Critical nitrogen dilution curves for winter oilseed rape (Brassica napus L.) along the whole crop cycle: A Bayesian analysis

Nitrogen (N) is the most limiting nutrient for crop growth. Determining the dynamics of N uptake by crops is essential to optimize N management and improve the sustainability of crop production, especially for winter oilseed rape (Brassica napus L.), a highly N-dependent crop. The concept of critical nitrogen concentration is an effective way to diagnose plant nitrogen status, defining the minimum concentration of nitrogen in the aerial parts needed to maximize shoot biomass at a given time in the crop growth. For winter oilseed rape the existing nitrogen dilution curve was developed in the 1990s, but the curve was established only until the beginning of flowering, the reproductive part not being covered. The main objective of this study is to determine new critical N dilution curves for winter oilseed rape over the whole crop cycle. To achieve this, we used data from 36 field trials, conducted between 2003 and 2020 under various N application rates and cultivars, and a new Bayesian statistical framework. For the vegetative phase, the critical curve (Ncvege=4.556 W-0.241) determined in this study was close to the curves found in previous studies. In addition, new critical N dilution curves were determined for the first time in winter oilseed rape, for the reproductive phase (Ncrepro =4.764 W-0.424) and for the entire growth period (Ncunique=7.364 W-0.582). Interestingly the parameters differed markedly from those recently obtained in spring canola. These new dilution curves should provide useful tools for determining the N nutritional status of winter oilseed rape over a longer period, and thus improve nitrogen nutrition diagnosis and enable optimal nitrogen management.