Microbial decomposition and plant bioavailability of dissolved organic sulfur in soil

Sulfur (S) is essential for plant growth, however, potential sulfur deficiency is happening worldwide. Soluble organic sulfur (DOS) is an important form of soil sulfur, and can be decomposed quickly into sulphate by microorganisms for the absorption of plants. Clarifying the pathways and influence factors of DOS microbial decomposition process and plant uptake is crucial for the regulation of plant S nutrition. Therefore, we reviewed the researches on plant bioavailability and microbial decomposition of DOS, focusing mainly on proteins, the S-containing amino acids methionine (Met) and cysteine (Cys). Soluble proteins can be decomposed to SO₄²⁻ within days, while S-containing amino acids can be decomposed within minutes to hours. After absorbed into microbial biomass, the carbon, nitrogen, and sulfur in S-containing amino acids would be released outside the microorganisms in form of CO₂, NH₄⁺, and SO₄²⁻ within minutes to hours, the microorganisms will utilize the released inorganic sulfur and nitrogen to meet their growth requirements cyclically. Microorganisms assimilate DOS (Met and Cys) rapidly, and the original molecular structures of DOS dictate the resulting biopolymers upon microbial assimilation, thereby influencing the rate of conversion between DOS and SO₄²⁻. The thiol group (H-S-C) of cysteine is susceptible to oxidation, resulting in a higher release of SO₄²⁻ after utilized by microorganisms, compared to methionine. The uptake of methionine and cysteine by wheat and rapeseed increase with the enhanced supply of sulfur in the field. Studies have found that they can absorb 0.63%−2.2% of added cysteine and 0.4%−2.1% of methionine. Met and Cys have a limited role in plant nitrogen uptake, but they play very important roles in sulfur nutrition, accounting for about 10% of total plant sulfur uptake. The DOS can be decomposed by soil microorganisms rapidly, and the produced SO₄²⁻ is a superior sulfur source for plant absorption. Even in the face of intense competition with soil microorganisms, plants are capable of absorbing intact molecular organic sulfur (Met and Cys), especially in soils rich in organic matter.