## 3 Results
### Development of the SQI\({}_{u}\)  
Regarding the development of the \(SQI_{u}\) in this study, after the redundancy elimination process, the Nmin indicator presented the closest relationship with respect to soil quality and the different concentrations of MPs used in the experiments. [PERSON] et al. [92] developed an \(SQI\) for contaminated soils from an open dump and used organic amendments (biosolids) for their rehabilitation. These researchers conducted C and N dynamics experiments over time to monitor the release and assimilation of nutrients in the presence of contaminated soils. An \(SQI_{w}\) was developed, finding significant differences \((p\leq 0.05)\) between the contaminated soil and the uncontaminated soil, obtaining a better quality in the uncontaminated soil. However, once they added biosolids as an organic amendment to the contaminated soils, their quality improved, without significant differences between them \((p>0.05)\). But most relevant of all is that they also found Nmin to be the indicator with the closest relationship to soil quality, which may be closely related to the presence of MPs in dumps. Additionally, [PERSON] et al. [93] compared three different methodologies that involved the development of \(SQI\)s using physicochemical and biological indicators for sodic soils with the addition of different doses of biosolids obtained from a local waste-water treatment plant, evaluating C and N dynamics over time. Their results indicated that the Nmin indicator showed a tendency to increase as a function of the applied dose of biosolids compared to the control soil (without biosolids) over time. One of the developed \(SQI\)s in their study was an \(SQI_{u}\), where the N-NH4+ indicator was the most related to soil quality, which is a secondary part of the mineralization process of N sources (Nmin) via nitrification. It should be emphasized that Nmin is a key indicator closely related to the mineralization of OM in agricultural soils,  
Figure 9: Percentage of PC variability. in soils with added organic amendments, and in soils with organic pollutants and with sources of C and N accessible to the biological phase of the soil. [PERSON] et al. [94] are in accordance with what was previously mentioned, i.e., that indicators involved in the N cycle (N\({}_{\rm min}\) in this study) are considered key to determining soil quality. Another key indicator related to the mineralization of OM in soil and, therefore, to the dynamics of N\({}_{\rm min}\) is C-CO\({}_{2}\). The widely used C-CO\({}_{2}\) indicator is either a direct indicator of microbial activity or is considered as a final product of a transformation process or the complete mineralization of OM or compounds used as primary sources of C and N. According to the [PERSON] analysis of variance (Figure 7C), there were significant differences in terms of the accumulated C-CO\({}_{2}\). Since increasing the concentration of MPs increases the C-CO\({}_{2}\) value, this indicator is closely related to nutrient mineralization into more accessible forms and, therefore, is indirectly related to soil quality. Once it was determined that the N\({}_{\rm min}\) indicator was the one with the closest relationship to soil quality, it was used in Equation (2) considering the function of the indicator as \"more is better\". The \(SQI_{u}\) was obtained through Equation (4), as follows:  
\[SQI_{u}\ =\ 0.832\times Si_{Nmin} \tag{4}\]  
With the above equation, the soil quality values were obtained for the different treatments. The \(SQI_{u}\) developed for the different treatments (Figure 11) indicated significant differences between T4 and the other treatments (\(p\leq 0.01\)). Nonetheless, there were no significant differences between treatments T1, T2, and T3 (\(p>0.05\)), having the following order in terms of the \(SQI_{u}\): T1 = T2 = T3 < T4. It should be noted that all soils were in the \"low-quality\" range (see Table 1), with values between 0.21 and 0.32 (Supplementary Table S1).