## 3 Results
### C and N Dynamics in Soil with MPs: Physicochemical Indicators
On the other hand, due to the dynamics of the TOC and TN values obtained in this study, the C/N ratio (Figure 4F) showed significant differences between treatments (_p_\(\leq\) 0.05) in the following order: T1 = T2 < T3 < T4. Treatment T4 was 25.4% above the control treatment T1. In all treatments, the C/N ratio was considered low, but it tended to improve with the addition of MPs. This was due to an increase in the TOC indicator when the concentration of MPs increased. In addition to TN, the inorganic N (N-NO2\({}^{-}\), N-NO3\({}^{-}\), and N-NH4\({}^{+}\)) was also determined in the different treatments and over time. In this regard, the N-NO2\({}^{-}\) in this study showed significant differences (_p_\(\leq\) 0.001) between the T3 and T4 treatments, presenting the following order in concentration: T1 = T2 < T3 < T4 (Figure 4G). Treatment T4 presented concentrations that were 12.1%, 15.8%, and 18.2% higher than T3, T2, and T1, respectively. This behavior may be closely related to the results obtained for N-NH4\({}^{+}\) in the present study, since it was reduced with the addition of MPs; with respect to this, several authors have concluded that, under high concentrations of MPs, the N cycle can be directed mainly towards nitrification [75,76]. It should be noted that there are few studies on the effect of MPs on the production of N-NO2\({}^{-}\) during N mineralization processes in soil. Similar to the results in the present study, [PERSON] et al. [76] observed that the addition of 2% (w w\({}^{-1}\)) PLA increased the concentration of N-NO2\({}^{-}\). These and other authors have concluded that MPs can alter soil biophysics and increase soil porosity, which could increase soil airflow and stimulate N-NH4\({}^{+}\) oxidation by providing sufficient dissolved O2. On the other hand, the N-NO3\({}^{-}\) indicator showed significant differences between treatments (_p_\(\leq\) 0.01), presenting the following order in concentration: T1 = T2 = T3 < T4 (Figure 4H). Treatment T4 was different from the other treatments, having a concentration 50% above the control treatment T1. Contrasting with this study, [PERSON] et al. [73] found that the addition of PP particles decreased N-NO3\({}^{-}\) production during the first three days, and then, between days 7 and 30, no difference was found between the treatments and the control. In this case, although the addition of MPs increased the soil porosity and O2 content, promoting the nitrification process, the low N-NH4\({}^{+}\) content in the soil was a limiting factor for its complete mineralization into N-NO3\({}^{-}\). On the other hand, [PERSON] et al. [8], when using PE (1% and 5% w w\({}^{-1}\)), only found differences in the N-NO3\({}^{-}\) between treatments on the first day of the experiment, and there were no significant differences during the following days. They used a temperature of 25 \({}^{\circ}\)C and a relative humidity of 75% in the dark, and they mentioned that higher temperatures and humidities could increase microbial activity, and, therefore, the concentration of compounds such as N-NO3\({}^{-}\) via nitrification. They also attributed this behavior to the short incubation period of their experiments (30 days). [PERSON] et al. [75] obtained results similar to those in the present study, where soils treated with PET MPs (7% and 28% w w\({}^{-1}\)) showed an increase in the nitrification process with respect to the control. In the present study, the increases in N-NO2\({}^{-}\) and N-NO3\({}^{-}\) in the treatments and over time could also be related to the decrease in N-NH4\({}^{+}\) (see Figure 4I) caused by the high concentrations of MPs (mainly 5% w w\({}^{-1}\)). The above may also be due to the fact that it has been reported that the incorporation of MPs affects the porosity of soil, which, in turn, increases the flow of O2 and promotes the microbial autotrophic activity present in the soil, which is reflected in a faster or more efficient oxidation of N-NH4\({}^{+}\), increasing N-NO3\({}^{-}\) via nitrification [75,77].