Selenium Biofortification of Vegetables Grown in Calcareous Soil: A Pot Experiment Using Se 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) 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. 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) exceeded that of fertilizer-derived Se (Se). Shoot concentrations of Se ranged from 58.2 to 115 µg kg, while ⁷⁷Se concentrations varied between 10.5 and 46.9 µg kg. 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 CaCO 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.