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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

A New Spectrophotometric Method for Rapid Semiquantitative Determination of Soil Organic Carbon

^a Water Management & Hydrological Sciences, Texas A&M Univ., 3408 TAMU, College Station, TX 77843-3408
^b Dep. of Geology and Geophysics, Texas A&M Univ., 3115 TAMU, College Station, TX 77843-3115
^c Texas Transportation Institute, 3135 TAMU, College Station, TX 77843-3135
^d Dep. of Agricultural Leadership, Education & Communication, Texas A&M Univ., 2116 TAMU, College Station, TX 77843-2116

^* Corresponding author (orharvey{at}tamu.edu).

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 = lc, where A is absorbance, is absorptivity, l is the path length, and c is the concentration of the absorbing species, and uses soil extract absorbance at 300 nm (A_300nm) 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 A_300nm. For soils containing up to 50 g kg^–1 SOC, the relationship could be described by the equation A_300nm = 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.

Abbreviations: EDTA, ethylenediamine tetraacetic acid • MLRA, major land resource area • SE_p, standard error of prediction • SOC, soil organic carbon • UV, ultraviolet