Near infrared reflectance spectroscopy (NIRS) is an accurate, precise, and rapid alternative to wet chemistry procedures for determining concentrations of major classes of chemical compounds in organic materials, such as plant foliage. The method utilizes reflectance signals resulting from bending and stretching vibrations in molecular bonds between carbon, nitrogen, hydrogen, and oxygen. Calibration is required to correlate the spectral response of each sample at individual wavelengths to known chemical concentrations from laboratory analyses.
Nitrogen, lignin, and cellulose concentrations for woody plant foliage: green leaf, leaf litter, and decomposing leaf litter have been measured in the laboratory according to procedures described by Newman et al. 1994. Sample preparation involves drying and grinding to a uniform particle size. Diffuse reflectance spectral data were acquired using a NIRSystems 6500 monochromator with a spinning cup module, scanning at wavelengths from 400 to 2498 nm with a bandwidth of 10 nm. Reflectance data is converted to absorbance, A = log (1/R). Calibration equations were developed using partial least squares regression on first difference transformation of the absorbance data for the entire spectral range (Bolster et al., in press). A total of 18 deciduous and 10 conifer species are represented in the green leaf calibration equation; 13 deciduous and 4 conifer species in the leaf litter calibration equation; 4 deciduous species in the litter decomposition calibration equation.
The calibration equations for woody plant foliage are used for prediction of nitrogen, lignin, and cellulose concentrations in unknown samples. Precision of the equation for prediction of unknown samples depends on whether the range of variation affecting the chemical and physical properties of the unknown samples is represented in the calibration samples. Universal equations developed for broad, or infinite populations may be used for a wide range of samples. The importance of truly universal equations is to increase the value of NIRS for the study of large-scale ecosystem processes and change through remote sensing.
Sample material should be dried, for example, we use an oven at 70o C, and then ground to a uniform particle size which will pass through a 1 mm mesh. The ground sample must be thoroughly mixed for homogeneity. For NIRS analysis of dried leaf samples at a 1 mm particle size, the minimum quantity is approximately 5 g. dry weight (with a volume of not less than 10 ml.).