Amino-acid metabolism in forest soil-Isolation and turnover of organic matter covalently labelled with ³⁵s-methionine

Organic matter was extracted with pyrophosphate-NaOH buffer, pH 8.0 from the 02 horizon of a hardwood forest and exposed to ¹⁵S-methionine for 18 h to allow the incorporation of the aminoacid into the extract. Compared with other amendments, supplementation of the incorporation medium with Na₂SO₄ yielded the highest recoveries of the labelled organic matter. This material was chemically recalcitrant, requiring treatment for 6 h at 121°C with 6 m NaOH for complete release of the incorporated methionine. A and B horizon soils from several forests were exposed to labelled material following dialysis to remove unlabelled components and a minor ³⁵S-labeIled component. All samples began to release ³⁵S-methionine after 48 h. Further metabolism of this amino-acid included mineralization and conversion to cysteine. A portion of the sulfate-S generated from mineralization was incorporated into organic matter and was recovered only by acid and alkali extraction. ³⁵S-labelled methionine and cysteine were also found in these latter fractions, indicating that these amino-acids had been directly incorporated into organic material during exposure.