Nitrogen influence on fresh-leaf NIR spectra

Simulations and measurements were used derive information on the form and strength of the nitrogen (N) “signal” in near-infrared (NIR) spectra of fresh leaves. Simulations across multiple species indicated that in total, protein absorption decreased NIR reflectance by up to 1.8% absolute and transmittance by up to 3.7% absolute, all other inputs held equal. Associated changes in spectral slope were generally of ±0.02% nm−1 absolute. Spectral effects were about an order of magnitude more subtle for a smaller, though potentially ecologically significant, change in N concentration of 0.5% absolute over measured. Nitrogen influence on spectral slope was fairly consistent across four empirical datasets as judged by wavelength dependence of N correlation, and there was reasonable agreement of observed and modeled slope perturbations with locations of known protein absorption features. Improved understanding of the form and strength of the N signal under differing conditions will support continued development of laboratory-based spectral measurement and analysis strategies for direct N estimation in individual fresh leaves. A pragmatic approach for canopy-level estimation by remote sensing, however, might additionally consider surrogate measures such as chlorophyll concentration or canopy biophysical properties.