On the Remote Sensing of Foliar Nitrogen in Plants

The concentration of nitrogen (N) in foliage is central to numerous biogeochemical processes and can serve as an indicator of carbon assimilation, species composition and linkages between terrestrial and aquatic ecosystems. Efforts to detect foliar N via remote sensing began decades ago and have been continually improved using a variety of methods and sensors. Despite this, the use of foliar N in regional- to global-scale analyses has lagged, in part because we lack instruments that provide applicable data at broad scales and because there is still no consensus on the spectral properties needed and the mechanisms that underlie foliar N detection. Here, we review the history of foliar N detection--from early laboratory based approaches to proposed methods using planned future sensors--and discuss recent findings that relate foliar N to broadband spectral features as well as high spectral resolution data. We also discuss recently revealed relations among foliar N and total shortwave albedo and address criticisms that have been directed at the use of remote sensing for foliar N detection. Our analysis is based on a combination of models and data collected over a wide range of North American research sites. Findings are presented in relation to both current and planned future sensors.