Quantification of lignite- and vegetation-derived soil carbon using ¹⁴C activity measurements in a forested chronosequence

¹⁴C activity measurements have been used in soil science for studying soil organic matter (SOM) dynamics as well as to separate carbon pools of different age. In this study, we used ¹⁴C activity measurements to quantify the contribution of organic carbon derived from lignite to the total carbon content of soils containing lignite and recently formed organic carbon in mixture. Samples were taken from the forest floor and mineral soils of a chronosequence of rehabilitated mine soils under pine forests (planted in 1966, 1981 and 1987). They were analyzed for their carbon content and ¹⁴C activity. All mineral soils show very old ¹⁴C ages. Old radiocarbon age is generated by contribution of lignite, which does not contain ¹⁴C. The lignite content of a sample was calculated by correcting the measured ¹⁴C activity for the ¹⁴C activity level of the recent plant-derived material contributing to SOM. The ¹⁴C activity of the recent carbon component was estimated in each stand by conducting a time series model using the Rothamsted model of soil carbon dynamics. The ¹⁴C activity of recent SOM was rather insensitive (± 4% relative) to the tested hypotheses on carbon dynamics in these young forest stands and allowed a precise estimate of the proportion of recent carbon. Lignite accounted for 80% to 93% of total carbon in the mineral topsoil (0-5 cm). The stock of recent soil carbon had a maximum increase between 20 and 32 years, and reached 36 ± 9 t/ha after 32 years, a value close to the current stock under ancient forests in the area.