## 4 Conclusions  
Phytotoxicity tests demonstrated that HANPs do not enhance quinoa seed germination or early root growth and, in some cases, exhibit an inhibitory effect. However, at Cd concentrations of 10 mg kg\({}^{-1}\) or higher, HANPs significantly contributed to early shoot development, improving the Ap\({}_{\text{Index}}\). Sequential chemical extraction revealed that HANPs do not significantly affect the most bioavailable soil fractions (F1-F3). However, the Cd content strongly complexed with organic matter (F5) decreased in soils treated with HANPs, while irreversibly adsorbed Cd (F6) increased, suggesting the formation of stable, insoluble Cd-phosphate complexes. Germination rates in pot experiments differed from those in phytotoxicity tests, likely due to the direct interaction of seeds with the soil in the former. No significant differences in the physiological characteristics of quinoa plants were observed between HANP-treated and untreated soils. However, Cd content in the aerial parts of quinoa plants significantly decreased in HANP-treated soils, indicating reduced Cd translocation, likely due to its immobilization associated with HANPs. The results confirm the potential of HANPs to reduce Cd bioavailability in contaminated soils and limit its accumulation in quinoa tissues. However, their limited efficacy in reducing highly bioavailable Cd fractions at elevated contamination levels highlights the need to combine their use with complementary remediation strategies, such as organic amendments. This study establishes a basis for understanding the potential of HANPs in agricultural remediation. Future investigations may focus on field assays to assess the long-term impact of HANPs on quinoa development, including their effects on seed safety and quality for human consumption, as well as the assessment of their combined use with other remediation strategies to optimize effectiveness under varying Cd contamination levels. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15020639/s1](https://www.mdpi.com/article/10.3390/app15020639/s1)]([https://www.mdpi.com/article/10.3390/app15020639/s1](https://www.mdpi.com/article/10.3390/app15020639/s1)), Figure S1: Location and sampling area of agricultural soil; Figure S2: Phytotoxicity test procedure (Microbiotests Toxkit) for quinoa seeds. Conceptualization, V.S.-M., P.P.-R. and D.A.-L.; methodology, P.P.-R. and D.A.-L.; validation, P.P.-R. and D.A.-L.; formal analysis, R.G.-F. and C.M.-C.; investigation, V.S.-M., P.P. [PERSON] and [PERSON]; data curation, [PERSON] and [PERSON]; writing--original draft preparation, [PERSON] and [PERSON]; writing--review and editing, [PERSON] and [PERSON]; visualization, [PERSON]; supervision, [PERSON]; project administration, [PERSON] and [PERSON]; funding acquisition, [PERSON] and [PERSON]L. All authors have read and agreed to the published version of the manuscript. This research was funded by the EnviNagro project (PID 2021-124497 OA-I00), which has received funding from Ministerio de Ciencia e Innovacion, Agencia y del Fondo Europeo de Desarrollo Regional (MCIN/AEI/10.13039/501,100,011,033/FEDER, UE). The financial support of the Conselleria de Cultura, Educacion e Universidade (Xunta de Galicia) is also recognized through the contract ED431C 2021/46-GRC, granted to the research group BV1 of the University of Vigo. [PERSON] acknowledges her predoctoral contract associated with Xunta de Galicia (ED481A-2024/019) funded by Conselleria de Educacion, Universidade e Formacion Profesional. [PERSON] acknowledges her predoctoral contract associated with Campus Agua of University of Vigo (AUGA131H6450211). [PERSON] has a postdoctoral contract Juan de la Cierva Incorporacion (IJC2020-044426-I) funded by Ministerio de Ciencia e Innovacion of Spain, the European Union NextGeneration EU/PRTR and the University of Vigo. [PERSON] holds a postdoctoral fellowship (ED481B-2022-081) financed by Xunta de Galicia.