Evaluation of nitrogen treatment effects on the reflectance of cotton at different spatial scales

The importance of assessing nitrogen (N) status in cotton is important from economic and environmental standpoints. In this study, visible and near-infrared reflectance (NIR) data were collected at cotton leaf-, canopy- and scene-scales at three levels of N treatments to determine the best spatial scale and growth stage that most effectively indicate N treatment effects. While N fertilization affected relative chlorophyll content, leaf area index (LAI), and ground cover (GC) simultaneously, these factors portrayed different effects on cotton reflectance measured at the three spatial scales. Leaf-scale measurement was mainly affected by chlorophyll content. Canopy-scale reflectance was controlled by chlorophyll content and LAI. Scene-scale reflectance was predominantly controlled by GC and to the least extent by chlorophyll content. In terms of visible reflectance, chlorophyll absorption decreased with decreasing N at all spatial scales. Nitrogen treatment effects were most apparent at 550 and 700 nm at the leaf-scale, 610 and 700 nm at the canopy-scale, and 685-690 nm at the scene-scale (after per cent GC exceeded 64%). Only measurements taken at the scene-scale demonstrated a consistent relationship between N fertilization and NIR (800-1000 nm). This information could be useful in the development of N-sensitive indices.