BOOK OF ABSTRACTS 133 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Thursday, 27 August 2026 / Hall C Soil Organic Matter, Caustobiolites, and Biochar SL64 Amorphous Humic Acid-MOF Composites as Controlled-Release Fertilizers: EPR-Assisted Structural Characterization and Nutrient Release Francisco Javier Górriz1,2, Javier Erro1,2, Georgios Theofilou3, María Garnica1,2, Yiannis Deligiannakis3, José María García-Mina1,2 1 Universidad de Navarra, Instituto de Biodiversidad y Medioambiente BIOMA, Irunlarrea 1, 31008 Pamplona, España. fgorriz@unav.es / jgmina@unav.es 2 Universidad de Navarra, Facultad de Ciencias, Departamento de Biología Ambiental, Grupo Biología y Química Agrícola, Irunlarrea 1, 31008 Pamplona, España 3 University of Ioannina, Department of Physics, Laboratory of Physical Chemistry of Materials & Environment, Panepistimioupoli, 45110 Ioannina, Greece ideligia@uoi.gr Phosphorus (P) fertilizers are commonly produced from rock phosphate, a finite and non-renewable resource. Their high-water solubility often results in low nutrient-use efficiency and may contribute to soil salinity and nutrient fixation [1]. To overcome these limitations, humic substances [2,3], controlled release formulations [4], and metal-organic frameworks (MOFs) [5,6] have emerged as promising alternatives. However, conventional crystalline MOFs usually exhibit limited pore size which can restrict macronutrient release [5]. In the present study, a series of humic acid- MOF (HA-MOF) composites were synthesized using fertilizergrade raw materials and different concentrations of Leonardite-derived humic acid to improve phosphorus availability and release behavior. Nutrient composition in HA-MOF was determined by ICP-OES and elemental analysis. Phosphorus was partially soluble in water and fully soluble in neutral ammonium citrate, indicating plant availability while reducing it susceptibility to fixation by soil cations. Humic acid content was determined by thermogravimetric analysis (TGA). X-ray diffraction (XRD) revealed the predominantly amorphous nature of the composites, while SEM-EDX confirmed the homogenous surface distribution of the elements. Fe-release kinetics as function of pH evaluated by Electron Paramagnetic Resonance (EPR), showing that iron was present as Fe (III). High-spin Fe3+(S=5/2) was associated with the bioavailable fraction, whereas low-spin Fe3+(S=1/2) was related to P- bound Fe and to the fraction protected from soil fixation. Increasing humic acid concentration enhanced the amorphous character of the HA-MOF materials and reduced their chemical stability, promoting a balance between nutrient bioavailability and protection. These findings support the potential of HAMOF composites as controlled-release phosphorus fertilizers.
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