Zinc- and magnesium-doped hydroxyapatite-urea nanohybrids enhance wheat growth and nitrogen uptake

Abstract

The ongoing and unrestrained application of nitrogen fertilizer to agricultural lands has been directly linked to climate change and reductions in biodiversity. The agricultural sector needs a technological upgrade to adopt sustainable methods for maintaining high yield. We report synthesis of zinc and magnesium doped and undoped hydroxyapatite nanoparticles, and their urea nanohybrids, to sustainably deliver nitrogen to wheat. The urea nanohybrids loaded with up to 42% nitrogen were used as a new source of nitrogen and compared with a conventional urea-based fertilizer for efficient and sufficient nitrogen delivery to pot-grown wheat. Doping with zinc and magnesium manipulated the hydroxyapatite crystallinity for smaller size and higher nitrogen loading capacity. Interestingly, 50% and 25% doses of urea nanohybrids significantly boosted the wheat growth and yield compared with 100% doses of urea fertilizer. In addition, the nutritional elements uptake and grain protein and phospholipid levels were significantly enhanced in wheat treated with nanohybrids. These results demonstrate the potential of the multi-nutrient complexes, the zinc and magnesium doped and undoped hydroxyapatite-urea nanoparticles, as nitrogen delivery agents that reduce nitrogen inputs by at least 50% while maintaining wheat plant growth and nitrogen uptake to the same level as full-dose urea treatments.

Figure 1

Powder X-ray diffraction (PXRD) analysis of the synthesized nanohybrids (HAU, MgHAU, and ZnHAU), bare nanoparticles (HAP, MgHAP, and ZnHAP), and urea.

Figure 2

Fourier Transform Infrared (FTIR) spectrum of the synthesized nanohybrids. (A) HAP, HAU, and urea) (B) MgHAP, MgHAU, and urea (C) ZnHAP, ZnHAU, and urea. The FTIR spectrums show urea (C–N, N–H, C = O) and hydroxyapatite (PO₄³⁻, P–O, O–P–O) distinct functional groups.

Figure 3

Scanning electron microscopy (SEM) images of the synthesized nanoparticles. (A) Hydroxyapatite (HAP), (B) Magnesium doped hydroxyapatite (MgHAP), (C) Zinc doped hydroxyapatite (ZnHAP), (D) Hydroxyapatite-urea (HAU), (E) Magnesium doped hydroxyapatite-urea (MgHAU), and (F) Zinc doped hydroxyapatite-urea (ZnHAU).

Figure 4

Thermogravimetric analysis of urea release from (A) HAU, (B) MgHAU, and (C) ZnHAU nanohybrids in a water environment (up to 100 min).

Figure 5

Wheat plants (at maturity) treated with nitrogen fertilizers, left to right: Control (Urea Fertilizer-100% RD_NPK), HAU (Hydroxyapatite-Urea 50% Suspension-50% RD_N + 100% RD_PK), MgHAU (Mg doped-Hydroxyapatite-Urea 50% Suspension-50% RD_N + 100% RD_PK), and ZnHAU (Zn doped-Hydroxyapatite-Urea 50% Suspension-50% RD_N + 100% RD_PK) in a pot experiment setup.

Figure 6

Wheat grain quality parameters evaluated after harvesting. (A) Protein, (B) Phospholipids, and (C) Proline were evaluated for the wheat grains after harvesting in the wheat crop treated with a quarter and half dose of nitrogen as nanohybrids (suspension and pellet), full dose of nitrogen as urea (as control), and without fertilizer (no fertilizer). All the nanohybrid treatments (HAU: Hydroxyapatite-urea, MgHAU: Mg-doped hydroxyapatite-urea, and ZnHAU: Zn-doped hydroxyapatite-urea) were compared with the control or 100% RD_NPK treatment. The values are provided as mean ± standard deviation, and statistical significance was calculated by one-way ANOVA with Dunnett’s multiple comparison test. The letters ‘*’, ‘**’, ‘***’, and ‘****’ represent ‘p < 0.05’, ‘p < 0.01’, ‘p < 0.001’, and ‘p < 0.0001’, respectively and ‘ns’ represents ‘not significant’.

Figure 7

Wheat leaf nitrogen, and soil available nitrogen estimation. (A) Leaf nitrogen after 40 days of germination, and (B) Soil available nitrogen after the wheat crop cycle were evaluated in the wheat crop and soil treated with a quarter and half dose of nitrogen as nanohybrids (suspension and pellet), full dose of nitrogen as urea (as control), and without fertilizer (No Fertilizer). All the nanohybrid treatments (HAU: Hydroxyapatite-urea, MgHAU: Mg-doped hydroxyapatite-urea, and ZnHAU: Zn-doped hydroxyapatite-urea) were compared with the control or 100% RD_NPK treatment. The values are provided as mean ± standard deviation, and statistical significance was calculated by one-way ANOVA with Dunnett’s multiple comparison test. The letters ‘*’, ‘**’, ‘***’, and ‘****’ represent ‘p < 0.05’, ‘p < 0.01’, ‘p < 0.001’, and ‘p < 0.0001’, respectively and ‘ns’ represents ‘not significant’.