General Remarks as Rationale for Future Experimentations

Our overall results highlighted some general remarks that are summarized as it follows:

1. On the basis of the long-term data set on field fluxes that was obtained for the Napoli site, we hypothesize a root effect on soil N2O fluxes. It appears that the N dependence of N2O fluxes could be strictly linked with crop phase and soil water content. During the early growth phase of the maize crop, where the root system was still poorly developed, nitrifiers and denitrifiers did not compete with roots for nitrogen supply.

2. Positive effects of compost addition on reduction of soil N2O fluxes was observed in both sites, though the long-term data set of Napoli suggests that the phenomenon may be also dependent on crop phase. This may be due to either a slow release of organic N from the humified compost material, or a greater N uptake by plants and its reduced availability to nitrifiers and denitrifiers. Moreover, an improved N uptake by roots may be facilitated by the bioactivity of humic molecules present in the soil solution. Residual effect of compost addition on N2O emissions during autumn-winter period (after maize crop) is allowed by the slow mineralization of N, that becomes more available for nitrifiers and denitrifiers because maize root system no longer compete for N. Based on this, it may be interesting to also grow a winter cover crop under the same soil treatments, in order to prove a reduced effect of residual compost.

3. Our experimental data confirmed the large literature results reporting on the positive effect of reduced plowing in reducing soil N2O and CO2 emissions.

4. We once more observed that unmanaged control plots (i.e., soils in natural conditions) in different textured conditions (clayey soil of Napoli and loamy soil of Torino) are less metabolically active, and release less CO2 and N2O than managed soils.

5. Although our results did not show an effect of the catalyst treatment on the CO2 and N2O emissions from soil, fluxes measurements should be extended to longer time before attempting a definite conclusion on the effect of such a soil treatment.

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