Sulphur-containing amino acid metabolism in surface horizons of a hardwood forest

The O1, O2 and A1 horizons of a hardwood forest mineralized and incorporated into organic matter ¹⁵S-labelled cysteine and methionine. Based upon assays of samples collected seasonally during 1 yr (n = 10–30), mineralization was the dominant process for cysteine whereas with methionine the reverse was true, except in the A1 horizon. Analysis of samples for existing amounts of organic S revealed that carbon-bonded S was a major component throughout the sampling period in all horizons. This S pool was fractionated further into sulphonate S and amino acid S. The former component represented, on a mean annual basis, 59, 44 and 28% of total S in the O1, O2 and Al horizons, respectively. In contrast, amino acid S comprised 22, 24 and 15% of the total S in these respective horizons. With Al horizon samples, decreases in existing amounts of amino acid S coincided with increases in cysteine mineralization suggesting that cysteine is an important component of this S pool in the Al horizon.