Soil organic sulfur dynamics in a coniferous forest

Sulfate microbial immobilization and the mineralization of organic S were m easured in vitro in soil horizons (LFH, Ae, Bhf, Bf and C) of the Lake Lafl amme watershed (47 degrees 17' N, 71 degrees 14' O) using (SO4)-S-35. LFH s amples immobilized from 23 to 77% of the added (SO4)-S-35 within 2 to 11 da ys. The (SO4)-S-35 microbial immobilization increased with temperature and reached an asymptote after a few days. The mineral soil generally immobiliz ed less than 20% of the added (SO4)-S-35, and an asymptote was reached afte r 2 days. An isotopic equilibrium was rapidly reached in mineral horizons. A two-compartment (SO4 and organic S) model adequately described (SO4)-S-35 microbial immobilization kinetics. The active organic reservoir in the who le soil profile represented less than 1% of the total organic S. The averag e concentrations of dissolved organic S (DOS) in the soil solutions leaving the LFH, Bhf and Bf horizons were respectively 334, 282 and 143 mug.L(-)1. Assuming that the DOS decrease with soil depth corresponded to the quantit ies adsorbed in the B horizons, we estimated that 12 800 kg.ha(-1) of organ ic S could have been formed since the last glaciation, which is about 13 ti mes the size of the actual B horizons reservoirs. Our results suggest that the organic S reservoirs present in mineral forest soils are mostly formed by the DOS adsorption resulting from incomplete litter decomposition in the humus layer. The capability of these horizons to immobilize SO4 from the s oil solution would be restricted to a 1% active fraction composed of microo rganisms. Despite their refractory nature, these reservoirs can, however, b e slowly decomposed by microorganisms and contribute to the S-SO4 export fr om the watershed in the long term.