Accurately monitoring Canopy Nitrogen Concentration (CNC) is a prerequisite for precision nitrogen (N) fertiliser management at the farm scale with carbon and N budgeting across the landscape and ecosystems. While many spectral indices have been proposed for CNC monitoring, their applicability and accuracy are often adversely affected by confounding factors such as aboveground biomass (AGB), crop type, growth stages, and environmental conditions, limiting their broader application and adoption; with AGB being one of the most dominant signals and confounding factors at canopy scale. The confounding effect can become more challenging as AGB is also physiologically linked with CNC across the growth stages. Additionally, the interplay between index form, selection of optimal wavebands and their bandwidths remains poorly understood for CNC index design. This study proposes robust and cost-effective 2- and 4-waveband multispectral (MS) CNC indices applicable across a wide range of crop conditions. We collected 449 canopy reflectance spectra (400–980 nm) together with corresponding CNC and AGB measurements across four growth stages of ryegrass (winter and summer), and five growth stages of barley (winter-spring) in Victoria, Australia, in 2018 and 2019. All possible waveband (400–980 nm) combinations revealed that the best combination varied between seasons and crop types. However, the visible spectrum, particularly the blue region, presented high and consistent performance. Bandwidths of 10–40 nm outperformed either very narrow (2 nm) or very broad bandwidths (80 nm). The newly developed 2-waveband index (416 and 442 nm with 10-nm bandwidth; R² = 0.75 and NRMSE = 0.2) and 4-waveband index (512, 440, 414 and 588 nm with 40-nm bandwidth; R² = 0.81 and NRMSE = 0.17) exhibited the best performance, while validation with an independent dataset (from a different growing period to those used in the model development) obtained NRMSE values of 0.25 and 0.24, respectively. The 4-waveband index provides enhanced performance and permits use of broader bandwidths than its 2-waveband counterpart.

准确监测冠层氮浓度 (CNC) 是农场规模精准氮 (N) 肥料管理以及整个景观和生态系统碳和氮预算的先决条件。虽然已经提出了许多用于 CNC 监测的光谱指数，但它们的适用性和准确性往往受到地上生物量 (AGB)、作物类型、生长阶段和环境条件等混杂因素的不利影响，限制了其更广泛的应用和采用；AGB 是冠层尺度上最主要的信号和混杂因素之一。由于 AGB 在整个生长阶段也与 CNC 存在生理联系，因此混杂效应可能变得更具挑战性。此外，对于 CNC 索引设计，索引形式、最佳波段的选择及其带宽之间的相互作用仍然知之甚少。本研究提出了稳健且经济高效的 2 波段和 4 波段多光谱 (MS) CNC 指数，适用于各种作物条件。我们于 2018 年和2019。所有可能的波段（400-980 nm）组合表明，最佳组合因季节和作物类型而异。然而，可见光谱，特别是蓝色区域，表现出较高且一致的性能。10-40 nm 的带宽优于非常窄的带宽 (2 nm) 或非常宽的带宽 (80 nm)。新开发的 2 波段指数（416 和 442 nm，10 nm 带宽；R ²  = 0.75，NRMSE = 0.2）和 4 波段指数（512、440、414 和 588 nm，40 nm 带宽；R ²  = 0.81 和 NRMSE = 0.17）表现出最佳性能，而使用独立数据集（来自模型开发中使用的不同生长期）进行验证时，NRMSE 值分别为 0.25 和 0.24。4 波段索引提供了增强的性能，并允许使用比 2 波段索引更宽的带宽。