Quantification of deep soil carbon by a wet digestion technique

Abstract

Two wet digestion methods were evaluated using pure kaolinite as background for quantifying small concentrations of carbon (<0.05% total organic carbon (TOC)) in deep kaolinitic regolith in south-western Australia. The limit of detection and limit of quantification of the Walkley–Black method (0.015 and 0.050% TOC respectively) were approximately five times lower than those of the Heanes method (0.085 and 0.281% TOC respectively). Both methods showed excellent linearity (R2>0.99) using prepared standards (lignin, humic acid, cellulose and chitin mixed with kaolinite and their combinations), in the concentration range 0.008–1.000% TOC. However, the percentage carbon recovery values were underestimated for chitin. The Walkley–Black method (TOCWB, %) was evaluated with 94 calibration and 27 validation deep soil samples (1–35m soil depth) and compared with a dry combustion (Elementar) technique (TOCactual, %). The predictive equation (TOCactual=1.66TOCWB+0.018) (R2=0.91) obtained from the calibration set agreed well with the benchmark dry combustion values (root mean square error=0.017) and is recommended for quantification of deep soil carbon in other kaolinitic regoliths.