Active Optical Sensing for Irrigated Durum Wheat: Separating Nitrogen and Water Effects.

Interest in active optical sensors (AOS) for assisting in nitrogen (N) management of crops like wheat (Triticum aestivum L.) has been widespread since the mid-1990s.  More recently, AOS have been tested to assess water status of crops in addition to plant N stress.  Researchers have used vegetation indices (VIs) that employ red-edge reflectance, instead of relying only on visible and near infrared bands, typically used in the normalized difference vegetation index (NDVI) calculation.  Some 2D VIs may be able to assess N status  while minimizing water effects.  A 2-year study was conducted on a Casa Grande sandy loam soil in Maricopa, AZ with durum wheat (Triticum durum Desf) with an overhead sprinkler system.  The study uniquely had 10 un-randomized levels of irrigation and five rates of N fertilizer.  The aim was to compare 12 vegetation indices (VIs) for their ability to distinguish irrigation and N fertilizer effects, and to determine how well the VIs estimated biomass, plant N, grain N, and yellow berry.  Crop Circle 470 AOS with interference filters at 800, 730, 670, 590,  and 530 nm were passed 1 m above the canopy throughout  the growing season.  Twelve VIs were calculated; NDVIA, NDVIR, NDVIG (amber, red, and green as the visible bands), and NDARE, NDRRE, NDRE, CCCI, DATT, PRI, CI, MTCI, and CIRE.  Canopy temperature was measured with Apogee infrared thermometers (IRTs).  Biomass at Zadoks 32 did not respond to N rate but did respond to water level. Zadoks 39 biomass responded to N and water ini both years.  Total N uptake was affected more by by N than by water.  The NDVIs showed very strong N response in all four growth-stage-years, and irrigation  response in three of four growth-stage-years.  The VIs with the best ability to respond strongly to N, but not to water effects were CCCI, DATT, and MTCI. CCCI also did best in esimating grain N. Canopy temperature measured by IRTs measured water stress with minimal N effects better than any of the VIs.