Tracing the mineralization rates of C, N and S from cysteine and methionine in a grassland soil: A 14C and 35S dual-labelling study.

Sulphur-containing amino acids (i.e. Cysteine (Cys) and methionine (Met)) constitute an important proportion of the soil organic sulphur. However, detailed information regarding the microbial transformation of Cys and Met at a molecular level remain poorly characterized. To trace the fate of carbon (C) and sulphur (S) derived from Cys and Met in an agricultural grassland soil, a14C and 35S dual-isotopic labelling approach was adopted. We also investigated whether their mineralization was affected by manipulating C (added as glucose), nitrogen (N), phosphorus (P) and S (added as NH 4 NO 3 , KH 2 PO 4 and K 2 SO 4) availability in soil solution. Our results showed that over a 7-day incubation period, 67.2–89.2% of the 14C derived from Cys and Met was respired as 14CO 2 , 2.7–19.5% had been immobilized in the soil microbial biomass; while the recovery of 35S in soil solution ranged from 6.4 to 9.9%, with the reminder retained in the soil microbial biomass. Overall, our results indicated that soil microbial communities possess a high capacity to utilize Cys and Met. Furthermore, using the 14C and 35S dual-labelling technique, we found that C and S derived from Cys and Met were microbially mineralized and immobilized at different rates, indicating that the cycles of these two elements were temporally decoupled at the molecular level. The addition of glucose-C increased 14CO 2 respiration from Cys and Met after 7 d, while in comparison inorganic N, P and S addition had less effect on 14C and 35S partitioning. Schematic presentation of carbon, nitrogen and sulphur partitioning from two S-containing amino acids in grassland soils after 7-d incubation. a: labelled amino acid uptake into soil microbial biomass; b: subsequent microbial mineralization. (35S FE : (35S in the 0.01 M CaCl 2 extract of fumigated soils - 35S uf) × 100/0.35/35S O (%)). [Display omitted] • 67.2–89.2% of 14C derived from Cys and Met were respired as 14CO 2 in 168 h. • 92.2–93.6% of 35S derived from Cys and Met retained in soil microbial biomass. • N from Cys and Met was firstly released as NH 4 +, then rapidly converted to NO 3 −. • Glucose addition led to significant increase in 14CO 2 respiration from Cys and Met. [ABSTRACT FROM AUTHOR]