东南烟稻轮作区烤烟临界氮浓度稀释曲线的建立与验证

  【目的】  建立东南烟稻轮作区烤烟临界氮稀释曲线，探讨氮素营养指数用于诊断和评价烤烟氮营养状况的可能性，为实现烤烟合理施用氮肥提供理论依据。  【方法】  两年两地共3个田间氮肥用量试验，每个试验共设6个氮水平 (N 0、45、90、135、180 和300 kg/hm2)，分析不同施氮量对移栽后不同天数烤烟地上部和叶片干物质积累量的影响。利用不同时期植株氮浓度和干物质积累量，建立地上部和叶片的临界氮浓度稀释曲线方程和干物质积累方程。结合基于无人机可见光谱平台的烤烟氮浓度无损测定方法，计算氮营养指数，判断烤烟氮营养丰缺情况。  【结果】  施用氮肥明显增加了烤烟地上部和叶片干物质积累量，不同氮肥处理间差异明显，烤烟地上部和叶片氮浓度随烤烟生长进程而降低；东南烟稻轮作区烤烟地上部及叶片的临界氮素浓度和干物质积累量符合幂指数的关系；模型进行独立验证时，氮限制组的数据均在临界氮素稀释曲线以下，而不受氮限制组的数据均在临界氮素稀释曲线附近。模型拟合的临界氮浓度和植株实际临界氮浓度呈线性相关，地上部和叶片的RMSE值分别为0.55和0.44，标准化均方根误差n-RMSE分别为25%和17%，模型具有较好的稳定性；烤烟氮素营养状况的判定可以通过应用临界氮浓度稀释曲线计算氮营养指数(NNI)实现。随着施氮水平的提高，叶片氮营养指数逐步升高，在氮施用量达到135 kg/hm2，叶片氮营养指数可在1以上，处于氮盈余状态。  【结论】  东南烟稻轮作区烤烟叶片临界氮素稀释曲线模型(N_c = 3.2339 × DM_leaves–0.475)和叶片氮营养指数能够诊断评价该区域烤烟氮营养状况，从而为优化烤烟的氮素管理提供指导。

Construction and verification of a critical nitrogen dilution curve for flue-cured tobacco under tobacco-rice rotation systems in southeast China

  【Objectives】  This study aims to construct a critical N dilution curve for flue-cured tobacco and use the same to rapidly diagnose and evaluate the N nutrition status of flue-cured tobacco.  【Methods】  Three field experiments were conducted in two locations for two years. Six N application rates (0, 45, 90, 135, 180, and 300 kg/hm2) were setup in each experiment. The shoot and leaf dry matter accumulation recorded on different days after transplanting were analyzed. The critical N dilution curve equation and dry matter accumulation equation of shoot and leaves were established using N concentration and dry matter accumulation in plants at different growth stages. Combined with the non-destructive determination of N concentration based on the visible spectrum platform of UAV, the N nutrition index (NNI) was calculated and used to determine the N nutrition status of flue-cured tobacco plants.  【Results】  N application (P < 0.05) increased shoot and leaf dry matter accumulation of flue-cured tobacco, with variation among the N treatments. The N concentration in shoot and leaves decreased across the growth stages of flue-cured tobacco. The power equation well described the relationship between the critical N concentration and dry matter accumulation in the shoot and leaves. Independent validation of the model showed that observations for the N deficient group were below the critical N dilution curve, while those of the sufficient N group were near the curve. The RMSEs of the simulated and actual critical N concentrations for the shoots and leaves were 0.55 and 0.44, and the corresponding n-RMSEs were 25% and 17%, showing the high stability of the model. The leaf NNI of flue-cured tobacco gradually increased with an increasing N application rate. The leaf NNI was higher than 1 when N application rate reached surplus level (135 kg/hm2).  【Conclusions】  The constructed leaf critical N dilution curve for flue-cured tobacco under tobacco-rice rotation system is Nc = 3.2339 × DM_leaves–0.475, which is accurate with low n-RMSEs. This is verified by leaf NNIs that coincide with the N application levels, suggesting the curve could be used to rapidly diagnose the N status of flue-cured tobacco in the study area.