Artificial humic substances as sustainable carriers for manganese: Development of a novel bio-based microfertilizer

Abstract

This study presents a new strategy for the production of sustainable manganese fertilizer based on artificial humic substances (AHS). AHS with different manganese concentrations (0‒20%) were synthesized from poplar bark under alkaline conditions via hydrothermal treatment. For the 20% manganese formulation, the interaction of manganese with AHS resulted in reduced solubility (from 25.2% to 12.3% organic carbon) and average molecular weight of humic acids (from 11.6 to 3.9 KDa), indicating preferential binding of the high-molecular-weight fraction. The formulation with 5% of manganese achieved optimal manganese loading without compromising the AHS solubility (19.4%). Structural analyses showed only minor changes in AHS in the presence of manganese, indicating that the main structural fragments of the AHS were preserved. Structural, morphological, and spectroscopic characterizations confirmed the formation of amorphous manganese complexes within the AHS matrix, primarily in the plant-available Mn(II) oxidation state. Plant bioassays showed increased manganese uptake with the application of AHS containing 5% Mn compared to MnCl2 alone (64 mg/kg vs. 40 mg/kg in dry cucumber biomass). Interestingly, unmodified AHS at higher concentrations (50 mg/L) further enhanced manganese (67 mg/kg) and iron (up to 209 mg/kg) uptake, highlighting the potential role of AHS in facilitating metal transport.