Fulvic acid increases iron bioavailability in aquaponic systems: Theoretical designs and practical considerations to prevent iron deficiency in plants

Abstract

Plants grown in aquaponics frequently develop iron deficiency, which is generally corrected via supplementation with iron chelates. However, the solid waste excreted by fish contains considerable amounts iron, which are lost in the system by precipitation of insoluble iron oxides. Thus, the present study proposed the use of fulvic acid (FA), a humic substance with metal complexing capacity, to prevent iron precipitation and enhance its bioavailability in the aquaculture wastewater. An aquaponic solution was added with levels of fulvic acid (0, 69, 138, 345, and 690 mg L−1), and analyzed for dissolved iron after incubation and centrifugation. Visual MINTEQ was used to characterize and quantify the effect of fulvic acid levels on iron precipitation, species of possible solid forms, and concentration of dissolved bioavailable iron forms. Dissolved iron concentration increased with the inclusion of fulvic acid in the nutrient solution, and the highest iron concentration (22.4 μM) was obtained by treating the solution with 690 mg L−1 of fulvic acid. Visual MINTEQ simulations demonstrated that, in the absence of fulvic acid, iron precipitated in the form of αFe2O3, and the concentration of available iron increased with the inclusion of FA due to its complexing properties. Solution pH had no effect on iron availability with the absence of fulvic acid; in its presence, the effect of pH was only significant at values below 4.5. This study shows that fulvic acid seems to play an important role in transporting low-soluble iron from fish tanks to plant roots in aquaponic systems.