35S-labeled methionine dynamics in a 62-year agricultural post-mining soil chronosequence

Global industrial sulphur (S) dioxide emissions between 1900 to 1980 led to excessive S deposition and associated soil acidification.  However, since introducing effective mitigation strategies, industrial S emissions have been significantly reduced, with concurrent reductions in S deposition. This has resulted in S deficiency in many croplands which now require supplementary S applications via fertilisers. We examined if such past differential atmospheric S inputs (‘legacy’) influence organic (or inorganic) S dynamics in current agricultural soils. We used a 62-year chronosequence of the reclaimed agricultural field after brown-coal mining (Inden, Germany) to sample topsoil (0-30 cm) from seven sites (representing the years 1956, 1971, 1985, 1995, 2005, 2011, and 2018). The dynamics of sulphur transformation were determined by adding ³⁵S labelled methionine (Met) at 6, 24 and 48 h in an incubation experiment. The ³⁵S-Met and ³⁵S-SO₄derived from labelled Metwere determined by measuring CaCl₂-extractable ³⁵S with or without BaCl₂, the difference between the total added ³⁵S-Met and the CaCl₂-extractable ³⁵S was recognized as the ³⁵S immobilised in the microbial biomass. Results showed that soil S concentrations declined in a curvilinear pattern over the full chronosequence, from 0.27 (in 1956) to 0.11 g S kg^-1 soil (in 2018). In contrast, soil C peaked in 1995 at 16 g C kg^-1 soil, with the lowest values in 1956 at 10 g C kg^-1 soil. For the site recultivated in 1985, transformation and S dynamics obviously differed from others. Here, compared with other sites, the ³⁵S-SO₄(inorganic S) concentrations (as % of the total ³⁵S-Met added) peaked at 12, 29, 38% respectively, and ³⁵S-Met (organic S) was the lowest at 35, 23, and 16%, respectively (at sampling times, 6, 24, and 48 h). The microbial biomass immobilized 53% of ³⁵S-Met added to the soils in less than 6 h, and gradually released it as ³⁵S-SO₄ as incubation time increased. We conclude that organic S transformation in the soils was driven by the C rather S content, possible through differences in microbial C biomass, As such the effect of the S legacy in the soils could not be confirmed.