Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 19, 2011
Publication Date: July 28, 2011
Citation: Eitel, J.U., Vierling, L., Long, D.S., Hunt Jr, E.R. 2011. Early season remote sensing of wheat nitrogen status using a green scanning laser. Agricultural and Forest Meteorology. 151:1338-1345. Interpretive Summary: Optical sensing of the crop canopy for determining chlorophyll content represents a new way to obtain information for nitrogen management in agriculture. Green laser scanning, a new technique which can measure the intensity of reflected light rapidly with extremely high resolution, is ideal for sensing growth and color differences in crop canopies early in the growing season. A green laser scanner was evaluated in the field for this purpose when the wheat crop was in the tillering stage of growth. Laser readings were more highly correlated with leaf chlorophyll than a hand-held chlorophyll meter and tractor-mounted light sensor. The results suggest superiority of green laser scanning over current techniques of basing nitrogen management in crops.
Technical Abstract: In-season, spatially variable nitrogen (N) fertilizer applications in agricultural systems can help to maximize crop N use efficiency and minimize N losses via hydrological leaching, runoff, and atmospheric volatilization. N fertilizer management often relies upon measurements of crop spectral reflectance using ground-based optical on-the-go sensors or hand-held chlorophyll meters. However, soil background reflectance can confound on-the-go sensing, especially during early crop growth stages, and hand-held chlorophyll meters are impractical for spatially explicit mapping at the field scale. Scanning laser technology is available that measures the intensity of the reflected laser light plus height information within a mm-scale ground instantaneous field of view at a very high sampling rate (up to 50000 points sec-1 in this study). We examined the ability to quantify foliar N status of spring wheat (Triticum aestivum L.) using a green (532 nm) terrestrial laser scanner during an early stem extension growth stage (Zadkoks growth stage 3.2). Laser data were processed by (1) removing soil background returns based on laser-determined height information, (2) standardizing green laser intensity based on white-reference panel readings, and (3) filtering noisy laser returns from leaf edges based on a laser return intensity threshold value. The return intensity of the reflected green laser light more accurately (r2 = 0.68, RMSE = 0.30 µg g-1) predicted foliar N concentration in contrast to conventional chlorophyll meter readings (r2 = 0.36, RMSE = 0.41 µg g-1) and spectral indices measured by a ground optical on-the-go sensor (r2 < 0.41, RMSE > 0.39 µg g-1). The results indicate that laser scanners are useful for measuring the N status of wheat during early growth stages, and provide justification for incorporating laser scanner based measurements into developing spatially-explicit estimates of foliar N during this critical growth period. Further research is needed to evaluate the operational practicality of a green scanning laser from a moving platform.