Nitrogen influence on fresh-leaf NIR spectra

Simulations and measurements were used derive information on the form and s trength of the nitrogen (N) "signal" in near-infrared (NIR) spectra of fres h leaves. Simulations across multiple species indicated that in total, prot ein absorption decreased NIR reflectance by up to 1.8% absolute and transmi ttance by up to 3.7% absolute, all other inputs held equal. Associated chan ges in spectral slope were generally of +/-0.02% nm(-1) absolute. Spectral effects were about an order of magnitude more subtle fur a smaller, though potentially ecologically significant, change in N concentration of 0.5% abs olute over measured. Nitrogen influence on spectral slope was fairly consis tent across four empirical datasets as judged by wavelength dependence of N correlation, and there was reasonable agreement of observed and modeled sl ope perturbations with locations of known protein absorption features. Impr oved 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 se nsing, however, might additionally consider surrogate measures such as chlo rophyll concentration or canopy biophysical properties. (C) 2001 Elsevier S cience Inc. All rights reserved.