## 3 Results  
The N budget in agricultural soils of the Po di Volano-Sacca di Goro watershed was not closed and it differed by less than 12% among the five years. N outputs (~8600 t N-year\({}^{-1}\)) amounted to ~63% of the N inputs (~13,600 t N-year\({}^{-1}\)), with a potential excess of ~5000 t N-year\({}^{-1}\) (~37% of N inputs), equivalent to 72 kg N-ha\({}^{-1}\)year\({}^{-1}\) (Table 2). Only N input from livestock manure showed a constant decrease from 2006 to 2010 (overall ~15%), while, for all the other budget items, the variability from year to year was erratic and without a clear temporal trend. Synthetic fertilizers represented the major contribution to  
\begin{table}
\begin{tabular}{c c c} \hline \hline  
all the N imported to agricultural land, averaging at 72% of total N input at the basin level, followed by biological fixation (20%), while the remaining amount was almost equally divided between livestock manure and atmospheric deposition. The proportion among the N input terms at the watershed scale reflected a similar situation at the municipal level, with synthetic fertilizers being in absolute the biggest N source in all the considered municipalities (up to 180 kg N:ha\({}^{-1}\).year\({}^{-1}\)), while biological fixation and livestock manure reached maximum values of 130 and 24 kg N:ha\({}^{-1}\).year\({}^{-1}\), respectively. Total N input in the municipalities of the studied area varied from 162 to 273 kg N:ha\({}^{-1}\).year\({}^{-1}\) with an average value of 194 kg N:ha\({}^{-1}\).year\({}^{-1}\) at the basin scale. N exported via crop harvest was the main output from agricultural soils among the three terms included in the budget, accounting for 95% of the sum of all items (Table 2). N lost to surface waters via runoff and N emitted to the atmosphere in the form of NH\({}_{3}\) were comparable terms (municipal values peaking at 5 kg N:ha\({}^{-1}\).year\({}^{-1}\)) but two orders of magnitude lower than the crop harvest (municipal values ranging from 108 to 160 kg N:ha\({}^{-1}\).year\({}^{-1}\)). Three cereals, i.e., maize, wheat, and rice, accounted more than 65% of the total N removed in crop biomass from cultivated land. Total N output in the municipalities of the studied area varied from 110 to 163 kg N:ha\({}^{-1}\).year\({}^{-1}\) with an average value of 121 kg N:ha\({}^{-1}\).year\({}^{-1}\) at the basin scale. Along the investigated five-year period, the water volumes entering into the canal network showed median NO\({}_{3}\)\({}^{-}\) concentrations of 1.70 and 2.80 mg N:L\({}^{-1}\) in the irrigation and non-irrigation periods, respectively (Figure 2). Nitrate concentrations in water drained from the study basin were substantially lower with median values of 1.20 and 2.30 mg N:L\({}^{-1}\) in the irrigation and non-irrigation periods, respectively. During the whole monitoring period, NO\({}_{3}\)\({}^{-}\) concentrations in the shallow aquifer were generally much lower than those measured in surface waters and similar between the irrigation and non-irrigation periods; median values never exceeded 0.1 mg N:L\({}^{-1}\), and only in some transient occasions did NO\({}_{3}\)\({}^{-}\) concentrations peak at 4-5 mg N:L\({}^{-1}\) (Figure 2). In all sites and sampling events, NO\({}_{3}\)\({}^{-}\) was constantly below the limit of 11.3 mg N:L\({}^{-1}\), the maximum permissible concentration in drinking water according to the World Health Organization [50]. The concentrations were quite homogeneous at the watershed scale with only a few hot spots in correspondence to groundwater samples belonging to sandy soils.