Real-time monitoring of leaf nitrogen (N) content by remote sensing can accurately diagnose crop nutrient status and facilitate precision N management. However, the methods used to estimate of vertically integrated leaf N content do not consider different cropping systems, in which the maize growth stages are not synchronized, resulting in decreased practical value of the results. The purpose of this study was to propose an optimized red-edge absorption area (OREA) index in which the prediction accuracy of vertically integrated leaf N content is improved within spring- and summer-sown maize canopies. The results showed that vertical distributions of N existed regardless of variations in the maize growth stages, that is, the leaf N density of the upper and middle layers was higher than that of the lower layers. These published vegetation indices (VIs) provided relatively good correlations with leaf N density at different layers across all of the datasets. When predicting leaf N density of each leaf layer, an optimal VI is generated, and inconsistent VIs will limit its practical application. To further overcome the drawbacks of the inconsistency of each VI when estimating the leaf N density at different layers, a new OREA index was proposed based on red-edge absorption area parameter. The OREA index showed the highest prediction accuracy with leaf N density for entire canopies (r2 = 0.811, RMSE = 0.374, RE = 13.17%) and canopies without top first leaves (r2 = 0.795, RMSE = 0.269, RE = 15.20%) compared with the other published VIs. It is concluded that the vertically integrated leaf N content under different field experiments can be accurately estimated by the OREA index.

中文翻译：

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使用冠层高光谱红边参数估计玉米中垂直整合的叶片氮含量

通过遥感实时监测叶片氮（N）含量，可以准确诊断作物营养状况，促进氮素精准管理。然而，用于估算垂直整合叶片N含量的方法没有考虑不同的种植系统，其中玉米生长阶段不同步，导致结果的实用价值降低。本研究的目的是提出一种优化的红边吸收区 (OREA) 指数，该指数可提高春播和夏播玉米冠层垂直整合叶片 N 含量的预测精度。结果表明，无论玉米生育阶段如何变化，氮素的垂直分布均存在，即上层和中层叶片氮浓度高于下层。这些已发布的植被指数 (VI) 在所有数据集的不同层提供了与叶 N 密度相对较好的相关性。在预测每个叶子层的叶子N密度时，会生成一个最优的VI，不一致的VI会限制其实际应用。为了进一步克服估计不同层叶氮密度时各VI不一致的缺点，提出了一种基于红边吸收面积参数的新OREA指数。OREA 指数显示整个冠层的叶氮密度最高（r2 = 0.811，RMSE = 0.374，RE = 13.17%）和没有顶部第一叶的冠层（r2 = 0.795，RMSE = 0.269，RE = 15.20%）与其他已发布的 VI。