Background

Plant sulphur (S) deficiency occurs worldwide; however, in comparison to other macronutrients (e.g., N, P), limited attention has been paid to the content, composition, bioavailability, and cycling of S in soil. An increased knowledge of S biogeochemical cycling, however, can aid soil S management and plant S nutrition.

Scope

This review discusses current knowledge on the bioavailability and decomposition of soil-soluble organic S, focusing mainly on proteins and two S-containing amino acids (methionine (Met) and cysteine (Cys)).

Conclusions

Proteins represent the major S input into soil with most held within insoluble organic matter and a lesser proportion present as dissolved organic S (DOS). The size of the DOS pool is typically much lower than that of the inorganic SO₄²⁻ pool, however, this reflects the rapid turnover and replenishment of this pool, which is orders of magnitude faster than the inorganic S pool, reflecting the importance of soil organic S cycling. Soluble proteins can be decomposed to SO₄²⁻ within minutes, and S-containing amino acids can be mineralised within seconds to hours. Microorganisms utilise S-containing amino acids in three steps: uptake into the microbial biomass within seconds; release of CO₂, NH₄⁺, and SO₄²⁻ within minutes to hours; and the re-utilisation of released inorganic S and nitrogen (N) by microorganisms and plants. Current evidence suggests that Met and Cys play limited roles in plant N nutrition due to intense competition from soil microbes and the supply of inorganic N in fertilisers, however, these amino acids can account for ca. 10% of total plant S uptake (intact form and the inorganic S derived from them). We conclude that direct uptake of S-containing amino acids by microbes, and to a lesser extent plants, is an effective and energy efficient way to bypass the SO₄²⁻ pool and that the production and consumption of DOS cycling represents the key central cog in soil S cycling.