How Does Soil Organic Matter Content Increase with Stand Age in Black Locust (Robinia pseudoacacia L.) Plantations Established on Overburden Deposits of Open-Pit Coal Mines?

This study examined the incorporation of organic carbon from above- and belowground primary production into soil organic matter following afforestation of backfilled overburden at the Bükkábrány lignite mine in northeastern Hungary. SOC accumulation was assessed in black locust (Robinia pseudoacacia L.) plantations of different ages (5, 9, 18, 23, and 29 years). In each age class, five randomly established 6 × 6 m plots were sampled; within each plot, soil samples were collected from five randomly selected boreholes at depths of 0–5, 5–15, and 15–25 cm.

At the initial stage, SOC content was close to zero. Over time, increasing plant biomass inputs (litter and root residues), together with enhanced microbial activity, resulted in continuous SOC accumulation. In older reclaimed stands, SOC levels approached those typical of natural forest soils, although full recovery depends on vegetation type, substrate properties, and climate.

At the Bükkábrány site, near-equilibrium SOC levels in the topsoil were reached within approximately 23 years, whereas it remains unclear whether deeper layers have equilibrated within three decades. Overall, SOC increased five- to tenfold between 5- and 29-year-old stands, depending on soil depth. After 29 years, SOC concentrations reached 4.0%, 2.7%, and 1.1% in the 0–5, 5–15, and 15–25 cm layers, respectively. In total, 6.1 kg m⁻² of organic carbon accumulated in the upper 25 cm over 29 years. These results demonstrate that fast-growing, nitrogen-fixing species such as black locust can rapidly enhance soil organic matter, classifying the studied soils among fast-accumulating reclaimed sites.