Recommendations for Nitrogen Fertilizer in Winter wheat Based on Nitrogen Nutrition Index

A critical nitrogen (Nc) concentration, defined as the minimum nitrogen (N) concentration required for maximum plant growth, could be used as an intermediate variable between remote sensing data and recommendation for N fertilizer. In this study, the critical N concentration dilution curve was established based on aboveground biomass (AGB) from data at jointing, booting, anthesis and filling stages. The quantitative correlations between normalized difference vegetation (NDVI) and nitrogen nutrition index (NNI) were established. Finally, an N recommendation model combined with the N fertilizer effect function and NDVI was established and verified by field test data. Results showed that the N concentration of winter wheat decreased gradually during the reproductive growth period, and The plant N concentration could be described by $N _{\mathrm{c}} =$ 6.27*AGB-0.54, with the R2 value of 0.80. Moreover, the thresholds of NDVI were 0.87, 0.91, 0.91 and 0.81 at jointing, booting, anthesis and filling stages, respectively, and the amounts of recommending nitrogen fertilizer were 2.63, 10.00, 11.11 and 10.00 kg N/hm2 when NNI value lowed 1% of corresponding period’s thresholds. Field experiments illustrated that jointing and booting stage could be used as the N fertilizer periods to guarantee winter wheat yield, and integrating Nc curve and hyperspectral data had advantages in N fertilizer recommendations.