## 4 Discussion
### DMPP Effects  
In the present study, the addition of DMPP had significant effects both in terms of gross ammonification and nitrification. DMPP addition significantly reduced both NH\({}_{4}\)\({}^{+}\) consumption and NO\({}_{3}\)\({}^{-}\) production in the CNTR and NZ treatments, suggesting that nitrification rates were reduced as expected, while NH\({}_{4}\)\({}^{+}\) production was unaffected. Lower ammonium consumption and nitrate production rates favour the preservation of the NH\({}_{4}\)\({}^{+}\) pool over time and hence the mitigation of N leaching and gaseous losses. The slightly lower nitrate production rates in NZ (compared to the CNTR) were maintained even after the addition of the nitrification inhibitor (Figure 3). The differences between these two treatments were, however, exiguous (even if statistically significant) and in summary, nitrate production was reduced by \(\sim\) 38% after the addition of DMPP in both treatments. As the sorption capacity of DMPP in this soil was already established to be slightly reduced by CHA zeolite application (especially NZ\({}_{\text{p}}\)) [58], the inhibition efficiency of DMPP was expected to be possibly even increased after CHA application. If the sorption of DMPP is reduced, it is reasonable to hypothesize a higher and faster inhibitory effect on zeolite amended soils. However, the results obtained in this short-term study highlighted no significant differences in DMPP efficiency compared to the CNTR. This indicates good compatibility of DMPP with CHA zeolites, as the inhibition efficiency was not altered. The authors of [55] tested DMPP in soils with similar properties and at similar WFPS, but they observed a lower efficiency in reducing gross nitrification (from 9.4 to 21.6%). However, other studies reported highly variable DMPP efficiencies, with reductions in net nitrification rates up to more than 90% in arable soils with pH ranging from \(\sim\) 5 to \(\sim\) 8 during the first days after DMPP addition [52]. Comparison of DMPP performance with other studies in the literature is hardly possible due to the large number of parameters influencing nitrification and potential inhibition, such as soil texture, pH, organic C [83], WFPS, nutrient levels, and incubation time. No significant effects of DMPP addition on the abundance of the investigated functional genes were observed (Table 3). This evidence supports the fact that in the present study DMPP had no detrimental effects on the abundance of soil microbes but only affected their activity. This finding is in agreement with a recent study by [84], where short-term effects on soil N-cycling genes and transcripts after the addition of nitrification plus urease inhibitor were evaluated. The authors did not observe major effects on the abundance or activity of target and non-target N-cycling microbial groups, despite reducing N\({}_{2}\)O emissions; most of the observed inhibition effects on genes and transcripts were only transient. In our study, DMPP significantly affected N\({}_{2}\)O emissions as already observed by other authors in the literature (Table 4) [38; 55; 55; 85]. The mitigation effect on this potent GHG is suggested to function in two ways, one is direct, by the inhibition of nitrification which generates N\({}_{2}\)O as a by-product, while reduced substrate (NO\({}_{3}\)\({}^{-}\)) availability for denitrification can also indirectly lower N\({}_{2}\)O emissions. In this study, a reduction of N\({}_{2}\)O emissions by more than 90% in the DMPP treated soils was observed. The authors of [86] measured similar reductions (99.2%) in N\({}_{2}\)O emissions but as for nitrification rates, the reported efficacy of DMPP in reducing N\({}_{2}\)O emissions is very broad and dependent on specific conditions of each case study. Additionally, as the source of N\({}_{2}\)O cannot be finally resolved (i.e., nitrification, denitrification, nitrifier denitrification, etc. ), the reduction/inhibition efficiency on N\({}_{2}\)O is even harder to compare in the literature.