## 4 Discussion  
The contribution of soil denitrification is rarely estimated experimentally, due to the difficulty of field measurements, which also makes it difficult to have enough spatial and temporal coverage to upscale the rates to the basin scale. It is also not accounted for indirectly, in N budgets at the basin scale, due to the difficulty in accounting for all the terms of the budget except denitrification. This is why the outcomes of the present work introduce some elements of novelty in the study of N dynamics in agricultural-dominated landscapes. The detailed N mass balance, performed both in the basin and in the hydrological network of the Po di Volcano-Sacca di Goro watershed, and facilitated by the high homogeneity of the territory, of crops, and by the artificial regulation of the canal network, allowed isolating and estimating the relevance of soil denitrification. In this basin, livestock densities are low, and cereals are the dominant crops, amended almost exclusively with synthetic fertilizers. NUE (nitrogen use efficiency), i.e., the proportion of all N inputs exported via crop harvest, reached ~60%, a value markedly higher than those calculated for other sub-basins of the Po river system characterized by higher availability of livestock manure and greater amounts of applied N that are potentially lost via leaching or runoff [25; 51; 52]. Synthetic fertilizers are more easily dosed and allow more accurately fulfilling crops' N requirements without generating large excesses and losses in the environment. However, on the other hand, their use implies a rapid dissolution of highly soluble N forms, such as NO3-, with increased risk of lateral or vertical transport. Since the investigated watershed is a recently reclaimed lowland, completely flat and with a slope mostly less than 0.05% [53], the proportion of soil N excess flowing into the canal network by surface runoff is a minor component of the N budget. On the contrary, groundwater NO3- pollution may be relevant, via vertical transport trough the vadose zone. In lowland areas, the coexistence of intensive agricultural activities and a shallow water table further stresses the need for detailed investigations into the role of soil denitrification in attenuating NO3- pollution. The specific knowledge of key factors governing N transport and transformation pathways in agricultural areas are not yet fully clarified. Several studies highlighted that, in agricultural irrigated watersheds, the generation of large N loadings does not necessarily translate into high exports to terminal waterbodies or coastal zones [9; 31; 52], and they demonstrated the relevance of small human-made waterways as elective sites for N processing and removal [19; 49; 54]. In the investigated basin, N removal via denitrification within the canal network represents only a small flux in the whole N budget on annual basis. Although specific studies, performed in vegetated canals of the same basin, demonstrated that emergent vegetation increases circa 10-fold the denitrification capacity of the same canals deprived of vegetation, stands of _Phragmites australis_ and _Typha litiglia_ are maintained, at present, only in <5% of the total network length and are subjected to moving at the beginning of the summer to preserve hydraulic performance [18; 19; 29]. On an annual basis, the watershed exports to the coastal zone nearly 77% more than the N load entering in the hydrological network from the Po river and basins located upstream. However, this amount (~320 t N-year-1), generated within the basin, is one order of magnitude lower than the missing N (~5000 t N-year-1) resulting from the calculation of the N soil budget. This N budget is conservative, since it does not account for sedimentary N export in drained peatlands, where a large N pool (mainly as ammonium) is accumulated [55] and surely contributes to the surface water N load.