Hydroxyapatite nanorods, hydrochar, biochar, and zeolite for controlled-release urea fertilizers

This study aimed to prepare, characterize, and investigate nitrogen (N) release pattern of urea-impregnated biochar (UB), hydrochar (UH), zeolite (UZ) as carriers, and urea-hydroxyapatite nanohybrids (U-HAP) in water and a calcareous waterlogged soil. The results indicated that the urea had interactions with carriers and hydroxyapatite nanorods which controlled the release of urea. The UH, UB, UZ, and U-HAP reduced the urea release by 4.5, 6.5, 9.0, and 11.5 times, respectively compared to the urea only. The kinetics of urea release from these fertilizers showed the best fit of the Korsmeyer-Peppas model following the Fickian diffusion law. Another experiment was carried out using 14 treatments including control, rice straw, hydrochar, biochar, and zeolite with and without urea application and UH, UB, UZ, and U-HAP fertilizers. The extractable-NH4+ was measured using 2 M KCl solution in different times (0, 0.5, 1, 3, 6, 12, 20, 30, 45, 70, and 100 days) after application of prepared fertilizers. The results showed that > 85% of the applied urea was converted to NH4+ within 12 days in the urea-alone treatment. However, 20 days after the application of the UH, UB, UZ, and U-HAP in soil, 89.9, 76.5, 75.0, and 58.1 percent of urea was converted to NH4+, respectively. After the application of all controlled-release urea fertilizers, the extractable-NH4+ concentration increased initially but after 20 days decreased with time and the rate of reduction was lower in the UB, UZ, and U-HAP as compared to other treatments. Also, the kinetics of extractable-NH4+ showed the best fit to the Extreme model. The results showed that the UB, UZ, and U-HAP slowly released urea which may reduce N loss and increase its use efficiency in calcareous paddy soils.