## 2 Materials and Methods
### N Budget in the Hydrological Network  
N inputs and outputs in the hydrological network of the Po di Volcano-Sacca di Goro watershed were included in the following equation:  
\[\left(\text{N}_{\text{Inflow}}+\text{N}_{\text{Urb}}+\text{N}_{\text{Ind}}+ \text{N}_{\text{Dep}}+\text{N}_{\text{Runoff}}\right)=\left(\text{N}_{\text{ Outflow}}+\text{N}_{\text{Rem}}\right), \tag{2}\]  
where N\({}_{\text{Inflow}}\) is N load imported in the basin by the canal network, N\({}_{\text{Urb}}\) is N load from urban areas, N\({}_{\text{Ind}}\) is N load from industrial areas, N\({}_{\text{Dep}}\) is atmospheric deposition on surfaces other than agricultural soils, N\({}_{\text{Runoff}}\) is N lost in runoff water to surface waterbodies, N\({}_{\text{Outflow}}\) is N load exported from the basin by the canal network, and N\({}_{\text{Rem}}\) is N removed by denitrification in the canal sediments.  
N loads imported through the irrigation water (N\({}_{\text{Inflow}}\)) and exported from the basin through the drainage water (N\({}_{\text{Outflow}}\)) were calculated combining discharge and N concentration datasets for the considered five-year period 2006-2010. Flow data were provided by the local water management authority (i.e., Consorzio di Bonifica Pianura di Ferrara) as daily and, in some cases, monthly volumes. Nitrogen species concentrations were measured at five stations (three for the inflow water and two for the outflow water, with frequency of sampling from monthly to fortnightly) located on the hydrological network of the Ferrara province and belonging to the official surface water monitoring network of the Emilia-Romagna Regional Agency for Environmental Protection (ARPAE). N loads were calculated by interpolating linearly the N species concentrations between two subsequent measurements [44; 45]. This method was revealed to be suitable for nitrate load estimation in small lowland watercourses characterized by sudden changes in hydrological regime [31; 46].  
N\({}_{\text{Runoff}}\) from agricultural soils to surface waters was estimated using the difference after all the other budget items were quantified by combining statistics on human pressures and surface water quality datasets for the five-year period 2006-2010. N load generated from urban areas (N\({}_{\text{Urb}}\)) was calculated by multiplying the resident population of the municipalities included within the basin by the per capita production of 12.5 g N\(\cdot\)day\({}^{-1}\)[47] (Table 1). About 85% of the population is connected to wastewater treatment plants with a tertiary treatment for N removal via denitrification, reaching on average 75% of the incoming loads.  
N load generated from industrial areas (N\({}_{\text{Ind}}\)) was calculated by considering the number of workers in the different N-polluting industrial sectors and their specific N production coefficients [48]. Industrial N load was calculated as a mean of the years 2001 and 2011, the last years for which data on industrial activities were available at the municipality scale (Table 1). N input from atmospheric deposition on surfaces other than agricultural soils (N\({}_{\text{Dep}}\)) was calculated by extending the areal rates of N deposition obtained as previously described in Section 2.2 to the total area of the basin with the exclusion of surfaces devoted to agriculture.  
[PERSON] and co-authors [49] previously estimated the current capacity to remove N via denitrification (N\({}_{\text{Rem}}\)) of the whole canal network of the Po river lowland (18,500 km) by means of a detailed upscale of large datasets of field measurements. The contribution of the sole canal network belonging to the Po di Volano-Sacca di Goro (1780 km) was extracted by GIS analysis.