Water and Nitrogen Effects on Active Canopy Sensor Vegetation Indices

Much of the previous evaluation of active crop canopy sensors for in-season assessment of crop N status has occurred in environments without water stress. The impact of concurrent water and N stress on the use of active crop canopy sensors for in-season N management is unknown. The objective of this study was to evaluate the performance of various spectral indices for sensing N status of corn (Zea mays L.), where spectral variability might be confounded by water-induced variations in crop reflectance. The study was conducted in 2009 and 2010 with experimental treatments of irrigation level (100 and 70% evapotranspiration [ET]), previous crop {corn-corn or soybean [Glycine max (L.) Merr.]-corn} and N fertilizer rate (0, 75, 150, and 225 kg N ha(-1)). Crop canopy reflectance was measured from V11 to R4 stage using two active sensors-a two band (880 and 590 nm) and a three band (760, 720, and 670 nm). Among the indices, the vegetation index described by near infrared minus red edge divided by near infrared minus red (DATT) and Meris terrestrial chlorophyll index (MTCI) were the least affected by water stress, with good ability to differentiate N rate with both previous crops. The chlorophyll index using amber band (CI), normalized difference vegetation index using red edge band (NDVI_RE) and the normalized vegetationi using the red band (NDVI_Red) showed more variation due to water supply, and had only moderate ability to differentiate N rates.