How useful is the mid-infrared spectroscopy in the assessment of black carbon in soils

Black carbon (BC), the recalcitrant continuum of products from incomplete combustion, includes char, charcoal and soot, being considered an important component of the global C cycle. However due to measurement uncertainties, the magnitude and distribution of BC is hardly known. In this study, a rapid and inexpensive spectroscopic technique, as it is mid-infrared spectroscopy in combination with oxidation procedures is proposed to quantify the recalcitrant aromatic fraction resistant, which can effectively determine the proportion of BC in soils. This method was tested by using a wide variety soil samples of various origin, composition and properties. Results were contrasted by those obtained by applying solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. Mid-infrared spectroscopy showed a very high predicting potential in the case of samples with large concentrations of BC by taking advantage of the relative optical density of the 2920 cm-1 C–H stretching band. In the case of soils with low BC contents, the application of Partial Least Square Regression to baseline-subtracted, second-derivative Fourier-Transformed Infra-red (FT-IR) spectra lead to significant (P<0.05) cross-validation models. By this procedure a considerable improvement in forecasting the aromatic fraction resistant to the chemical oxidation steps (BC-like material) was obtained.