## 3 Results and Discussion
### Results of Statistical Analysis  
Statistical data analysis provides a detailed understanding of the significant effects of hydroxyapatite nanoparticles (HANP) and added Cd concentrations on soil properties, metal immobilization, and quinoa response. The soil characterization results indicated that the sandy-loam-textured soil had a low organic matter content and moderately acidic pH, which significantly influenced the mobility of Cd in the soil. The low ECEC, where exchangeable Al\({}^{3+}\) predominates, limits the availability of sorption sites for Cd retention.  
Sequential chemical extraction showed that Cd distribution among geochemical fractions varied significantly between treatments. Soils treated with Ca but not with HANPs showed a high proportion of Cd in the most bioavailable fractions (F1-F3), ranging from 36.6% to 60.4% of total Cd, depending on the Cd concentration applied. In contrast, soils treated with HANPs showed a redistribution of Cd towards less bioavailable fractions, particularly the irreversibly adsorbed fraction (F6). At 50 mg kg\({}^{-1}\) Cd, the F6 fraction increased significantly from 5.86 mg kg\({}^{-1}\) in untreated soils to 8.96 mg kg\({}^{-1}\) in soils treated with HANPs. This change shows the ability of HANPs to promote the formation of stable Cd-phosphate complexes, reducing metal mobility and environmental risks of contamination.  
In phytotoxicity tests, statistical comparisons revealed significant differences in germination percentage (G%), root and shoot length, and GIndex and APIndex between treatments. In untreated soils, Cd concentrations above 10 mg kg\({}^{-1}\) significantly reduced shoot length and ApIndex values compared to the control. For example, ApIndex decreased from 100% in control soils to 70% at 50 mg kg\({}^{-1}\) Cd. However, the application of HANPs mitigated these effects at moderate contamination levels. At 10 mg kg\({}^{-1}\) Cd, ApIndex increased significantly from 84% in untreated soils to 119% in HANP-treated soils. Root growth was particularly sensitive to Cd toxicity, with GIndex values below 90% at 10 mg kg\({}^{-1}\) Cd or higher. HANPs partially improved root growth inhibition, as evidenced by GIndex values, which remained significantly lower than those of the control.  
Pot assays confirmed the mitigating effects of HANPs on Cd toxicity under controlled conditions. Germination rates in pots decreased significantly with increasing Cd concentrations, particularly above 10 mg kg\({}^{-1}\). Fresh and dry shoot and root biomass also showed significant reductions at higher Cd levels, regardless of HANP application. However, HANP-treated soils showed a statistically significant improvement in plant height and Cd content in shoots compared to untreated soils at equivalent Cd concentrations. For example, at 25 mg kg\({}^{-1}\) Cd, HANP reduced Cd accumulation in shoots from 153.8 mg kg\({}^{-1}\) in untreated soils to 56.0 mg kg\({}^{-1}\) in treated soils, highlighting its potential to limit Cd translocation to aboveground plant tissues.  
The statistical analysis indicates the efficacy of HANPs in mitigating Cd toxicity through its immobilization and redistribution to less bioavailable soil fractions. Significant differences between treatments demonstrate that nanoparticles effectively reduce Cd bioavailability, improve plant growth parameters, and limit Cd translocation under moderate levels of contamination. However, their lower effectiveness under high levels of Cd concentrations suggests the need for complementary strategies, particularly for soils with elevated Cd concentrations.