Black carbon in soils: the use of benzenecarboxylic acids as specific markers

A simple method is presented for the estimation of black carbon in soil samples. Our method uses benzenepolycarboxylic acids (BPCA) as specific markers for black carbon. The analytical procedure includes acid digestion, oxidation, sample cleanup, derivatization, and gas chromatography. To test the usefulness of BPCA as markers for black carbon, we compared the yields of BPCA from charred wood with those from “humic substances” produced in the laboratory by enzymatic browning, non-enzymatic browning, and aerobic incubation of barley straw. The BPCA yield from charred wood was 257 mg BPCA-C g−1 C, whereas no BPCA were formed from the humified materials. Soils known to have received charred residues gave much higher yields of BPCA than their adjacent counterparts without char application. We conclude that BPCA can be used as a specific measure of black carbon in soils.