CARBON AND NITROGEN ANALYSIS OF SOIL FRACTIONS USING NEAR-INFRARED REFLECTANCE SPECTROSCOPY

A nondestructive method to determine total C and N concentrations in soil size fractions is desirable when a limited sample is available. Near-infrared reflectance spectroscopy (NIRS) was used to determine the total C and N concentrations in silt (50.0-2.0-mu-m) and coarse clay (2.0-0.2-mu-m), separated from 12 surface soils, by regressing the diffuse reflectance of near-infrared radiation with constituent concentrations determined using combustion techniques. The correlation coefficients (R2) of the calibration equations were 0.93 for C and 0.89 for N, and the standard errors associated with NIRS predictions were 6.2 g kg-1 soil for total C and 0.6 g kg-1 for total N. Equation development with only silt samples improved the accuracy of NIRS calibration equations. Coefficients of variation [CV = (standard error of performance divided-by mean of the combustion procedure) X 100] for validation sample sets ranged from 14 to 19%, which is within the acceptable range for determining inorganic elements in plant tissues. We conclude that NIRS can be used to predict C and N concentrations in soil size fractions.