## 2 Materials and Methods
### N Budget in Agricultural Soils
Reduced inorganic N forms were not considered as they were already taken into account in the budget as local redeposited N after NH\({}_{3}\) volatilization (see the following description of the budget item N\({}_{\text{Vol}}\)). Indeed, deposition of reduced N likely reflects local recycling as NH\({}_{3}\) is short-lived in the atmosphere [38]. For each year of the investigated period, rainfall data were recorded at seven weather stations homogeneously distributed across the watershed by the Hydro-Meteorological Service of the Environmental Protection Agency of the Emilia-Romagna Region (Table 1). N content of wet depositions was measured in 2008 on occasion of several relevant rainfall events (\(n=51\), rainfall intensity >3 mm), within the framework of the EC project EU-WATER. Four stations homogeneously distributed across the watershed and localized in agricultural areas representative of the four main soil types were sampled. As variability of rainfall N content among stations and sampling events was small, a constant average value of 0.5 mg N\(\cdot\)L\({}^{-1}\) was assigned to the average precipitation amount of each year within the study period (range 550-650 mm). The obtained areal rates of N deposition were in agreement with those adopted by the Italian Code of Good Agricultural Practices (DM 19 April 1999, decree of the Italian Ministry of Agricultural and Forest Policies) reporting a range of total N deposition in the Po River plain between 10 and 20 kg N\(\cdot\)ha\({}^{-1}\cdot\)year\({}^{-1}\) with 45% on average in reduced inorganic N form. N output from crop harvest (N\({}_{\text{Harv}}\)) was calculated by means of (a) surface of each crop type cultivated in the study area, (b) N uptake coefficient, and (c) yield of the harvested portion of each crop (Table S2, Supplementary Materials). For N-fixing crops, the N amount exported from agricultural land was assumed equal to that fixed in the above-ground biomass. As crop residues are usually left on the fields, N in crop residues was not accounted for as an output in the budget. Crop data, i.e., municipal surfaces and provincial yields for each crop type, were provided by the Agriculture Department of Ferrara Province, while crop N uptake coefficients were retrieved from a local inventory acquired within the framework of the Rural Development Program 2007-2013 of the Emilia-Romagna Region. N\({}_{\text{Runoff}}\) to surface waters was estimated using the difference by computing a detailed N balance in the hydrological network of the Po di Volano-Sacca di Goro watershed, as described in Section 2.3. N output from NH\({}_{3}\) volatilization (N\({}_{\text{Vol}}\)) of synthetic fertilizers was calculated by employing experimental data obtained in laboratory batch experiments [39]. Briefly, N loss via NH\({}_{3}\) volatilization was measured after urea application on the four main soil types of the study area under the standard fertilization regime and simulating the in situ microclimate (i.e., wind speed, humidity, and air temperature). Total NH\({}_{3}\) volatilization fluxes of 1.8%, 0.7%, 0.4%, and 0.2% of the average fertilization rate (240 kg N\(\cdot\)ha\({}^{-1}\)) were obtained for sand, silty loam, peat and silty clay, respectively [39]. Due to low livestock density in the Po di Volcano-Sacca di Goro watershed, measurements of N loss via NH\({}_{3}\) volatilization following manure spreading are not available. After an exhaustive revision of the literature, volatilization factors of 13% and 33% were associated with spreading operations of chicken manure and other livestock manure [40; 41; 42], respectively. The employed volatilization factors were coherent to direct emission estimations performed in lowland areas of Northern Italy [38; 43]. Due to the short NH\({}_{3}\) life span in the atmosphere, a fraction of the volatilized N amount was considered redeposited within the area where it was emitted (60%) [41] and, as a consequence, only the remaining portion was treated as N output in the soil budget.