The dramatic increase of anthropogenic reactive nitrogen (N) inputs in watersheds with intensive agriculture and animal farming has demonstrated negative effects for inland water and groundwater chemical and biological quality, drinking water supplies, ecosystem integrity and functioning and human health (Van Grinsven et al. 2006; Galloway et al. 2008; Rivett et al. 2008; Schlesinger, 2009; Sobota et al. 2015; Huang et al. 2017). Such negative effects are amplified by the human-derived alteration of the hydrological cycle at the watershed scale and by climate change (Galloway et al. 2008; Overeem et al. 2013; Woolway and Merchant, 2019; Woolway et al. 2020). Among the underlying mechanisms are water abstraction for irrigation or industrial purposes or climate change-related drought reducing river discharge and its capacity to dilute and process N loads (Palmer et al. 2008). Low discharge promotes also hyporheic anoxia and ammonium recycling from sediments (Hlaváčová et al. 2005). Hydrological extremes include also short-term, heavy precipitations resulting in high discharge events transferring large N loads from cultivated areas saturating riverine denitrification capacity (Viaroli et al. 2018; Magri et al. 2019).
Nitrogen budgets calculated for agricultural soils within a river basin allow to evaluate the potential risk of diffuse N pollution (Oenema et al. 2003; Soana et al. 2011). In agricultural soils, N inputs associated with organic or synthetic fertilizers, atmospheric deposition or biological fixation can be either temporarily retained in crops or released to the atmosphere as gaseous losses. Nitrogen inputs in excess to temporary retention or permanent loss can be transferred via runoff to adjacent aquatic ecosystems (Howarth et al. 1996; Seitzinger et al. 2006; Pinardi et al. 2018, 2020). If soil system budgets in arable land produce reliable snapshots of N pools and fluxes in cultivated areas, the detailed reconstruction and partitioning of N pools and fluxes within watersheds is a challenging objective. For example, seasonally variable water inputs to agricultural soils via precipitation and irrigation affect soil N leaching, horizontal and vertical transport and transformation, N use efficiency as well as river-groundwater interactions and associated N exchange (Schaefer and Alber, 2007; Howarth et al. 2012; Sinha and Michalak 2016). Moreover, in intensively cultivated floodplains the hydrological cycle has been regulated by the realization of infrastructures as dams and networks of canals that help buffering climatic anomalies and ensure water availability for crops. In Italy for example, the Alpine sector of the Po River basin hosts large dams that regulate the release of water from deep subalpine lakes (Maggiore, Como, Iseo, Idro and Garda Lakes) to their emissaries (Ticino, Adda, Oglio, Chiese and Mincio Rivers). Winter water retention in subalpine lakes occurs at the cost and drawbacks of reduced water discharge and contributes to the downward vertical migration of groundwater, often resulting in downwelling river-groundwater interactions (i.e. the river feeds the groundwater) (Rotiroti et al. 2019; Severini et al. 2021). On the contrary summer irrigation, besides representing a vehicle for N transport, produces opposite effects, often reversing the direction of river-groundwater interactions (i.e. upwelling, the groundwater feeds the river). These practices, that characterize anthropogenic, intensively cultivated, and hydraulically regulated watersheds with permeable soil, introduce marked seasonality in N budgets (Lin et al. 2019; Racchetti et al. 2019).
Many authors reported a significant correlation between annual N input to croplands and river N export (Neff et al. 2003; Yan et al. 2010; Xu et al. 2013; Strokal et al. 2014; Tong et al. 2017), but they did rarely account for the seasonality of N input and export (McCrackin et al. 2014; Chen et al. 2019). Studies targeting N budgets in agricultural watersheds are generally conceived at the annual scale for mainly practical reasons, as agricultural census data are collected and published by national statistical institutions with annual frequency. Such an approach from one side allows to calculate N use efficiency in cropland and potential N loss, but from the other side, it misses temporal resolution and precludes the understanding of seasonal variations of the array of N-related processes, potentially regulated also by climate change. For example, human activities (e.g., crop production) and altered hydrology may influence the seasonality of N river export (Basu et al. 2010; Compton et al. 2020), together with the seasonal evolution of temperature that influences N losses, retention and removal processes (e.g., denitrification) (McCrackin et al. 2014). Understanding how seasonal variations in human activities and hydrology influence N budgets in agricultural soils and N transport by rivers is important to better understand the mechanisms underlying N transformations along the terrestrial-aquatic path, improve agricultural practices to increase N use efficiency and decrease N pollution, and eventually forecast how climate change will affect N dynamics (Mas-Pla and Menció, 2019). This important set of objectives is a difficult target at the scale of whole watersheds due to scarce resolution of available data and spatial heterogeneity (e.g. pedology, land use, etc). Smaller scales of analysis, targeting specific and homogeneous river and watershed sectors, seem much more promising (McCrackin et al. 2014; Chen et al. 2019; Compton et al. 2020).
Different studies carried out at large temporal and spatial scales (Soana et al. 2011; Pinardi et al. 2018; Viaroli et al. 2018; Lassaletta et al. 2021) have highlighted the presence of hot-spots within watersheds that represent outliers in N budgets (e.g., with very large N excess or very low N use efficiency). They also emphasized the presence of hot-moments within watersheds, that are specific periods during which N mass transfer peaks as a combination of decreased uptake, increased runoff or variation of the water table level, resulting in the reactivation of river-groundwater interaction (Rosenzweig et al. 2008; Preisendanz et al. 2020; Taherisoudejani et al. 2018). The analysis of N hot-spots and hot-moments in watersheds require specific studies, focusing on small spatial and temporal scales.
In Northern Italy, the Po River valley is an alluvial plain heavily exploited by human activities such as agriculture, animal farming, industry, and tourism. Land use change and hydrological alterations determined high pressure on both surface and groundwater (May, 2013; Pérez-Martín et al. 2014; Lasagna and De Luca, 2019) and a wide portion of the plain is classified as vulnerable to nitrate pollution (Martinelli et al. 2018). The main aim of this study is to analyze the seasonal evolution of dissolved inorganic N loads in a fluvial segment of the Mincio River, a tributary of the Po River, characterized by natural banks, gravel bottom with submerged vegetation, and regulated discharge. This segment crosses a transitional area between permeable and non-permeable soils, characterized by springs and classified as an area of river-groundwater interactions (Balestrini et al. 2021). Due to its hydrogeological features and the large water availability, the considered sub basin is a hotspot of intensive agriculture and animal farming and represents a key study area to analyze if and how the seasonality of agricultural practices affects N dynamics.
In this sector of the Po River, groundwater in the phreatic and shallow aquifer has a short residence time as compared to semiconfined or confined deeper aquifers. This is supported by fast (few days) surface-groundwater dynamics of micro-pollutants (Balderacchi et al. 2016) and low concentrations of total dissolved solids (Martinelli et al. 2018). Results of Balderacchi et al. (2016) suggest also fast response of shallow aquifers to changing conditions; as such they allow to trace agricultural practices (e.g., use of herbicides or fertilization) and they respond quickly to hydrologic variations (e.g., drought, precipitations, irrigation). It can be assumed that macrocontaminants as nitrates undergo the same fast transfer mechanisms, also due to their elevated solubility and absence of interaction with soil and sediment.
The main hypotheses of this work are that river-groundwater interactions affect N transport in specific river sectors and vary seasonally due to combination of irrigation practices and inorganic nitrogen excess in soil. We also hypothesized that the seasonal dynamics of such variable interactions can be captured analyzing comparatively seasonal N budget in agricultural soils and seasonal riverine N transport.