Authenticity testing of organically grown vegetables by stable isotope ratio analysis of oxygen in plant-derived sulphate

Abstract

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

“…The available methods of sulphate quantification are, for example, ion chromatography (Blake‐Kalff, Harrison, Hawkesford, Zhao, & McGrath, 1998; Novak et al, 2019), turbidimetry (Eriksen, 2005), or gravimetry (Mayer et al, 1995). Here, gravimetry was chosen to streamline the sample preparation by measuring sulphate content while providing precipitate for direct isotope ratio analysis.…”

“…The sulphate extraction was conducted on soils from the fertilizer incubations, greenhouse soils (both pre-and post-harvest), and on radish leaves. A protocol for plant and fertilizer sulphate extraction for isotope ratio analysis (δ 18 O sulphate ) was used as described by Novak et al (2019).…”

“…Therefore, it represents readily available sulphate for plant uptake. The water-based soil sulphate extraction was modified from the plant protocol by Novak et al (2019).…”

“…Across the water isotopic gradient, the soil and plant δ 18 O sulphate values remained different between NPK and organic manure treatments (Figures 1 and 3). Therefore, the fertilizer type can be traced in plants and soils using oxygen isotope ratios of sulphate (Novak et al, 2019). The lack of significant O exchange between water and fertilizer sulphate ( Figure 1e) also highlights the possibility of using 18 O-labelled sulphate as a tracer for studies of sulphate absorption and circulation in plants.…”

“…For calibration and correction, in-house standards IA-R061 (barium sulphate, δ 34 S = 20.3‰), IA-R025 (barium sulphate, δ 34 S = 8.5‰), and IA-R026 (silver sulphide, δ 34 S = 3.4‰) were used. Manchester, UK) as described byNovak et al (2019).2.10 | Correction for initial sulphateTo determine the δ 18 O of the soil sulphate formed de novo via miner-18 O sulphate(measured) is the δ 18 O value of the sulphate that was extracted at the end of the incubation period, F is the fraction and δ 18 O sulphate(initial) is δ 18 O value of the initial water-soluble sulphate in the soil-fertilizer mixture at the beginning of the incubation period, (1 − F) reflects the fraction of sulphate derived from mineralization of organic matter, and δ 18 O sulphate(formed) is δ 18 O value of sulphate formed via mineralization. Individual treatments that had higher amount of initial sulphate than the amount of the extracted sulphate at the end of the incubation (F ≥ 1) were excluded from the correction as this indicated sulphate immobilization.…”