## 3 Results
### C and N Dynamics in Soil with MPs: Physicochemical Indicators  
According to the [PERSON] analysis of variance (Figure 4A), the pH indicator showed significant differences between treatments (\(p\leq 0.001\)); in general, the following order was established: T1 = T2 = T3 \(<\) T4. Treatment T4 presented pH values that were 1.9%, 2.6%, and 1.5% higher than treatments T1, T2, and T3, respectively. [PERSON] et al. [67], in 31-day experiments with sandy soils, found that different MPs in the form of fragments and foams (0.4% w w\({}^{-1}\)) tended to increase the soil pH, a result similar to that found in the present study. Figure 2: (**A**) Face masks used to obtain the MPs, (**B**) outer layer of the face mask at 10\(\times\) magnification, and (**C**) inner layer of the face mask at 10\(\times\) magnification. Figure 4: _Cont._  
Figure 3: FTIR spectra of the MPs used in the experiments. A possible explanation for this behavior is that once these MPs are incorporated into soil, there is an increase in aeration and porosity [15], which causes a structural change in the soil matrix and, consequently, in its microbial activity. Also, the leaching of chemical compounds that come from MPs can alter the soil biota and, therefore, its pH [68]. [PERSON] et al. [14] used polyester (PES) fibers (0.4% w w\({}^{-1}\)) in a sandy loam soil and noted a significant 4.0% increase in pH (\(p<\) 0.01), attributing this increase to a change in soil aggregates caused by the PES fibers, which resulted in a more complete decomposition of OM, causing greater microbial activity. Contrary to the results of the present study, [PERSON] et al. [69] observed a decrease in pH when using polyethylene (PE) particles in a sandy loam soil (1% w w\({}^{-1}\)), where the pH in the control treatment was 6.10, whereas the pH in the treatment with PE was 5.90. However, they observed an increase in pH when using the same concentration of MPs but from polylactic acid (PLA), presenting a slight increase in pH from 6.10 to 6.20. Therefore, it can be established that changes in the pH of soils that contain MPs depend not only on the concentration, but also on the chemical nature of the MPs, their interaction with the microorganisms present in the soil,  
Figure 4: Friedman analysis of variance for physicochemical indicators with respect to treatments. Tl, 0% MPs; T2, 0.5% MPs; T3, 1% MPs; and T4, 5% MPs; (**A**) pH, hydrogen potential; (**B**) EC, electrical conductivity (dS m\({}^{-1}\)); (**C**) TOC, total organic carbon (%); (**D**) OM, organic matter (%); (**E**) TN, total nitrogen (%); (**F**) C/N, carbon–nitrogen ratio; (**G**) N-NO\({}_{2}\), nitrites (mg N-NO\({}_{2}\) kg\({}^{-1}\) dry soil); (**H**) N-NO\({}_{3}\)\({}^{-}\), nitrates (mg NO\({}_{3}\)\({}^{-}\) kg\({}^{-1}\) dry soil); (**I**) N-NH\({}_{4}\)\({}^{+}\), ammonium (mg N-NH\({}_{4}\)\({}^{+}\) kg\({}^{-1}\) dry soil); (**J**) N\({}_{\min}\), mineralizable N (mg N kg\({}^{-1}\) dry soil); and (**K**) NI, nitrification index. The dotted line represents the mean of the indicator. Same letters indicate that there is no significant difference between treatments using a pairwise Wilcoxon rank-sum comparison test with Bonferroni adjustment (\(p\leq\) 0.05). and the type of soil. Moreover, there is evidence that MPs can also change the abundance of N-NH\({}_{4}\)+ oxidizing bacteria, influencing the mineralization of nitrogen compounds via nitrification [70], accompanied by a change in pH by releasing H+ ions in the process [7]. Regarding the EC indicator, its values presented the following order: T2 < T1 = T3 = T4 (Figure 4B), showing significant differences between treatments (\(p\leq 0.05\)). Treatment T2 presented a value below the rest of the treatments, with a value 6.5% lower than that of T1. There are few studies that have evaluated the effect of MPs on EC. [PERSON] et al.