## 3 Results
### C and N Dynamics in Soil with MPs: Physicochemical Indicators
With respect to N-NH4\({}^{+}\) (Figure 4I), there were no significant differences between the treatments (_p_\(>\) 0.05). However, N-NH4\({}^{+}\) tended to decrease with the addition of MPs, mainly at high concentrations. This behavior may be closely related to the N-NO2\({}^{-}\) and N-NO3\({}^{-}\) indicators, since these tend to increase in concentration when increasing the dose of MPs (Figure 4G,H). This indicates that the direction of the nitrogen cycle tended towards nitrification processes. In similar studies, [PERSON] et al. [73] also found no differences in the concentration of N-NH4\({}^{+}\) during 30 days of experimentation on a silty textured soil with PP (7% and 28% w w\({}^{-1}\)). They attributed this behavior to a greater accumulation of DOC when increasing the concentration of MPs, showing that heterotrophic microbial growth (the immobilization of N-NH4\({}^{+}\)) dominates over autotrophic growth (the oxidation of N-NH4\({}^{+}\)). Likewise, the results obtained by [PERSON] et al. [8] were similar to the results of the present study when they used PE (1% and 5% w w\({}^{-1}\)), and they found no differences in the values of N-NH4\({}^{+}\) between treatments and the control during the 30 days of the experiment. They attributed this behavior to the short time of the experiment, since MPs do not decompose in a short time and do not show effects on the inorganic N of soil. On the other hand, contrary to the results of this study, [PERSON] et al. [75] found overall decreases in N-NH4\({}^{+}\) of 67.93% and 79.84% when using PET at concentrations of 7% and 28% (w w\({}^{-1}\)), respectively, compared to a control treatment. They attributed this behavior to the fact that PET can chelate N-NH4\({}^{+}\), since it has carbonyl (=O) and hydroxyl (-OH) groups on its surface [75,78]. They also found that the concentration of N-NO3\({}^{-}\) in their experiment increased when adding PET MPs, so they attributed the decrease in N-NH4\({}^{+}\) to the acceleration of the nitrification process, which, in turn, affected the transformation of N in the soil. Also, [PERSON] et al. [76] found that the concentration of N-NH4\({}^{+}\) was significantly reduced between days 12 and 15 of their experiment on paddy field soil using 2% (w w\({}^{-1}\)) PLA concentrations over 70 days. They concluded that PLA MPs did influence the transformation of soil N, and their influence was different depending on high or low C conditions, since, in treatments supplemented with C sources, the concentration of N-NH4\({}^{+}\) tended to be lower than the control. [PERSON] et al. [75], in similar studies, also considered that the conditions for the transformation of N in the soil were favorable to increase the mineralization processes via nitrification. Considering the above data, it can be established that the effects of MPs on the mineralization of N sources and their transformation into more assimilable forms (N-NO3\({}^{-}\) and N-NH4\({}^{+}\)), the type of MPs, their form, and their concentration play fundamental roles in the physicochemical and biological properties of soil. Regarding the net mineralization of inorganic N, represented as N\({}_{\text{min}}\), significant differences were observed between treatments (\(p\leq 0.01\)), showing the same order in concentration as N-NO3\({}^{-}\) (T1 = T2 = T3 < T4), with T4 presenting a concentration of N\({}_{\text{min}}\) that was 23.3% higher than the control treatment T1 (Figure 4). It is important to emphasize that this indicator evaluates to what degree the mineralization of the N compounds present in soil develops in the presence of MPs.