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.