## 4 Discussion
### DMPP Effects
DMPP behaved differently in the CZ treatment compared to CNTR and NZ (Figures 2 and 3, Tables 2-4). In this treatment, when DMPP was added, gross production of NH\({}_{4}\)\({}^{+}\) was further stimulated, while NO\({}_{3}\)\({}^{-}\) gross production was completely inhibited in the first 24 h (reduction of 100% vs the same treatment without DMPP), but a fraction of NO\({}_{3}\)\({}^{-}\) was still being consumed. If emissions of N\({}_{2}\)O-N of each treatment are based on the total amount of N added (Table 5), it emerges that in CZ, notwithstanding the strong N surplus, DMPP reduced the emissions of N\({}_{2}\)O-N by 72% compared to the fertilized control without DMPP. Even though the percentage for reduction of N\({}_{2}\)O-N emission with DMPP was higher for CNTR and NZ (with 95, and 96% respectively), the highest total amount of N\({}_{2}\)O-N was inhibited in the CZ treatment (1.754 ug N). The absolute efficiency in inhibiting nitrification and N\({}_{2}\)O emissions in CZ treatment agrees with other studies in which it was established that DMPP performance is generally higher in treatments that receive organic fertilizers (i.e., the best performance for DMPP has been found to occur when applied in combination with ammonium sulphate or organic fertilizers) and at high application rates [85]. For CZ, both conditions were met, since CZ\({}_{\text{p}}\) amendment contains additional N originating from animal slurry (approximately \(\sim\)427 kg N ha\({}^{-1}\) organic fertilizer) and thus increased the total N input compared to the other treatments that received only chemical NH\({}_{4}\)NO\({}_{3}\). DMPP addition also had a significant effect on CO\({}_{2}\) emissions in both NZ and CZ treatments, reducing the fluxes significantly (Table 4), while no significant effects were found in the CNTR. Significant short-term effects of CHA zeolites on soil CO\({}_{2}\) emissions were already encountered in a previous incubation experiment when zeolites were applied in combination with urea fertilizer (CO\({}_{2}\) emissions reduced by \(\sim\)21% to approximately \(\sim\)30%) [22]. To understand the mechanisms behind this CO\({}_{2}\) emission reduction in zeolite amended soil, further focused experiments are required. The effects of DMPP on soil CO\({}_{2}\) emissions and, hence, on C-mineralization and heterotrophic respiration are poorly investigated and contrasting evidence is found in the literature. For example, the authors of [38] found no significant DMPP effect of soil CO\({}_{2}\) emission in a lab-incubation experiment performed on loam and clay soils with different labile C availability, while the authors of [54] measured an average reduction of 28% in a three-year field experiment, suggesting significant effects of DMPP on C-mineralization. NO\({}_{3}\)\({}^{-}\) consumption and the enhanced NH\({}_{4}\)\({}^{+}\) production coupled with the total inhibition of NO\({}_{3}\)\({}^{-}\) production may suggest the possibility of an enhanced breakdown of organic N into NH\({}_{4}\)\({}^{+}\) or DNRA. In addition, the significantly lower abundance of _nosZ_ and _nirS_ genes even after DMPP addition in CZ treatment compared to the CNTR or NZ treatments (Table 3) suggest lower denitrification, and hence NO\({}_{3}\)\({}^{-}\) consumption might be driven by DNRA, as the little fraction of labile OC added with CZ\({}_{\rm p}\) could have stimulated respiration and hence the reduced soil redox potential promotes a shift to NO\({}_{3}\)\({}^{-}\) consumption via DNRA [76]. Although the effects of DMPP on DNRA rates in soils is a nearly unknown subject, in the very recent work of [38], the authors found that DNRA rates increased by a factor \(>\) 5 when DMPP was applied to a sandy horticultural soil but had no significant effects on a loamy pasture soil. The authors suggest that the increased labile C availability promoted NO\({}_{3}\)\({}^{-}\) reduction through DNRA. This evidence further supports our hypothesis that DNRA was probably an important process occurring in CZ treatment after DMPP addition.