The oxygen isotopic signature of soil‐ and plant‐derived sulphate is controlled by fertilizer type and water source

The oxygen isotope signature of sulphate (δ18Osulphate) is increasingly used to study nutritional fluxes and sulphur transformation processes in a variety of natural environments. However, mechanisms controlling the δ18Osulphate signature in soil–plant systems are largely unknown. The objective of this study was to determine key factors, which affect δ18Osulphate values in soil and plants. The impact of an 18O-water isotopic gradient and different types of fertilizers was investigated in a soil incubation study and a radish (Raphanus sativus L.) greenhouse growth experiment. Water provided 31–64% of oxygen atoms in soil sulphate formed via mineralization of organic residues (green and chicken manures) while 49% of oxygen atoms were derived from water during oxidation of elemental sulphur. In contrast, δ18Osulphate values of synthetic fertilizer were not affected by soil water. Correlations between soil and plant δ18Osulphate values were controlled by water δ18O values and fertilizer treatments. Additionally, plant δ34S data showed that the sulphate isotopic composition of plants is a function of S assimilation. This study documents the potential of using compound-specific isotope ratio analysis for investigating and tracing fertilization strategies in agricultural and environmental studies.