Zinc- and Magnesium-Doped Hydroxyapatite Nanoparticles Modified with Urea as Smart Nitrogen Fertilizers

Excess urea fertilizer application to the agricultural fields causes severe environmental deterioration. Researchers actively seek safer alternatives, such as nanoparticles with unique properties, to reduce chemical inputs without compromising agricultural output. In the present study, three variants─hydroxyapatite–urea, magnesium-doped hydroxyapatite–urea, and zinc-doped hydroxyapatite–urea nanohybrids─have been synthesized in a two-step method and characterized as slow-release nitrogen fertilizers. Doping with Zn and Mg reduces the hydroxyapatite nanoparticles’ size and accommodates a higher amount of urea molecules. As per the Hixson–Crowell model equation, the urea molecules were slowly released from the nanohybrids for up to two weeks in the soil environment. With zinc and magnesium integrated into hydroxyapatite, the synthesized nanohybrids serve as a multinutrient complex of nitrogen, calcium, phosphorus, magnesium, and zinc nutrients. We found that nanohybrids containing 50% nitrogen doses maintained wheat crop yield and nitrogen nutrient uptake equivalent to a urea fertilizer containing 100% nitrogen doses, which helped mitigate ammonia emissions from the agricultural fields. The nanohybrid-supplemented soil enhanced the soil dehydrogenase and urease enzyme levels, suggesting no adverse impact of nanohybrids on soil health. We present comprehensive experimental evidence for the synthesis and application of the nitrogen nanohybrids for agricultural production and for cutting off nitrogen input by up to half to mitigate environmental repercussions. This study unlocks paradigms for designing and applying climate-friendly smart fertilizers in sustainable agriculture.