Effects of cultivation and long-term superphosphate applications on pasture soil sulphur mineralisation and availability in the field

The mineralisation of soil organic sulphur (S) by soil micro-organisms constitutes a significant source of S for pasture plant growth. Cultivation and fertiliser applications affect microbial activity which in turn affects soil S mineralisation and the release of plant-available S. A field trial was conducted with cultivated and uncultivated subplots superimposed on the main plots of a long-term pasture trial in New Zealand receiving annual applications of superphosphate (0, 188, 376 kg ha⁻¹) since 1952. The aim was to provide information on soil S mineralisation and availability as affected by cultivation and superphosphate applications under field conditions. Carrier-free ³⁵SO₄-S was applied to all plots in the field, allowed to pre-condition for two weeks before perennial ryegrass (Lolium perenne L.) seeds were sown to cultivated sub-plots, while in uncultivated subplots existing pasture was cut and removed and then allowed to re-grow. Five plant harvests followed by soil sampling each time were conducted over a period of one year. Herbage and soil samples were analysed for total S and ³⁵S and different extractable soil S and ³⁵S fractions (Hi-reducible S, C-bonded S). Results obtained showed that cultivation and fertilisation significantly affected plant dry matter yield, soil S mineralisation and plant S uptake. These effects were affected by seasons. Plant S uptake was strongly related to soil S supply to plants and the plant S uptake provided a better measure of soil S availability to plants than changes in the extractable soil S fractions examined in the current study.