Effective indicators of plant nitrogen (N) nutrition are needed to improve N management in grasslands. This is particularly the case for mixtures that rely on N fixation by legumes as a major N input, because no reference tool such as the nitrogen nutrition index (NNI) exists under these conditions. The aims of this study were to test the reliability of a plant‐based index, the N concentration of upper leaves in the canopy (Nup), as a possible alternative for NNI in both pure and mixed grasslands. Data were gathered from four experiments covering a range of pure and mixed grasslands under different N fertilization levels. A cross‐validation of Nup predictions against NNI in pure stands, and against two NNI‐derived indices in mixtures, was performed. The Nup values appeared to be linearly related to NNI in pure stands of both grasses and legumes. The relationship was identical for the two groups of species and explained up to 86% of NNI variability. In mixtures, Nup also displayed a linear relationship with the two other tested indices, explaining 65% and 78% of variability. The conclusions of the three indices diverged with respect to strongly unbalanced mixtures, where the assumptions regarding the computation of NNI‐derived indices were not met. Excluding these situations, the overall relationship between Nup and NNI proved to be identical for mixtures and pure stands. The results suggest that Nup is a valid criterion for plant N nutrition which applies to a broad range of grassland species and to mixture conditions.

中文翻译：

---

冠层上部叶片中的氮浓度是纯草地草场和混合草地草场中植物氮素营养的可靠指标

需要有效的植物氮（N）营养指标来改善草原的氮素管理。对于依赖豆类固氮作为主要N输入的混合物，尤其如此，因为在这些条件下不存在诸如氮营养指数（NNI）之类的参考工具。这项研究的目的是测试以植物为基础的指数（冠层中上部叶片的氮浓度（Nup））的可靠性，以作为纯草地和混合草地中NNI的可能替代方法。数据来自四个试验，覆盖了不同氮肥水平下的一系列纯草地和混合草地。对纯林分中的NNI和混合物中两个NNI派生的指数进行了Nup预测的交叉验证。在纯草和豆科植物中，Nup值似乎与NNI呈线性关系。两组物种之间的关系相同，并解释了高达86％的NNI变异性。在混合物中，Nup还显示了与其他两个测试指标的线性关系，解释了65％和78％的变异性。对于强烈不平衡的混合物，这三个指数的结论不尽相同，因为它们不满足有关计算NNI衍生指数的假设。排除这些情况，混合物和纯林的Nup和NNI之间的整体关系被证明是相同的。结果表明，Nup是植物N营养的有效标准，适用于广泛的草地物种和混合条件。Nup还显示了与其他两个测试指标的线性关系，解释了65％和78％的变异性。对于强烈不平衡的混合物，这三个指数的结论不尽相同，因为它们不满足有关计算NNI衍生指数的假设。排除这些情况，混合物和纯林的Nup和NNI之间的整体关系被证明是相同的。结果表明，Nup是植物N营养的有效标准，适用于广泛的草地物种和混合条件。Nup还显示了与其他两个测试指标的线性关系，解释了65％和78％的变异性。对于强烈不平衡的混合物，这三个指数的结论不尽相同，因为它们不满足有关计算NNI衍生指数的假设。排除这些情况，混合物和纯林的Nup和NNI之间的整体关系被证明是相同的。结果表明，Nup是植物N营养的有效标准，适用于广泛的草地物种和混合条件。Nup和NNI之间的整体关系对于混合物和纯林而言是相同的。结果表明，Nup是植物N营养的有效标准，适用于广泛的草地物种和混合条件。Nup和NNI之间的整体关系对于混合物和纯林而言是相同的。结果表明，Nup是植物N营养的有效标准，适用于广泛的草地物种和混合条件。