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

The oxygen isotope signature of sulphate (δ ¹⁸ O _sulphate ) is increasingly used to study nutritional fluxes and sulphur transformation processes in a variety of natural environments. However, mechanisms controlling the δ ¹⁸ O _sulphate signature in soil-plant systems are largely unknown. The objective of this study was to determine key factors, which affect δ ¹⁸ O _sulphate values in soil and plants. The impact of an ¹⁸ O-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, δ ¹⁸ O _sulphate values of synthetic fertilizer were not affected by soil water. Correlations between soil and plant δ ¹⁸ O _sulphate values were controlled by water δ ¹⁸ O values and fertilizer treatments. Additionally, plant δ ³⁴ S 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.
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