Tracing S dynamics in agro-ecosystems using 34S

Abstract This review attempts to provide a comprehensive coverage of the past use of the stable isotope 34 S 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 34 S introduced at artificially enriched or depleted levels or as a tracer at 34 S 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 34 S 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 34 S-labelled fertilizers (S o and SO 4 2− ) have been reported, and there appears to be an anomaly between the relative recoveries of co-applied 15 N- and 34 S-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 34 S as a tracer in plant physiology and soil faunal metabolism were also reviewed. Future opportunities for the applications of 34 S to trace S dynamics in agro-ecosystems were identified.