## 2 Materials and Methods
### N Budget in Agricultural Soils
The area of each municipality included within the Po di Volcano-Sacca di Goro watershed boundaries was calculated by clipping the shape-file of the Italian administrative boundaries (National Statistics Institution, 2010; [[https://www4.istat.it/it/archivio/24516](https://www4.istat.it/it/archivio/24516)]([https://www4.istat.it/it/archivio/24516](https://www4.istat.it/it/archivio/24516))) with the shape-file of the Po River sub-basins (geoportal of the Environmental Protection Agency of the Emilia-Romagna Region; [[https://www.arpae.it/dettaglio_generale.asp?id=1177&idlivello=1527](https://www.arpae.it/dettaglio_generale.asp?id=1177&idlivello=1527)]([https://www.arpae.it/dettaglio_generale.asp?id=1177&idlivello=1527](https://www.arpae.it/dettaglio_generale.asp?id=1177&idlivello=1527))). Budget calculations relied on simple equations converting agricultural census data into N fluxes, but the approach got its robustness from the use of high-spatial-resolution datasets, site-specific agronomic coefficients, and N cycling measurements gathered from the study area. Input, output, and the overall budget were expressed in units of mass per time (t N-year\({}^{-1}\)) and on a per-area basis, after normalization for the UAA (kg N-ha\({}^{-1}\).yr\({}^{-1}\)). N input from livestock manure (N\({}_{\text{Man}}\)) was calculated by means of (1) livestock density data (divided into eight major categories and 30 sub-categories according to type, age, and purpose), (2) live weight of each livestock category, and (3) N excretion rate of each livestock category corrected for the N amount volatilized as NH\({}_{3}\) during animal housing and manure storage (DM 4 July 2006, decree of the Italian Ministry of Agricultural and Forest Policies about agronomic utilization disciplinary; Table S1, Supplementary Materials). Livestock data at the municipal level were acquired from the National Veterinary Information System of Livestock Registry and the National Statistics Institution(Table 1). Manure was assumed to be applied as fertilizer to agricultural land within the municipality where each farm was located. N input from synthetic inorganic fertilizers (N\({}_{\text{Fert}}\)) was calculated by means of (a) the surface of each crop category cultivated in the study area, and (b) local crop-specific fertilizer recommendations according to the soil types (Table S2, Supplementary Materials). Specific N application rates were retrieved from a detailed inventory conducted within the framework of the EC project EU-WATER (\"Transnational integrated management of water resources in agriculture for the European WATER emergency control\", contract n. SEE/A/165/2.1/X). Municipal crop surfaces divided into the main soil types in which each crop category is cultivated were extracted from a high-resolution land-use map provided by the Agriculture Department of Ferrara Province (Table 1). Twenty crop categories were considered, accounting for more than 95% of the harvested surface in the whole basin. N input from biological fixation (N\({}_{\text{Fix}}\)) was calculated by means of (a) surface of each N-fixing crop type (alfalfa and soybean) and (b) areal rates of symbiotic N fixation. Rates of biological N fixation were estimated by multiplying the production per unit of surface (i.e., yield) by the N uptake coefficient of the harvested portions (Table S2, Supplementary Materials), corrected for a multiplicative factor expressing the ratio of total biomass produced to harvested biomass. According to [PERSON] and co-authors [37], values of 1.7 and 1.3 were used for alfalfa and soybean, respectively. Crop data, i.e., municipal surfaces and provincial yields for each crop type, were provided by the Agriculture Department of Ferrara Province. Spatialized data on atmospheric N deposition on agricultural soils (N\({}_{\text{Dep\_UAA}}\)) were obtained by integrating annual wet deposition and average rainfall N content (inorganic oxidised N forms, i.e., nitrate plus nitrite, and organic N).