Effect of elements availability on the decomposition and utilization of S-containing amino acids by microorganisms in soil and soil solutions

Sulfur-containing amino acids, methionine (Met) and cysteine (Cys), are important components of soil-soluble organic S and N. The effects of elemental availability and microbial density (soil or soil solution) on amino acid decomposition remain unclear.

We traced the decomposition and utilization of Met and Cys by microorganisms in soil and soil solutions using ¹⁴C, ¹⁵N, and ³⁵S labelling. Further, how microbial keep its stoichiometric homeostasis after uptake Cys and Met under various element availability was also explored.

Low concentrations of Met and Cys (50 µM) can be decomposed by microorganisms in both soil and soil solutions rapidly. In soil, Met and Cys undergo three steps: uptake into microorganisms (38.4% for Cys; 56.9% for Met at 2 min), the release of CO₂, NH₄⁺, and SO₄²⁻, and the re-utilization of inorganic ions. In the soil solution, the Met and Cys were also decomposed rapidly, while the produced inorganic ions were not utilized by microorganisms, which may be due to the limited amounts of microorganisms. The Cys and Met uptake process instantly occurred and was driven by microbial carbon demand in both soil and soil solution. The microbial N and S demand regulated the re-utilization process of released inorganic ions. Soil microbes balance their S, N, and C after uptake of Met and Cys, however, this balance was disturbed by high S addition, unlike additional C or N, which may be due to the lower demand of S.

Low-demand elements might regulate soil microbial stoichiometry balance, and a specific fertilization strategy that balanced the high- and low-demand elements can enhance nutrients use efficiency.