Composites of Biopolymers and ZnO NPs for Controlled Release of Zinc in Agricultural Soils and Timed Delivery for Maize

Zinc (Zn) micronutrient deficiency is a widespread problem affecting crops worldwide. Unlike conventional ionic fertilizers (Zn as salt or chelated forms), Zn-based engineered nanomaterials (ENMs) have the potential to release Zn in a controlled manner, reducing Zn losses through leaching upon application to soil. In this work, composites made of biopolymers (microcrystalline cellulose, chitosan, and alginate) and ZnO nanoparticles [NPs; 4–65% (w/w) Zn] were prepared. Their potential for Zn controlled release was tested in four agricultural soils of distinct pH and organic matter content over 30 days. While conventionally used Zn salts leached from the soil resulting in very low CaCl2-extractable Zn concentration, Zn in ZnO NPs was less labile, and ZnO biopolymers maintained a better constant supply of CaCl2-extractable Zn than all other treatments. ZnO NPs/alginate beads prepared by cross-linking with CaCl2 presented the slowest Zn release kinetics. As assessed with maize plants grown in poor Zn acidic soil (LUFA 2.1, pH 5.2), this constant Zn release from ZnO NPs/alginate beads resulted in a steadier Zn concentration in soil pore water over time. Results further indicated that ZnO NPs/alginate beads could meet the maize Zn needs while avoiding the early stage Zn toxicity induced by conventional Zn supplies, demonstrating that these ENMs are a sustainable way to supply Zn in a controlled manner in acidic soils. The impact of plant exudates on Zn bioavailability in the soil under maize-root influence (rhizosphere) is also discussed, underlying the need to study the fate of micronutrients in the rhizosphere to better predict its long-term bioavailability in bulk soils.