Estimation of nitrogen content in herbaceous plants using hyperspectral vegetation indices

In this chapter, we review the use of hyperspectral data for the assessment of nitrogen content in herbaceous plants (crops and pastures) by illustrating the major datasets and methods employed for this purpose. In the introduction and state of the art sections, general background is provided together with information on the precursory studies and instruments used for N content measurement and monitoring. Moreover, the most recent and advanced instruments for field measurements/estimates of leaf and canopy nitrogen are described. A brief summary of the major platforms used for proximal and remote sensing of nitrogen content is given; hand-held and airborne platforms are by far the most exploited ones at the local to regional scale. Satellite hyperspectral data are still underexploited, and future missions could further stimulate large-area applications. In Section 8.3, the theoretical background relating reflectance of vegetated surfaces in the visible to shortwave infrared wavelengths and chlorophyll/ nitrogen content are given; these are the bases of the retrieval approaches illustrated in Sections 8.4 and 8.5. Section 8.5, in particular, is fully dedicated to vegetation indices, which have been developed for and applied to hyperspectral data; this review is organized based on the formalization of the indices and their content (e.g., simple ration, normalized difference, chlorophyll content, soil adjusted, red-edge).