A new spectrophotometric method for rapid semiquantitative determination of soil organic carbon

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Most methods to determine soil organic C (SOC) content are constrained by the time or equipment required, thereby limiting their use in determining the spatial variability of SOC across large areas. In this study, a new spectrophotometric method for the rapid determination of SOC was developed. The method is based on Beer's law, A = {varepsilon}lc, where A is absorbance, {varepsilon} is absorptivity, l is the path length, and c is the concentration of the absorbing species, and uses soil extract absorbance at 300 nm (A300nm) as a proxy for SOC. For organic C extractions, 1 mol L–1 HCl followed by a 0.25 mol L–1 NaOH–0.1 mol L–1 Na4P2O7·10H2O solution was used (1:250 soil/extractant ratio). Evaluation of the method using 146 soil samples from 11 Major Land Resource Areas (MLRAs) across Texas indicated that SOC was linearly related to A300nm. For soils containing up to 50 g kg–1 SOC, the relationship could be described by the equation A300nm = 5.0 SOC (r2 = 0.89). The standard error and relative prediction deviation associated with the SOC values determined using the newly proposed method were 2.9 g kg–1 SOC and 3.2, respectively. Calibration sample size, MLRA, soil texture, and inorganic C content had no significant effect on SOC extractability or method performance. In addition to being rapid, the method was accurate, stable, easy to execute, amendable to field use, and shows potential for use across large geographic areas comprised of soils from different parent materials and land uses.

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