## 3 Results
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Po di Volano-Sacca di Goro** & \multicolumn{3}{c}{**N Budget**} & \\
**Watershed** & **t N.year\({}^{-1}\)** & **kg N:ha\({}^{-1}\).year\({}^{-1}\)** & **\%** \\ \hline
**INPUT** & & & \\ \hline Livestock manure (N\({}_{\text{Man}}\)) & 603 (43) & 9 (1) & 5 \\ Synthetic fertilizers (N\({}_{\text{Fert}}\)) & 9875 (1029) & 140 (12) & 72 \\ Biological fixation (N\({}_{\text{Fix}}\)) & 2733 (323) & 39 (5) & 20 \\ Atmospheric deposition (N\({}_{\text{Dep\_UAA}}\)) & 437 (298) & 6 (4) & 3 \\ \hline
**\(\Sigma\) input** & **13,648 (987)** & **194 (10)** & \\ \hline
**OUTPUT** & & & \\ \hline Crop harvest (N\({}_{\text{Harv}}\)) & 8149 (685) & 116 (9) & 95 \\ NH\({}_{3}\) volatilization (N\({}_{\text{Vol}}\)) & 171 (7) & 2 (1) & 2 \\ N lost in runoff water (N\({}_{\text{Runoff}}\)) & 298 (203) & 4 (2) & 3 \\ \hline
**\(\Sigma\) output** & **8618 (749)** & **121 (10)** & \\ \hline
**\(\Delta\)N\({}_{\text{Soil}}\) = \(\Sigma\) input - \(\Sigma\) output** & **5029 (789)** & **72 (11)** & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Nitrogen budget in the agricultural soils of the Po di Volano-Sacca di Goro watershed (average of the five-year period 2006–2010; standard deviation is reported in brackets). Budget items are expressed as tons of N imported or exported per year (t N-year\({}^{-1}\)) in the whole area and normalized for the utilized agricultural area (kg N:ha\({}^{-1}\).year\({}^{-1}\)). The percentage of each input or output with respect to the total is also reported. The canal network imported \(\sim\)420 t N annually, of which 88% occurred during the six-month period from April to September, when huge water volumes are withdrawn from the Po River to sustain irrigation practices. The annual N load exported to the Sacca di Goro lagoon accounted for \(\sim\)740 t N, of which 66% occurred during the non-irrigation period (Figure 3). The study watershed hosts \(\sim\)120,000 inhabitants and N loads from urban and industrial areas entering into the canal network were estimated at \(\sim\)200 t N-year\({}^{-1}\) and 67 t N-year\({}^{-1}\), respectively. The greatest input term in the N budget of the hydrological network was the N load generated in upstream catchments and imported through the pumping stations. N loads from point sources (urban and industrial areas and atmospheric deposition on surfaces other than agricultural soils) accounted for \(<\)5% of the total inputs due to farming activity. N removal via denitrification in the canal network of the basin was estimated at \(\sim\)390 t N-year\({}^{-1}\) (Figure 3), of which \(\sim\)75% occurred from April to September when canals were active for irrigation. Figure 3: Schematic showing the N fluxes in the soil–water system of the Po di Volano-Sacca di Goro watershed. N input to agricultural soils: livestock manure (N\({}_{\rm Man}\)), synthetic fertilizers (N\({}_{\rm Fert}\)), biological fixation (N\({}_{\rm Fix}\)), and atmospheric deposition (N\({}_{\rm Dep\_UAA}\)). N output from agricultural soils: crop harvest (N\({}_{\rm Harv}\)), ammonia volatilization (N\({}_{\rm Vol}\)), leaching to the shallow unconfined aquifer (N\({}_{\rm Aqu}\)), N lost in runoff water (N\({}_{\rm Runofi}\)), and denitrification (N\({}_{\rm Den}\)). N input to the canal network: N load imported by the canal network (N\({}_{\rm Inflow}\)), atmospheric N deposition on surfaces other than agricultural soils (N\({}_{\rm Dep}\)), N load from urban areas (N\({}_{\rm Urb}\)), N load from industrial areas (N\({}_{\rm Ind}\)), and N\({}_{\rm Runofi}\). N output from the canal network: N load exported from the basin (N\({}_{\rm Outflow}\)) and N removed by denitrification in the canal sediments (N\({}_{\rm Rem}\)). All terms are expressed in t N-year\({}^{-1}\) (standard deviation is reported in brackets). N\({}_{\rm Inflow}\) and N\({}_{\rm Outflow}\) are also reported by splitting the contribution of the irrigation period (from April to September) and the non-irrigation period (from October to March).