Combined Spectral Index to Improve Ground-Based Estimates of Nitrogen Status in Dryland Wheat

Spectral indices are useful for estimating crop yield potential and basing in-season N fertilizer applications. The normalized difference vegetation index (NDVI) is positively related to crop N status and leaf area index (LAI) under N limited conditions. However, under water limited conditions, variations in LAI may be driven by soil moisture rather than plant-available N, which will confound spectral estimates of crop N status. This study evaluated the performance of spectral indices for ground sensing of crop chlorophyll a+b content (C-ab) and N status in dryland wheat (Triticum aestivum L.). Sensitivity of spectral indices to Cab and LAI was assessed using reflectance spectra simulated by the PROSPECT+SAIL model. Simulations showed the modified chlorophyll absorption ratio index and second modified triangular vegetation index in ratio (MCARI/MTVI2) to be sensitive to Cab and resistant to LAI. This conclusion was tested in dryland fields with measured canopy reflectance at Zadoks growth stages 57 to 60. NDVI and other simple indices were highly correlated with LAI (r(2) <= 0.84) and less well correlated with chlorophyll meter readings (r(2) <= 0.46) and flag leaf N (r(2) <= 0.29). MCARI/MTVI2 was poorly correlated with LAI (r(2) = 0.01) and more highly correlated with chlorophyll (r(2) = 0.70) and flag leaf N (r(2) = 0.54), agreeing with PROSPECT+SAIL. Use of MCARI/MTVI2 may improve ground estimates of crop N status where LAI variability is associated with water availability. This information is potentially useful for decisions whether to apply foliar N to enhance grain protein concentration of wheat.