## 4 Discussion
### Effects of CHA at NH4\({}^{+}\)-Enriched State (without DMPP Addition)  
NH4\({}^{+}\)-N and NO3\({}^{-}\)-N concentrations were particularly high in CZ because of the additional N input brought by CZ\({}_{\text{P}}\) (Table 2). Net changes in the pool sizes of both NH4\({}^{+}\) and NO3\({}^{-}\) were positive throughout the incubation, with a predominance of net production of NH4\({}^{+}\)-N. The predominance of ammonification over nitrification processes in CZ\({}_{\text{P}}\) is more evident by looking at gross rates (Figure 2). In CZ treatment, NH4\({}^{+}\)-N production was more than three-fold of NO3\({}^{-}\)-N production (-75 mg NH4\({}^{+}\)-N kg\({}^{-1}\) day\({}^{-1}\) dm compared to the \(<\) 25 mg NO3\({}^{-}\)-N kg\({}^{-1}\) day\({}^{-1}\) dm, respectively). Contrary to the other treatments, in CZ NH4\({}^{+}\), production exceeded consumption. The possible origin of the NH4\({}^{+}\) excess is likely related to the increased breakdown of organic N or to Dissimilatory Nitrate Reduction to Ammonium (DNRA) although we did not directly measure these two processes. In particular, DNRA is still a poorly understood process in the N-cycle, which consists of NO3\({}^{-}\) reduction via NO2\({}^{-}\) to NH4\({}^{+}\) with the last step catalyzed by the cytochrome c nitrite reductase (_NrfA_). DNRA is a process that does not contribute to N removal but instead concurs in retaining N in the soil matrix [75]. DNRA and denitrification are known to occur simultaneously under a wide range of environmental conditions and are generally boosted at WFPS higher than 60%; hence, it is assumed to occur at low soil redox and under high C/NO3\({}^{-}\) ratio. However, this latter assumption has found contradictory evidence in the literature [76]. The addition of a small fraction of labile organic C through CZ\({}_{\text{P}}\) (Table 1) may have stimulated both processes (hydrolysis of urea and/or DNRA), contributing to the increasing of NH4\({}^{+}\) levels in this treatment. CZ treatment without DMPP also shows  
Figure 4: Relationship between _nirS_ and _nosZ_ functional genes and N2O emissions in treatments with and without DMPP addition. CNTR (red circles), NZ (dark-yellow circles), CZ (blue circles) like in Figure 1.  
higher CO\({}_{2}\)-C emission rates (Table 4), indicating higher heterotrophic respiration and probably a higher decomposition rate of soil organic matter, which also generates NH\({}_{4}\)+. The higher heterotrophic respiration observed for CZ may reduce oxygen availability in air-filled pore space and hence conditions favourable for DNRA. Further investigation is required to better identify the mechanisms of gross NH\({}_{4}\)+ production in CZ amended soils. Another relevant aspect observed in CZ involves the amount of N\({}_{2}\)O-N emitted over the incubation and the amounts of _mirS_ and _nosZ_ functional genes (Figure 4, Tables 3 and 4). In this treatment, the emissions of N\({}_{2}\)O-N were huge, ten-fold higher than that recorded in the CNTR. Archaeal and bacterial _amoA_ were not significantly different from the CNTR, while a significant reduction in the abundance of _nosZ_ and _mirS_ was recorded (Tables 3 and 4). It is known that the potent GHG N\({}_{2}\)O-N is a byproduct of both nitrification and denitrification reactions [77]. High N\({}_{2}\)O emissions, thus, indicate normally high rates of denitrification or nitrification. In our experiment, this increase in N\({}_{2}\)O is not attributable to enhanced nitrification processes since the gross production of NO\({}_{3}\)\({}^{-}\) was lower than the rates accounted for in the CNTR. Significantly lower copy numbers of _mirS_ and _nosZ_ in CZ compared to CNTR provides additional support as more N is lost in the form of N\({}_{2}\)O by partial denitrification rather than full denitrification (Figure 4).