FATE OF S-35 LABELED URINE SULFATE IN URINE AFFECTED AREAS OF PASTURESOIL UNDER FIELD CONDITIONS

The fate of S-35-labelled sheep urine sulphate in the soil and its pla nt uptake were measured in field experiments over a 12 month period in 1990/91 in Canterbury, New Zealand. Urine was applied in either summe r or winter. After the summer application, there was a marked increase in pasture growth and S uptake which lasted for a period of c. 90 day s. Plant uptake of applied S-35 was very rapid and c. 77 % was recover ed in the pasture herbage within 65 days of application. By the end of the experiment, c. 80 % had been recovered in the herbage. A portion of applied S-35 was incorporated into soil organic forms mainly as C-b onded S. After 86 days, 30 % of the applied S-35 was in the C-bonded f orm but during the rest of the experiment this declined to 17 % as the S-35 was remineralized and absorbed by the growing pasture plants. Fo llowing the winter urine application, the increase in pasture dry matt er production and S-32 uptake was much less marked than that for the s ummer application. Twenty five percent of the applied S-35 was recover ed in pasture herbage within 65 days and this increased to 70 % by the end of the experiment. Some of the applied S was incorporated into so il organic forms and, by 83 days, 20 % of the applied S-35 was present in C-bonded form. Through mineralization this declined to 7 % by the end of the experiment. Over the initial 41 days of the experiment, c. 25 % of the ''S was lost due to leaching.