Plant sulphur (S) deficiency is a worldwide concern. However, the mechanisms controlling the immobilization and mineralization of low-molecular weight organic S by soil microorganisms remain unclear. Therefore, we investigated the assimilation of carbon (C) and S by soil microorganisms using uniformly ¹⁴C- or ³⁵S-labelled cysteine and methionine. The decomposition of cysteine and methionine in the soil occurred in three steps. First, the microbial biomass (MB) rapidly immobilised the added cysteine-S (55%–63%) and methionine-S (81%–84%) in less than 30 min. Subsequently, S in the MB was released as ³⁵S-sulphate (release of S into the soil peaked at 1 h [21.4%] and 24 h [17.3%] after adding cysteine and methionine, respectively). Lastly, the released ³⁵SO₄²⁻ was reutilised by microorganisms. The amount of ¹⁴CO₂ and ³⁵SO₄²⁻ released from methionine was much lower than that from cysteine. The addition of excess glucose-C or inorganic nitrogen and S had little effect on cysteine and methionine uptake rate, but had a major effect on microbial C use efficiency (CUE) and internal S partitioning and the subsequent release of SO₄²⁻. We conclude that the microbial community cycles S-containing amino acids at a high rate, irrespective of soil S and N status with a large proportion of the C used in catabolic processes.

植物硫（S）的缺乏是全世界关注的问题。然而，控制土壤微生物对低分子量有机硫的固定化和矿化的机制仍不清楚。因此，我们研究了土壤微生物对碳（C）和S的同化作用，使用¹⁴ C或³⁵ S标记的半胱氨酸和蛋氨酸统一。土壤中半胱氨酸和蛋氨酸的分解过程分为三个步骤。首先，微生物生物量（MB）在不到30分钟的时间内快速固定了添加的半胱氨酸-S（55％–63％）和蛋氨酸-S（81％–84％）。随后，MB中的S释放为³⁵ S硫酸盐（分别添加半胱氨酸和蛋氨酸后，S在土壤中的释放分别在1 h [21.4％]和24 h [17.3％]时达到峰值）。最后，发布³⁵ SO ₄ ^2-被微生物再利用。从蛋氨酸释放的¹⁴ CO ₂和³⁵ SO ₄ ^2-的量远低于半胱氨酸。添加过量的葡萄糖-C或无机氮和硫对半胱氨酸和蛋氨酸的吸收率几乎没有影响，但对微生物的C利用效率（CUE）和内部的S分配以及随后的SO ₄ ^2-释放具有重大影响。我们得出的结论是，微生物群落以很高的速率循环含S的氨基酸，而与土壤S和N的状态无关，而分解代谢过程中所用的C的比例很大。