Organic Matter Dynamics in Reclaimed Lignite Mine Soils under Robinia pseudoacacia L. Plantations of Different Ages in Germany

In temperate regions, cultivation of Robinia pseudoacacia L. has recently received considerable attention because it is a fast-growing species for biomass and bioenergy production, while acting as a potential carbon (C) sink to counterbalance carbon dioxide (CO₂) emissions and an alternative to agricultural crops on marginal sites. The objective of our work was to compare total organic carbon (TOC), total nitrogen (TN), and organic C fractions in postlignite mining soils under different development stages of R. pseudoacacia. Soil samples from three different depths (0–3, 3–10 and 10–30 cm) were taken in plantations 2, 3, 4, and 14 years old (R2, R3, R4, and R14, respectively). The TOC and TN contents increased with increasing tree age in all layers (P < 0.01). In the top 30 cm, TOC and TN stocks ranged from 11.7 to 59.8 Mg C ha^?1 and from 0.30 to 2.61 Mg N ha^?1 at R2 and R14, respectively. The rate of C sequestration was calculated to be 4.0 Mg C ha^?1 year^?1. Microbial biomass C and N were strongly correlated to TOC (r² = 0.96 – 0.81; P < 0.001) and TN contents (r² = 0.92 – 0.91; P < 0.001). The light fraction C (C_LF) accounted for 15–30% and the heavy fraction C for 70% of TOC in all layers. In the 0- to 3-cm layer, C_LF increased by 0.5 g kg^?1 year^?1. The results indicate that plantations of R. pseudoacacia are an attractive alternative to increase soil C contents in reclaimed lignite mining soils. In the short term, microbial biomass C and light fraction C are sensitive and provide an appropriate measure to assess soil C changes caused by cultivation of R. pseudoacacia.