SULFUR MINERALIZATION IN 5 DANISH SOILS AS MEASURED BY PLANT UPTAKE IN A POT EXPERIMENT

Reductions in atmospheric S deposition have created S deficiency in so me agricultural soils. In such soils the release of S from organic mat ter may be important for the supply of S to plants. In this study net mineralisation of S was measured during two successive growth seasons as S removed by plant material and leachates. Pots were subjected to n atural conditions and cropped with either Italian rye grass (Lolium mu ltiflorum Lam.) both years or spring oilseed rape (Brassica napus L.) followed by spring barley (Hordeum vulgare L.). Sulphur was the limiti ng factor for dry matter yield in the pots. Sulphur concentrations in Italian rye grass, rape and barley varied from 0.055 to 0.21, 0.06 to 0.078 and 0.07 to 0.17%, respectively. Net mineralisation was 3.3-6.7 mu g S per g soil year(-1) (1.7 to 3.1% of total organic S), and could not prevent S deficiency in the crops; differences among soils develo ped early in the experiment. Net mineralisation of S was not significa ntly correlated with total organic S, C-bonded S or sulphate-ester S c ontent of the soils but with microbial activity in the soils (r=0.90). Net mineralisation was significantly affected by the type of cropping system but differences between systems were small compared to differe nces in net mineralisation among soils. It may be concluded that reduc tions in atmospheric deposition of S must, at least partly, be replace d by S fertilisation to maintain crop yield and quality.