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REMOTE SENSING

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

^a Dep. of Forest Resources, Univ. of Idaho, Moscow, ID 83844-1133
^b USDA-ARS Columbia Plateau Conserv. Res. Center, PO Box 370, Pendleton, OR 97801
^c USDA-ARS Hydrology and Remote Sensing Lab., Bldg 007, Rm. 104, BARC-West, Beltsville, MD 20705-2350

^* Corresponding author (dan.long{at}ars.usda.gov).

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 C_ab 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 C_ab 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 (r² 0.84) and less well correlated with chlorophyll meter readings (r² 0.46) and flag leaf N (r² 0.29). MCARI/MTVI2 was poorly correlated with LAI (r² = 0.01) and more highly correlated with chlorophyll (r² = 0.70) and flag leaf N (r² = 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.

Abbreviations: BER, band equivalent reflectance • C_ab, chlorophyll a+b content • CCCI, canopy chlorophyll content index • GNDVI, green normalized difference vegetation index • GI, greenness index • LAI, leaf area index • MCARI, modified chlorophyll absorption ratio index • MCARI/MTVI2, the ratio of the modified chlorophyll absorption ratio index to the second modified triangular vegetation index • MTVI2, second modified triangular vegetation index • N, nitrogen • NIR, near infrared • NRI, nitrogen reflectance index • NDVI, normalized difference vegetation index • OSAVI, optimized soil-adjusted vegetation index • SR, simple ratio • SPAD, soil plant analysis development • TCARI, transformed chlorophyll absorption in reflectance index • TVI, triangular vegetation index

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Received for publication November 9, 2007.