Spatio-Temporal Amino Acid-Sulphur Decomposition Characteristics in Recultivated Agricultural Soils after Lignite Mining

Recultivation aims to compensate for negative ‘legacy’ effects on soils from past open-cast mining when restoring land to productive agricultural systems. But little is known about the belowground sulphur (S) cycle that develops along a soil chronosequence after recultivation, especially for the deeper part of the soil. We, therefore, investigated the spatio-temporal characteristics of organic S depletion by adding the 35S-labelled methionine to the surface (0-30 cm) and subsurface soil (30-60 cm and 60-90 cm) after recultivated for 1, 8, 14, 24 and 48 years, respectively. Overall, total organic carbon (TOC), nitrogen (TN) and sulphur (TS) in surface soil significantly increased after 48 years of recultivation. However, no significant changes in nutrient content were observed in subsurface soils along the soil chronosequence. Recultivation age had no large effect on the microbial uptake amount of organic S (35S-MB) in surface soil. However, recultivation ages (≥ 8 years) led to fast immobilisation of 35S and less 35S being released as sulphate throughout the profile to maintain microbial stoichiometric equilibrium in older soils. Specifically, in year 1 more than 30% of the 35S was metabolised as sulphate throughout the profile, it declined to 9% (0-30 cm in year 8), 4% (30-60 cm in year 14) and 13% (60-90 cm in year 14), respectively. Weak correlation relationships (Pearson analysis) between sulphate and soil TOC, TN and TS in subsurface soil indicated that the microbial metabolisms of organic S as sulphate in subsoil were regulated by more complex factors rather than solely soil nutrient availability. Significant changes in the organic S partition were observed at age 8 after the soil was recultivated with N-fixing alfalfa. Whereas, after that (≥ 14 years), soil organic S partition showed no significant changes when a conventional tillage and cropping system had been well established. In conclusion, the adapted recultivation management preserves soil S stock through the reduction in microbial metabolisms of organic S as sulphate rather than by increasing microbial S uptake.