Fly Bioash Ameliorates Acid Luvisol and Increases Sunflower (Helianthus annuus L.) Yield in Field Conditions without Compromising the Risk of Radioactive Contamination

Fly bioash (FBA) as a by-product of biomass-fuelled facilities exhibits alkaline properties and is enriched with phytonutrients, thereby offering the potential to effectively ameliorate acidic and nutrient-deficient soils. However, concerns about health risks due to a potential FBA radioactive contamination are still not well studied, notably under field conditions. This study examined pH changes and concentrations of natural (U-238, Th-232, Ra-226, K-40) and anthropogenic (Cs-137) radionuclides after application of very alkaline (pH > 12) FBA in: (i) highly acid (pH(KCl) = 4.1) Luvisol and (ii) sunflower (Helianthus annuus L.) seeds, grown in organic farming and rain-fed conditions. FBA (originated from a modern cogeneration, fuelled on certified deciduous forest wood chips) was applied at increasing doses; 0, 4.5, 8.6, 13, and 17.2 t/ha. After 54 months of application, FBA significantly increased soil pH(KCl) by up to 1.8 unit and the seed yield by 15%, compared with no amended Control, without compromising soil electrical conductivity (salinity). The activity concentrations (A(c)) of all observed radionuclides, measured using high-resolution gamma-ray spectrometry, were not altered under FBA application, neither in the surface (0-30 cm) Luvisol horizon nor in the sunflower seed. Moreover, the A(c) of U-238, Th-232, and Cs-137 in the seed were below detection limit, whereas the A(c) of K-40 and Ra-226 were lower by up to 2.6 and 61 times, respectively, than their corresponding A(c) in the soil treatments. The radiological footprint of FBA exhibited lower A(c) for most of the observed radionuclides compared with both (i) Croatian non-arable topsoils (with reductions of U-238 3.6 times, Th-232 1.8 times, Ra-226 1.7 times, and Cs-137 1.5 times) and (ii) widely used mineral N/P/K fertilisers in conventional agroecosystems (with reductions of U-238 12.5 times; Ra-226 1.3 times, and K-40 2.4 times). Our findings provide evidence that the application of FBA as a soil conditioner does not pose radiological health or environmental risks, contributing to more sustainable agri-food production and circular bioeconomy. However, it is essential to conduct further studies to comprehensively investigate the effects of FBA application on soil and crop quality across diverse environmental conditions and extended spatiotemporal scales.