Transformations and plant uptake of urine-sulphate in urine-affected areas of pasture soil

The fate of sheep urine sulphate in the soil and its plant uptake was monitored using ³⁵S-labelled sulphate-S in undisturbed pasture microplots in two glasshouse experiments. The extent of macropore flow of simulated urine immediately following a sheep urination was also investigated at 5 pasture sites in the field. Immediately following urination to pasture microplots in the glasshouse, the amounts of urinederived ³⁵S recovered in the 0–2.5, 2.5–7.5, 7.5–15 and 15–30 cm soil layers were 38, 28, 18 and 9%, respectively. In the field study on 5 contrasting soils, a similar pattern was found with 55–70, 20–35 and 13–20% of simulated urine being recovered in the 0–5, 5–10 and 10–15 cm soil layers, respectively. There was insignificant loss below 15 cm. If urine had moved via simple displacement in these soils the wetting front would have reached only 2.0–2.5 cm in depth suggesting that significant downward movement of urine via macropore flow occurs after urination. In a 15-day period following urine application to a pasture soil there was a rapid rate of incorporation of ³⁵S into organic forms, while between 15 and 64 days the rate of incorporation declined. After 7 days, 27% of added ³⁵S had been incorporated into organic forms with 19% being C-bonded S and 8% Hl-reducible S. This rapid incorporation was attributed to the large and active microbial biomass present in the rhizosphere. Since urine application depressed pasture growth, due to ‘urine burn’, less than 10% of applied ³⁵S was absorbed by pasture plants over a 64-day period. A second experiment using microplots of contrasting soil types, confirmed that the majority of the ³⁵S incorporated into the organic form was present as C-bonded S. Results showed that of the ³⁵S remaining in the 0–2.5 cm layer 35 days after application, 20–40% was present as sulphate, 10–20% as Hl-reducible S and 50–60% as C-bonded S. Plant uptake of S accounted for only 7–12% of applied ³⁵S over the 35-day period.