Stable isotopic composition of perchlorate and nitrate accumulated in plants: Hydroponic experiments and field data

Natural perchlorate (ClO₄ ⁻) in soil and groundwater exhibits a wide range in stable isotopic compositions (δ³⁷Cl, δ¹⁸O, and Δ¹⁷O), indicating that ClO₄ ⁻ may be formed through more than one pathway and/or undergoes post-depositional isotopic alteration. Plants are known to accumulate ClO₄ ⁻, but little is known about their ability to alter its isotopic composition. We examined the potential for plants to alter the isotopic composition of ClO₄ ⁻ in hydroponic and field experiments conducted with snap beans (Phaseolus vulgaris L.). In hydroponic studies, anion ratios indicated that ClO₄ ⁻ was transported from solutions into plants similarly to NO₃ ⁻ but preferentially to Cl⁻ (4-fold). The ClO₄ ⁻ isotopic compositions of initial ClO₄ ⁻ reagents, final growth solutions, and aqueous extracts from plant tissues were essentially indistinguishable, indicating no significant isotope effects during ClO₄ ⁻ uptake or accumulation. The ClO₄ ⁻ isotopic composition of field-grown snap beans was also consistent with that of ClO₄ ⁻ in varying proportions from irrigation water and precipitation. NO₃ ⁻ uptake had little or no effect on NO₃ ⁻ isotopic compositions in hydroponic solutions. However, a large fractionation effect with an apparent ε (¹⁵N/¹⁸O) ratio of 1.05 was observed between NO₃ ⁻ in hydroponic solutions and leaf extracts, consistent with partial NO₃ ⁻ reduction during assimilation within plant tissue. We also explored the feasibility of evaluating sources of ClO₄ ⁻ in commercial produce, as illustrated by spinach, for which the ClO₄ ⁻ isotopic composition was similar to that of indigenous natural ClO₄ ⁻. Our results indicate that some types of plants can accumulate and (presumably) release ClO₄ ⁻ to soil and groundwater without altering its isotopic characteristics. Concentrations and isotopic compositions of ClO₄ ⁻ and NO₃ ⁻ in plants may be useful for determining sources of fertilizers and sources of ClO₄ ⁻ in their growth environments and consequently in food supplies.