## 1 Introduction  
Nitrogen (N) pollution is a still unsolved global environmental issue and with multiple implications in terms of water quality deterioration, biodiversity loss, human health problems, global carbon-cycle alterations, and climate change [1; 2; 3]. Reactive N sources from intensive agriculture are recognized as one of the most serious threats in industrialized and emerging countries, determining eutrophication and groundwater nitrate (NO\({}_{3}\)\({}^{-}\)) contamination [4; 5; 6]. However, uncertainty remains on processes and paths, since about 75% (range 40-95%) of the N load generated within worldwide catchments is assessed not to be delivered to terminal waterbodies, via river export, but internally retained [7; 8; 9]. This high retention capacity may mask both permanent (i.e., denitrification in soils and aquatic environments) and temporary removal processes (i.e., storage in the vadose zone-groundwater system). Denitrification, the microbial-mediated reduction of NO\({}_{3}\)\({}^{-}\) to di-nitrogen gas (N\({}_{2}\)) under anaerobic conditions, is globally considered the primary biogeochemical process via which mineral N is returned to the atmosphere [10; 11; 12]. Whether N is denitrified and lost to atmosphere as N\({}_{2}\) or accumulates in the vadose zone and in groundwater [13; 14] makes a huge difference in an environmental perspective and from the point of view of policymakers and stakeholders. Thus, assessing the fate of missing N is a major scientific challenge for defining codes of good agricultural practices and implementing effective management strategies aimed at protecting water quality in human-impacted watersheds. N surplus from fertilizers in intensive agricultural systems usually promote NO\({}_{3}\)\({}^{-}\) leaching and accumulation in groundwater, especially in lowland areas where surface runoff is minimal due to the flat topography. On the other hand, rainfall or irrigation events and poor drainage are expected to promote denitrification in fertilized fine-textured soils by decreasing redox conditions and triggering the creation of anaerobic microsites prone to anaerobic respiration pathways [15; 16; 17]. The interesting question is to evaluate the net effect of these two potentially opposite features in agricultural-dominated watersheds. Due to the complexity of field measurements, soil denitrification is rarely addressed as the main focus of research in watershed N budgets, and it is generally estimated as a residual term, i.e., the unaccounted complement of the N balance, with a great deal of uncertainty [10]. However, whereas N inputs, outputs, and storages can be accurately quantified by using high-resolution local datasets, mass balance approach may provide a reliable estimate of denitrification at the catchment scale and a benchmark value for field measurements. The aim of the present work was to evaluate the relative importance of soil denitrification in buffering N excess at the watershed scale in a deltaic, intensively cultivated, and irrigated sub-basin of the Po River plain, the Po di Volcano-Sacca di Goro. This basin was selected as a case study for the following reasons: (1) the homogeneous cropping system, i.e., maize and winter cereal rotation in about two-thirds of the agricultural lands developing in a completely flat territory; (2) the N fertilization supplied almost exclusively as synthetic urea and ammonium nitrate; (3) the capillary network of canals distributing irrigation water in the spring-summer period; (4) the extensive information available on hydrogeological and microbial drivers regulating N processing and transport in surface waters [18; 19], soil water [20; 21], and groundwater [22; 23]. Agricultural statistics, water quality data, and locally derived measurements of several N fluxes and pools were combined to set up a comprehensive and detailed N budget along the terrestrial-freshwater continuum.