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 (δ37 Cl, δ18 O, and Δ17 O), 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 (15 N/18 O) 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.