Amino acid-sulphur decomposition in agricultural soil profile along a long-term recultivation chronosequence.

The significance of sulphur (S) availability for crop yield and quality is highlighted under the global S deficiency scenario. However, little is known about the temporal trend in belowground organic S mineralisation when restoring land to productive agricultural systems, particularly for the deeper soil parts. Therefore, we investigated the decomposition of ³⁵ S-labelled methionine in surface (0-30 cm) and subsurface soil (30-60 cm and 60-90 cm) over a 48-year recultivation chronosequence (sampled after1, 8, 14, 24 and 48 years). Soil total sulphur (TS) significantly (p < 0.05) increased in surface soil but not in subsurface soils after 48 years of recultivation. Overall, the immobilisation of ³⁵ S-methionine ( ³⁵ S-MB) in subsurface soils relative to year 1 significantly decreased over the chronosequence but did not change in the surface samples. The ³⁵ S-MB values in subsurface soils were positively corrected with soil carbon (C) stoichiometry (Pearson correlation, p < 0.05), suggesting the immobilisation of methionine was likely constrained by microbial C demand in deep soil. Compared to year 1, ³⁵ S-SO ₄ ^2- released from ³⁵ S-methionine significantly declined throughout the older (≥ 8 years) soil profiles. Significant (p < 0.05) changes in the organic ³⁵ S partition ( ³⁵ S immobilisation and ³⁵ S released as sulphate) were observed in year 8 after the soil was recultivated with N-fixing alfalfa or fertilisers. Whereas, after that (≥ 14 years), soil organic S partition remained affected when conventional tillage and agricultural crops dominated this site. Indicating that the effect of recultivation on organic S decomposition depends on the manner of recultivation management. Our study contributes to an improved understanding of amino acid S and organic S mineralisation under severe anthropogenic disturbance.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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