Black carbon as a significant component of aromatic carbon in surface soils and its importance is enhanced in volcanic ash soil profiles

Abstract

Black carbon (BC) constitutes a refractory carbon (C) pool in soil and other environments. Estimating the amount and dynamics of BC in soil is important in terms of the global C cycle. The goal of this study was to characterize the contribution of BC to total and aromatic C in a variety of soils using the benzene polycarboxylic acid (BPCA) method and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Thirteen A-/Ap (plowed layer)-horizon soils that belong to widely distributed soil series (Humic Andosols, Dystric Cambisols, Gleyic Cambisols, Eutric Fluvisols, and Dystric Planosols) under typical land uses (forest, grassland, upland field, and paddy field) in Japan and two Andosol profiles were analyzed. Three humic acid samples including the green fraction, which contains 4,9-dihydroxyperylene-3,10-quinone (DHPQ) as a chromophore and has mycorrhiza as a possible source (referred to as Pg), were also subjected to the BPCA procedure. The BC content in the 13 A-/Ap-horizon soils was 0.3–53 gC kg−1 or 5.7–48% of the total soil C (BC%). The BC content correlated positively with the total C content and with aromatic C content, while the relationship between BC% and soil order or land use was not clear. An analysis of Pg suggested the potential contribution of DHPQ to the yield of benzenehexacarboxylic acid in the BPCA method. The variations in BC content and C composition of two Andosol profiles indicated a greater stability of BC than the other types of C, including monocyclic aromatic C. The contribution of BC to aromatic C in their lower layers was estimated to be ca. 55–70% based on a regression equation when assuming that 70–90% of the BC was aromatic C.