Intact Soil Core Total, Inorganic, and Organic Carbon Measurement Using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is an emerging elemental analysis technology with the potential to provide rapid, accurate, and precise analysis of soil constituents. We evaluated the accuracy of LIBS in measuring soil profile C for field-moist, intact soil cores by interrogating 78 intact soil cores from three Montana agricultural fields. Samples from each core were analyzed in the laboratory for total C (TC), inorganic C (IC), and soil organic C (SOC). Partial least squares 2 (PLS2) regression calibration models were derived using 58 cores (227 samples) and independently validated at the field scale with the remaining 20 cores (79 samples). We obtained the best LIBS validation predictions for IC (r(2) = 0.66, standard error of prediction [SEP] = 5.3 g kg(-1), ratio product differential [RPD] = 1.7) followed by TC (r(2) = 0.63, SEP = 6.0 g kg(-1), RPD = 1.6) and SOC (r(2) = 0.22, SEP = 3.2 g kg(-1), RPD = 1.1). Although the SEP for SOC was less than for TC and IC, low SOC variance limited our ability to evaluate LIBS SOC prediction capabilities. Regression coefficients from LIBS PLS2 models suggested a reliance on stoichiometric relationships between C and other elements related to total and inorganic C in the soil matrix (e. g., Ca, Mg, and Si) to discriminate TC from IC. Results indicate that LIBS spectral data collected on intact soil cores can be calibrated to accurately estimate and differentiate between soil total and inorganic C concentrations at the field scale.