## 4 Discussion
### Effects of CHA in Natural State (without DMPP Addition)  
Both NH4+-N and NO3-N concentrations in all treatments were high after the addition of the N fertilizer (Table 2). The NZ treatment showed consistent analogies with the CNTR. Net and gross ammonification rates were very similar (Figures 2 and 3) as well as the amount of _amoA_ AOA and _amoA_ AOB functional genes (Table 3). In both treatments, the processes associated with the consumption of NH4+-N (e.g., volatilization, immobilization, and oxidation) exceeded those responsible for NH4+-N production (e.g., mineralization of organic N, DNRA). The observed lack of significant differences in functional genes between CNTR and NZ agrees with the study of [74], which reported no differences in the abundance of functional genes involved in nitrification (_amoA_ and _nxrA_) and denitrification (_narG_, _nirK_, _nirS_, and _nosZ_) after natural zeolite addition to sludge composite in aerated reactors.  
The consumption of NH4+-N was very similar to that of the NO3-N production, indicating that the NH4+-N consumed over the 24 h of incubation was converted into NO3- by nitrification processes (Figures 2 and 3). However, in NZ, the production of NO3- was slightly lower than that in the CNTR (Figure 3) (reduction of ~1.8 mg kg-1 day-1), suggesting an effect of CHA zeolite on short-term nitrification. This was the only significant effect recorded on NZ treatment, considering that N2O-N and CO2-C emissions were also very similar to the CNTR (Table 4). A plausible hypothesis is that given the weak bounds of NH4+ with the CHA framework, the microbial biomass was able to access the fraction of N adsorbed by zeolites quite fast, causing a slight delay in short-term NO3-N  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Sample** & \
\begin{tabular}{c} **N2O-N Emissions** \\ **\(\mathbf{\mu_{\text{g}}}\) N2O-N g–1 day–1** \\ \end{tabular} &
\begin{tabular}{c} **CO2-C Emissions** \\ **\(\mathbf{\mu_{\text{g}}}\) CO2-C g–1 day–1** \\ \end{tabular} \\ \hline \multirow{3}{*}{No DMPP} & CNTR & 0.127 \(\pm\) 0.108 b & 21.67 \(\pm\) 19.55 b \\  & NZ & 0.196 \(\pm\) 0.148 b & 25.15 \(\pm\) 6.55 b \\  & CZ & 1.876 \(\pm\) 0.226 a & 46.43 \(\pm\) 4.89 a \\ \hline \multirow{3}{*}{With DMPP} & CNTR & **0.0061 \(\pm\) 0.0031 c** & 32.62 \(\pm\) 11.81 ab \\  & NZ & **0.0049 \(\pm\) 0.0034 c** & **16.98 \(\pm\) 1.55 c** \\  & CZ & **0.122 \(\pm\) 0.068 b & **28.88 \(\pm\) 1.94 b** \\ \hline \hline \end{tabular}
\end{table}
Table 4: N2O-N and CO2-C emission rates throughout incubation. Values are expressed as mean \(\pm\) standard deviation (4 replicates). Different superscript letters express significant differences as a result of ANOVA (for N2O-N, In transformed), Kruskal-Wallis (for CO2-C), and Tukey-HSD statistical tests (\(p<0.05\)). Values in bold highlight a significant effect of DMPP.  
production rates. Coherently, gross NO3\({}^{-}\)-N consumption was also very similar in CNTR and NZ, indicating that processes responsible for NO3\({}^{-}\)- removal were unaffected by CHA zeolite application in the short term (Figure 3). This is further supported by comparable N2O-N emissions as well as _nirS_ and _nosZ_ functional genes (Figure 4 and Table 3), which are responsible for reducing nitrite and nitrous oxide into NO and N2, respectively, in the process of denitrification. This finding suggests that the addition of CHA as soil amendment does not significantly affect the abundance of genes from microbes in the short term but instead may have some significant effects on their activity.  
It is important to consider that other typologies of zeolites, characterized by stronger binding between NH4\({}^{+}\) and the framework structure, may show different behaviours and more pronounced effects on nitrification rates.