Novel multivariate analysis for soil carbon measurements using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS), a rapid and potentially field-deployable technology for estimating total C in soil, represents a novel approach to address important issues in soil science and C management. Our study has shown that models relating LIBS signal intensity at 247.85 nm to total C concentration determined by dry combustion vary as a function of elemental and textural characteristics of the soil, and, to a lesser extent, wavelength and excitation energy of the laser. To better quantify these sources of variation, two wavelengths and three excitation energies were used to analyze soils from various locations. The emission line of C at 247.85 nm was pronounced at an excitation wavelength of 532 nm and energy of 45 mJ, but it was largely obscured by the 248.9-nm Fe line at 1064 nm and excitation energies of 90 and 135 mJ. Univariate analysis revealed linear but soil-specific correlations between the signal intensity at 247.85 nm and total C concentration. A single and robust calibration model correlating LIBS spectra, collected at a laser wavelength of 532 nm and an excitation energy of 45 mJ, to C concentration in all samples was obtained using a multivariate approach. Several emission lines in addition to the strong C line contributed significantly to the multivariate model. These results show that multivariate analysis can be used to construct a robust calibration model for LIBS spectra and therein provide a reliable estimate of total soil C. Such results must be confirmed for a broader range of soils, yet crop and soil scientists, C managers, and instrument developers should find these results encouraging.