Review PaperTracing S dynamics in agro-ecosystems using 34S

- 34S has played a minor role as a tracer in agro-ecology compared to 15N or 13C.
- 34S at enriched, depleted or natural abundance levels can be used as tracers.
- Natural variations in 34S abundance of soils, plants and fertilizers are reviewed.
- Estimates of the relative efficiencies of 34S- and 15N-fertilizers are required.
- S deficiencies in agriculture will increase as will the need for 34S tracer studies.

This review attempts to provide a comprehensive coverage of the past use of the stable isotope 34S as a tracer of S dynamics in agro-ecosystems, and to identify where deficiencies exist in our current knowledge of S cycling in agriculture. Applications include the use of 34S introduced at artificially enriched or depleted levels or as a tracer at 34S natural abundance. Isotopic fractionation occurs during the majority of S transformations in the soil-plant system, but they are generally of lesser magnitude compared with fractionation accompanying several N transformations. Significant variation occurs in the natural abundance of 34S in soils, crops, mineral and organic fertilizers, but apart from long-term agronomic studies such variation has been successfully used only as a qualitative rather than a quantitative tracer. Relatively few studies of the efficiencies of 34S-labelled fertilizers (So and SO42−) have been reported, and there appears to be an anomaly between the relative recoveries of co-applied 15N- and 34S-enriched fertilizers by crops, as well as leaching losses, which warrant further investigation. We also examined long-term agronomic experiments in Europe where archived soil and plant samples allowed for the relative contributions of soil and atmospheric sources of S to crops and pastures to be evaluated. Studies involving the use of 34S as a tracer in plant physiology and soil faunal metabolism were also reviewed. Future opportunities for the applications of 34S to trace S dynamics in agro-ecosystems were identified.