## 3 Results
### C and N Dynamics in Soil with MPs: Physicochemical Indicators
[71] used PE in uncultivated soil and, although they observed a decrease in this indicator when adding MPs (0.2% and 2% w w\({}^{-1}\)), a similar result to the T2 treatment (0.5% w w\({}^{-1}\)) in the present study, they did not find significant differences (\(p>0.05\)) with respect to the control. Nonetheless, other studies have indicated that MPs can adsorb ions with different charges, releasing various additives, such as bisphenol A, perfluorinated compounds, polycyclic aromatic hydrocarbons, and naphthalene, among others [7; 72]. Therefore, MPs can alter the ionic equilibrium in the soil solution and directly or indirectly influence the EC. However, the specific mechanism by which this happens is still unknown, and further research is needed [71]. Regarding the TOC and OM indicators, there were significant differences between treatments (\(p\leq 0.05\)), showing a tendency to increase with respect to the concentration of MPs added to the soil (Figure 4C,D). The TOC and OM indicators presented the following order in concentration: T1 < T2 < T3 < T4. It is important to highlight that treatment T4 presented values 14.28% above the control treatment T1 for both indicators. Similarly, [PERSON] et al. [73] found that a concentration of 28% (w w\({}^{-1}\)) PP MPs promoted the accumulation of dissolved OM in sandy loam soil. Their explanation for this behavior was that high concentrations of MPs facilitate the release of nutrients into the soil solution, which is caused by microorganisms via mineralization. Furthermore, [PERSON] et al. [74] obtained results similar to those obtained in the present study with regard to the TOC indicator, establishing a proportional relationship between this indicator and the concentration of MPs (PP) used in the experiments (0.1%, 0.3%, and 1% w w\({}^{-1}\)). In the same study, it was emphasized that treatments with 0.3% and 1% (w w\({}^{-1}\)) of added MPs showed values significantly higher than the control (\(p\leq 0.01\)). It was established that these phenomena could be attributed to the fact that MPs are a habitat and food source for microorganisms, which results in greater microbial activity, and, since PP is a material rich in organic C, the TOC quantification methods used were able to detect it, as the PP used in the study contained 85.7% C. Moreover, [PERSON] et al. [8] used PE (1% and 5% w w\({}^{-1}\)) in 30-day experiments and found that the concentration of dissolved organic C (DOC) tended to increase with the MP concentration used in the experiments. However, although C from MPs can increase the concentration of TOC in soil, it has not yet been regarded as a true nutrient [74]. In the same context, in the present study, inverse behavior was observed for the TN indicator (Figure 4E) compared to the TOC indicator, where TN showed a decrease when increasing the concentration of MPs during the experiment, with no significant differences observed between treatments (\(p>0.05\)), showing the following order in TN concentration: T1 = T2 = T3 = T4. Similarly, [PERSON] et al. [74] did not find significant differences in TN between a control treatment and treatments with PP and polyvinyl chloride (PVC) MPs when using concentrations of 0.1%, 0.3%, and 1% (w w\({}^{-1}\)) in sandy soil. Although their experiments lasted 3 months, it was concluded that it would be appropriate to conduct a follow-up of this indicator for a longer time. [PERSON] et al. [75] used concentrations of 7% and 28% (w w\({}^{-1}\)) of polyethylene terephthalate (PET) in silty loam soil for 30 days, and, in the treatment with a higher concentration of MPs (28%), they observed a TN concentration that was 21.07% below the control treatment. This behavior can be observed in the present study in the T4 treatment with 5% (w w\({}^{-1}\)). Even though there were no significant differencesbetween treatments in the present study, a decrease in TN was observed when increasing the concentration of MPs, possibly due to the degree of total N mineralization into inorganic forms.