Early season remote sensing of wheat nitrogen status using a green scanning laser

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 50,000 points s−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 (Zadoks 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 than 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.

► Early season crop foliar N assessment is critical to limit N runoff and volatilization.
► Foliar N was examined using a green scanning laser and traditional techniques.
► Green laser return intensity accurately predicted foliar N concentration.
► Chlorophyll meter values and spectral indices showed weak relationships to foliar N.
► Green lasers may allow improved spatiotemporal N management.