napus L.)

Greater synchrony between populations and individual plants is dependent on the interactions between plant density and nitrogen (N) fertilization and is crucial to achieve food and environment security. A two-factor designed experiment of transplanted winter oilseed rape was conducted in the 2012-2013 and 2013-2014 growing seasons in central China. Five transplanted plant densities and six N rates were used to study the influences of plant density and N fertilizer application rate on seed yield, N uptake and apparent N surplus. Optimal plant density was the prerequisite to obtain high seed yield and the contribution of plant density to seed yield was minor when it exceeded the optimal density. N fertilization offset the compensatory effect between plant density and pods per plant, enhance seed yield further. Seed yield increased linearly with the increase of seed N uptake, however, it did not increase as the non-seed parts (stem + pod wall) N uptake increased when seed yield exceeded the critical values in the high plant density treatments. Higher plant densities promoted shoot N uptake and reduced soil mineral N residues and apparent N surplus in root zone. Compared with the recommended transplanted density in this region (9.0 × 104 plant ha−1), the optimal N fertilizer application rate could be cut down with 10.3%-23.7% for the target yield (3000 kg ha−1) and apparent N surplus would be declined with 41.5%-92.1% when the density suitably increased to 10.0-12.0 × 104 plant ha−1; On the contrary, when the density was decreased to 7.0-8.0 × 104 plant ha−1, increasing N fertilization rate with 13.9%-34.8% also could gain the target yield, but the apparent N surplus would increase with 58.1%-148.0%. Consequently, varied combinations between plant densities and N fertilizer application rate are useful to fulfill high seed yield. Reducing N fertilizer rates under suitable higher plant densities are the optimal N management strategy to achieve high yield with lower environment risks.