In precision farming, accurate estimation of canopy nitrogen concentration (CNC) is valuable for effective crop growth monitoring and nitrogen (N) fertiliser management. To date, many canopy multispectral indices have been proposed as indicators for CNC; however, many of these indices have also shown sensitivity to biomass and their performance drops at high biomass levels. Dependence on growth stage, season, or other environmental conditions limits their efficacy as generalized CNC indices. The objectives of this study were to assess the robustness of popular CNC indices across a wide range of biomass levels and fertiliser application levels; and for two contrasting seasons – winter and summer. To achieve this, we analysed the efficacy of seven canopy nitrogen indices, including canopy chlorophyll content index (CCCI), together with eleven other commonly used spectral indices. We used canopy level solar-induced hyperspectral reflectance data acquired using a hand-held optical spectroradiometer across four growth stages in winter (May-June 2018) and four in summer (January-February 2019) from an experimental field of irrigated perennial ryegrass with variable N application in Victoria, Australia. The field contained 40 plots, each with one of eight different N treatments. Almost all the indices exhibited similar correlation to CNC (%) when applied to individual stages (days) in both winter and summer; however, relationships between CNC and individual indices varied significantly between stages. We obtained similar results for canopy biomass. When the data across the entire range of growth stages and seasons were combined, the correlations between most canopy nitrogen indices and CNC became weak (R² < 0.25, 0.9% ≤ RMSE ≤ 1.0%). PRI exhibited the highest correlation with CNC (R² = 0.58, RMSE = 0.7%) for the combined data set. Even so, PRI’s association with CNC and canopy biomass changed with the season. Most indices responded to both CNC and biomass simultaneously, and this confounds the estimation of CNC due to strong but growth stage-specific relationships between CNC and canopy biomass. This study shows that it is important to consider a wide range of conditions when evaluating multispectral CNC indices.

在精准农业中，准确估算冠层氮浓度（CNC）对于有效的作物生长监测和氮（N）肥管理非常有价值。迄今为止，已经提出了许多冠层多光谱指数作为CNC的指标。然而，许多这些指标也显示出对生物质的敏感性，并且在高生物质水平下其性能下降。对生长阶段，季节或其他环境条件的依赖限制了它们作为通用CNC指标的功效。这项研究的目的是评估广泛的生物量水平和肥料施用水平下流行的CNC指数的稳健性。和两个截然不同的季节-冬季和夏季。为此，我们分析了7个冠层氮指数（包括冠层叶绿素含量指数（CCCI），以及其他十一种常用的光谱指数。我们使用从手持式光谱仪获得的冠层级太阳诱发的高光谱反射率数据，该数据来自冬季多年生黑麦草可变实验场的四个生长阶段（2018年5月至6月）和夏天四个夏季（2019年1月至2019年2月）。在澳大利亚维多利亚州的N应用程序。该字段包含40个地块，每个地块有八种不同的N处理之一。在冬季和夏季的各个阶段（天）应用时，几乎所有指数都显示出与CNC（％）相似的相关性；但是，CNC和各个索引之间的关系在各个阶段之间有很大的不同。我们获得了冠层生物量相似的结果。如果将整个生长阶段和季节的数据进行合并，²  <0.25，0.9％≤RMSE≤1.0％）。 对于组合数据集，PRI与CNC的相关性最高（R ² = 0.58，RMSE = 0.7％）。即便如此，PRI与CNC和冠层生物量的关联也随季节而改变。大多数指数同时响应CNC和生物量，由于CNC和冠层生物量之间存在强烈但特定于生长阶段的关系，这混淆了对CNC的估计。这项研究表明，在评估多光谱CNC指标时，必须考虑广泛的条件。