Construction of Critical Nitrogen Dilution Curve Based on Stem Dry Matter in Rapeseed

There are mathematical equations to distinguish nitrogen (N) status based on the concept of critical nitrogen (Nc), the lowest N concentration required to achieve maximum growth. The Nc demonstrates the ideal N status in crops. Whenever crop N concentration is greater than the Nc, at that time N is in surplus, but if it is less, then N is inadequate. The contribution of the stem dry matter (SDM) toward total crop dry matter is significantly higher than that of other tissues, therefore, it is the most determinant factor for N dilution of the whole plant. The objectives of this research were to construct a Nc dilution curve based on SDM and to evaluate the plausibility of this curve for assessment of the N nutrition status of spring rapeseed.

A field experiment was conducted with seven levels of N fertilizer application (0, 50, 100, 150, 200, 250 and 300 kg N ha-1) in which the amount of the SDM and stem N concentration (SNC) of two spring rapeseed cultivars (Hyola 401 and Dalgan) were measured during the growing season. The approach proposed by Justes et al. (1994) was used to construct the SDM basis Nc dilution curve. The N nutrition index (NNI) on each sampling date was calculated by dividing actual SNC by stem Nc concentration. The accumulated N deficit (Nand) in rapeseed stems for each sampling date was determined by subtracting the N accumulation in stems under the Nc condition from actual N accumulation in stems under different N fertilizer application rates.

Overall, both SDM and SNC showed an increasing and decreasing trend with increasing crop age, respectively, regardless of fertilizer treatment. The Nc dilution curve of both cultivars during the growing season was described by the equation Nc = 5.71SDM-0.29. Both NNI and Nand criteria were able to diagnosis the N deficiency or excess in the spring rapeseed cultivars studied. At all of the crop growth stages, there was a significant positive correlation between the difference value of the N application rate (∆N) and the difference value of NNI (∆NNI). For both cultivars studied, however, the positive correlation between ΔN and differences in Nand among N fertilizer treatments (ΔNand) was not significant at early growth stages. In any case, for accurate management of N fertilizer during the growing season, it is sufficient to determine only one of the two NNI or Nand criteria per day from planting to silique formation, and then the N fertilizer required for maximum growth is calculated based on the equations presented to describe the relationships between ΔN and ΔNNI or between ΔN and ΔNand.

The N concentration in the stems decreased with advancing maturity, while a higher N application rate generally exhibited a higher stem N concentration. The present Nc dilution curve based on SDM provides an insight into N nutrition status in spring rapeseed crop and can serve as a novel tool to improve N fertilization management in spring rapeseed.