Long-term effects of land use and fertiliser treatments on sulphur transformations in soils from the Broadbalk experiment

Sulphur transformations were monitored in a unique set of arable, grassland and woodland soils from the Broadbalk Classical Experiment, which started in 1843. In an open incubation experiment with periodic leaching, 14–35 mg SO42−-S kg−1 was mineralised in 28 weeks at 25°C, equivalent to 4.4–8.3% soil organic S. Cumulative amounts of S mineralised increased linearly during the 28 weeks, indicating constant rates of mineralisation. The rate of mineralisation was the greatest in the woodland soil (170 μg SO4-S kg−1 day−1), followed by the grassland (120 μg SO4-S kg−1 day−1) and the arable soil from the farmyard manure (FYM) plot (110 μg SO4-S kg−1 day−1). Three soils from arable plots receiving different inorganic fertiliser treatments but no FYM had similar rates of S mineralisation (~70 μg SO4-S kg−1 day−1). In an incubation experiment with 35SO42−, addition of glucose greatly enhanced S immobilisation. In 132 days, the woodland and grassland soils immobilised more S than the arable soils, with or without glucose amendment. Immobilisation and mineralisation of S occurred concurrently, and both were stimulated by glucose addition. The results show that S mineralisation and immobilisation were influenced strongly by the type of land-use and long-term organic manuring, whereas annual application of sulphate-containing fertilisers for over 150 years had few effects on short-term S transformations.