Stable carbon isotope signature and chemical composition of organic matter in lignite-containing mine soils and sediments are closely linked

Abstract

We studied the chemical and stable carbon isotope composition of soil organic matter at a mine site, rehabilitated for soil development 40 years ago. The site contains two types of organic matter: fresh plant material and lignite inherent to the parent substrate. A transect was sampled, consisting of mine soil rehabilitated with forest, partially submerged sediment and submerged sediment. Our objective was to assess whether the stable carbon isotope signature is related to the chemical composition of the organic matter. We analysed the chemical composition using 13C cross polarization, magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy and lignin parameters determined after CuO oxidation. The organic matter was most enriched in 13C in the bulk soil of the forest plot and partially submerged plot. The stable isotopic signature (δ 13C) was related to the chemical composition of the sample as seen with 13C CPMAS NMR spectroscopy. The ratio (alkyl + aromatic)/(O-alky + carbonyl C) was strongly correlated with the δ 13C value regardless of the site conditions (aerobic soil or submerged sediment). Such a correlation was also observed between δ 13C and lignin content (V + S + C, where V = vanillyl, S = syringyl, C = cinnamyl oxidation products), as well as parameters indicating lignin diagenetic state [(Ac/Al)V, S/V and C/V, where Ac = acids, Al = aldehydes]. Taking δ 13C as an indicator for lignite contribution, we explain the relationships by an increasing influence of lignite contribution not only on bulk chemical composition but also on lignin parameters when the organic matter shows 13C enrichment.