Carbon and sulphur tracing from soil organic sulphur in plants and soil microorganisms

In soils, cysteine and methionine represent the predominant constituents of the low molecular weight organic S, but their role in plant nutrition and soil S cycling is unclear. Cysteine and methionine uptake by pot-cultivated wheat and oilseed rape and their cycling were evaluated using ¹⁴C and ³⁵S labelling. About 0.16%–0.30% of ¹⁴C from cysteine and methionine was absorbed by the plants after 6 h, indicating that the plants could utilise organic S, with oilseed rape showing higher ability than wheat to take up intact organic S and its derivative inorganic S. Plants utilised organic S with much lower efficiency than inorganic S because most organic S was decomposed by microorganisms. Nitrogen (N) addition enhanced plant S uptake, as high N uptake stimulates S immobilization. About 28%–33% and 67–71% of ¹⁴C from cysteine and methionine, respectively, were retained in the microbial biomass (MB), a much higher proportion of ¹⁴C from cysteine was released as ¹⁴CO₂ from the soil than from methionine. Further, 16%–25% and 61%–72% of ³⁵S from cysteine and methionine, respectively, were retained in the MB, and 35%–42% and 5%–9% were released from it as SO₄²⁻. Microbial carbon (C) and S use efficiency for cysteine was lower than for methionine, whereas plants utilised higher amounts of cysteine-derived S as sulphate. The higher microbial utilisation rate of methionine by soil microorganisms, compared with cysteine, may reduce the bioavailability of this compound in the soil solution, and, consequently, its uptake by plants.