Near-infrared reflectance spectroscopic analysis of soil C and N

Near-infrared reflectance spectroscopy (NIRS) is a rapid and nondestructive analytical technique that can be used to quantify various soil properties. The objective of this study was to evaluate the ability of NIRS to evaluate independently organic C, inorganic C, and total N content of diverse soil materials. Samples (n = 108) were prepared by mixing soil with CaCO3, humic acid, and/or compost materials. About 30% of the samples were selected randomly for the validation set, and the remaining samples were assigned to the calibration set. NIR spectra of these samples were correlated with measured values of organic C, inorganic C, total C, total N, and C:N ratios using partial least squares regression. Leave-one-out cross-validation analysis yielded r(2) values between the measured and predicted soil properties higher than 0.86 for all tested properties. Similar results were obtained from analysis of the validation set (r(2)>0.85). The successful prediction of total N and C:N ratios for the studied samples indicates that NIRS predictions of total N for soils are based not on the strong correlation between levels of C and N but rather on an independent response to soil N. The results indicate that MRS can be used to quantify independently and simultaneously organic C, inorganic C, and total N for soils with diverse C and N compositions.