Selenium Biofortification of Vegetables Grown in Calcareous Soil: A Pot Experiment Using ⁷⁷Se as a Tracer

Dietary selenium (Se) is vital for human health and can be provided through consumption of Se-rich vegetables. Soil Se is often poorly available and so biofortification using Se-enriched fertilizers is used to enhance dietary intake. This study aimed to (a) evaluate the feasibility of biofortifying vegetables, commonly grown in the calcareous soils of Kurdistan, with a single application of Se (10 g ha^-1) as selenate and, (b) trace the fate of applied Se using an enriched stable isotope, ⁷⁷Se. A randomized block pot experiment was conducted with five vegetable species: celery, chard, lettuce, radish, and spring onion. Soils were spiked with 2 µg ⁷⁷Se per pot, simulating 10 g ha^-1. Plants were harvested after 8 weeks, and both plant tissues and soils underwent sequential extraction and isotopic analysis to determine Se fractionation and source apportionment. Across all species, plant uptake of native soil Se (Se_s) exceeded that of fertilizer-derived Se (Se_f). Shoot concentrations of Se_s ranged from 58.2 to 115 µg kg^-1, while ⁷⁷Se_f concentrations varied between 10.5 and 46.9 µg kg^-1. Post-harvest soil analyses indicated immobilization of applied ⁷⁷Se: 55% transitioned to organically bound forms, 40% became recalcitrant, and only 5% remained in plant-available fractions. The study underscores the challenges of Se biofortification in calcareous soils, where interaction with CaCO₃ may reduce Se availability. Variations in Se uptake among vegetable species highlight the importance of application timing. To enhance biofortification efficacy for fast-growing leafy vegetables, mid-season or foliar Se applications are recommended to counteract rapid soil immobilization.