Diagnosis of Nitrogen Nutrition and Recommended Model of Topdressing for Transplanting Cotton after Wheat Harvest in Jianghan Plain

【Objective】 To establish topdressing models based on the appropriate nitrogen diagnostic indices for transplanted cotton following wheat harvest in Jianghan Plain with the view of providing the basis for cotton nitrogen accurate management. 【Methods】 A field plot experiment was carried out with different nitrogen application rates (0, 90, 180, 270, 360, 450 kg/hm²) in 2019, the relationships between SPAD value (SPAD) in the fourth leaf from the top, the nitrate content (NIT) of petiole, 24 hyperspectral parameters and nitrogen application rate at early stage were analyzed in budding, flowering and boll-forming stages, and the appropriate nitrogen diagnostic indices of transplanted cotton following wheat harvest in high yield field of Jianghan Plain were determined, and the recommended models of topdressing were established combined with nitrogen effect function. 【Results】 The results showed that the SPAD and NIT increased significantly with the increase of nitrogen application, but no significant increase was observed when the nitrogen application increased to 360kg/hm² (excluding the NIT in bud stage), and the reflection peak formed in the green light band of the canopy spectrum slowed down with the increase of nitrogen application. The SPAD, NIT, and the 2 hyperspectral parameters (RSI and mND705) were all significantly related to nitrogen application rate at early stage (R² was higher than 0.8, RMSE was lower than 1), which could be used as the appropriate nitrogen diagnostic indices of transplanted cotton following wheat harvest in high yield field of Jianghan Plain. With the 95% of the potential yield as the critical yield, the critical values of nitrogen diagnostic indices of SPAD, NIT, RSI and mND705 were 34.802, 2.307, 1.526 and 0.549 in budding stage respectively, and the critical values were 34.841, 4.174, 1.589 and 0.619 in flowering and boll-forming stage respectively. 【Conclusion】 According to the unary quadratic relationship between the appropriate nitrogen diagnostic indices and nitrogen application rate at early stage, the total nitrogen application rate during the whole growth stage, a topdressing recommendation model based on the sui Table nitrogen diagnostic indices is established in the bud stage or the flowering and boll-forming stage.