Sulphur cycling in Danish agricultural soils: Inorganic sulphate dynamics and plant uptake

Predicting the S requirements of plants has become increasingly important in European agriculture primarily because of decreasing atmospheric S emissions. I have examined inorganic sulphate fluctuations during the year and its relation to plant uptake and S requirements. In a field study, three soil types (4, 7 and 10% clay) were mixed with carrier-free 35SO4S and combinations of sulphate-S and cellulose in PVC cylinders cropped with perennial rye-grass. Fertilizer S addition increased sulphate concentrations, but in a few months, they decreased again because of plant uptake, leaching and incorporation into organic matter. In the absence of cellulose, total dry matter production in rye-grass roots and tops increased up to twofold by S application, and a significant correlation was observed between soil inorganic sulphate at the start of the experiment and plant S uptake (r = 0.72). The presence of a readily available C source inhibited the uptake of both S and 35S. C addition equivalent to 10 t straw ha−1 caused immobilization and rapidly decreasing amounts of soil inorganic sulphate. Thus, plant yield, S-uptake and leaching losses were lower than in soils without C addition. During the season, the contribution of fertilizer-S to plants decreased from 46 to 26% in material sampled 84–182 d after sowing, irrespective of soil type and C addition. The decrease in the proportion of S in plants originating from fertilizer was caused by plant uptake of unlabelled soil S.