Estimating Corn Leaf Chlorophyll Concentration from Leaf and Canopy Reflectance

Farmers must balance the competing goals of supplying contributions of background reflectance, while spectral vegetation indices that combined reflectances of near-infrared adequate N for their crops while minimizing N losses to the environment. To characterize the spatial variability of N and other visible bands (MCARI and NIR/Green) were responsive to both leaf chlorophyll concentrations and backover large fields, traditional methods (soil testing, plant tissue analysis, and chlorophyll meters) require many point ground reflectance. Pairs of these spectral vegetation indices plotted together produced isolines of leaf chlorophyll samples. Because of the close link between leaf chlorophyll and leaf N concentration, remote sensing techniques have concentrations. The slopes of these isolines were linearly related to leaf chlorophyll concentration. A limited test the potential to evaluate the N variability over large fields quickly. Our objectives were to (1) select wavelengths sensiwith measured canopy reflectance and leaf chlorophyll data confirmed these results. The characterization of leaf chlorotive to leaf chlorophyll concentration, (2) simulate canopy reflectance using a radiative transfer model, and (3) propose phyll concentrations at the field scale without the confounding problem of background reflectance and LAI variability a strategy for detecting leaf chlorophyll status of plants using remotely sensed data. A wide range of leaf chlorophyll levels holds promise as a valuable aid for decision making in managing N applications. Published by Elsevier Science was established in field-grown corn (Zea mays L.) with the application of 8 N levels: 0%, 12.5%, 25%, 50%, 75%, Inc. 100%, 125%, and 150% of the recommended rate. Reflectance and transmittance spectra of fully expanded upper INTRODUCTION leaves were acquired over the 400-nm to 1,000-nm wavelength range shortly after anthesis with a spectroradiometer Nitrogen is an essential element for plant growth and is frequently the major limiting nutrient in most agricultural and integrating sphere. Broad-band differences in leaf specsoils. Profitable corn (Zea mays L.) production systems tra were observed near 550 nm, 715 nm, and .750 nm. Crop require inputs of large quantities of N. Nitrogen fertilizer canopy reflectance was simulated using the SAIL (Scattering in excess of a crop’s nutritional needs may move into surface by Arbitrarily Inclined Leaves) canopy reflectance model water and groundwater and contribute to eutrophication for a wide range of background reflectances, leaf area indices of lakes and streams (Wood et al., 1993). Farmers must (LAI), and leaf chlorophyll concentrations. Variations in balance the competing goals of supplying enough N to background reflectance and LAI confounded the detection their crops while minimizing the loss of N to the environof the relatively subtle differences in canopy reflectance ment, which represents both a threat to water quality and due to changes in leaf chlorophyll concentration. Spectral an economic loss. The economic penalties of reduced yields vegetation indices that combined near-infrared reflectance from supplying inadequate N are substantial. and red reflectance (e.g., OSAVI and NIR/Red) minimized Traditionally, soil testing, plant tissue analysis, and long-term field trials have been used for assessing N avail*USDA, ARS, Remote Sensing and Modeling Laboratory ability for crops. More recently, chlorophyll meters (e.g., †USDA, ARS, Instrumentation and Sensing Laboratory SPAD-502,1 Minolta Osaka Co., Ltd., Japan) have been ‡Raytheon STX, Lanham, Maryland Address correspondence to C. S. T. Daughtry, Remote Sensing and Modelling Lab., USDA/ARS, 10300 Baltimore Avenue, Beltsville, MD 20705-2350, USA. E-mail: [email protected] 1 Company and product names are used for clarity and do not imply any endorsement by USDA to the exclusion of other comparable products. Recevied 13 December 1999; revised 4 March 2000.