FORMS OF SULFUR IN SHEEP EXCRETA AND THEIR FATE AFTER APPLICATION ON TO PASTURE SOIL

Sheep were fed S-35-labelled grass/clover herbage and the excreta was collected. About 74% of excretal S was in the form of urine and the re mainder was in dung. Sulphate-S accounted for 74% of the urine S and t he remainder was in HI-reducible form. In dung, 80% of the S was C-bon ded organic S and 20% was present as sulphate-S. S-35-labelled dung wa s applied to undisturbed pasture microplots and the degradation and S release followed over a 9 month period under glasshouse conditions. Du ring the first 34 days about 24% of S-35 was leached from the dung, ma inly as sulphate-S and possibly some labile organic S. The C-bonded S in the dung was notably resistant to mineralisation. Only 14% of the a pplied S-35 was recovered by pasture plants. S-35-labelled urine with either a high (1130 mg S litre-1) or a low S concentration (280 mg S l itre-1) was applied in the field and its fate followed over a 10 month period. During the experiment 57 and 79% of the applied S-35 was reco vered in pasture herbage in the high and low urine treatments, respect ively. S-35 was also incorporated into soil organic matter and this re ached a peak about 120 days after urine application when 52 and 30% of applied S had been incorporated in the high and low urine S treatment s, respectively. With time, these levels declined as about 50% of the organic S-35 was mineralised and subsequently absorbed by the pasture plants. The results demonstrate that excreted S is recycled rapidly in pasture soils when it originates from urine. However, the bulk of S i n dung appears to be relatively inert at least over a 9 month period.