Agricultural practices regulate the seasonality of groundwater-river nitrogen exchanges

Abstract Soil system budgets are generally performed aggregating annual nutrient inputs and outputs over arable land to infer their use efficiency and water pollution risk in watersheds with intensive agriculture and animal farming. They are seldom partitioned into seasonal budgets and matched with seasonal transport of nutrients in adjacent river reaches. In this work we calculated seasonal soil nitrogen (N) budgets in a Mincio River sub-basin (Northern Italy), and we analyzed the dissolved inorganic N net export in the river reach draining such sub-basin. Our results suggest pronounced seasonal differences of soil system budgets with N excess in winter and even more in spring, equilibrium among sources and sinks during autumn and N deficit during summer. Seasonal inorganic N loads transported by the river were not correlated with soil system budgets as they peaked in late summer and were at their minimum in early spring. Fertilization uncoupled to significant uptake supports N excess in winter and spring, whereas crop uptake uncoupled to N inputs supports summer N deficit. Within the river reach potential nitrification cannot explain nitrate accumulation in the water, suggesting alternative dynamics, related to the seasonality of irrigation practices, driving the local hydrology. Flood irrigation results in large soil nitrate solubilization, horizontal and vertical transport and in upward vertical migration of the groundwater piezometric head during the spring and the summer. River water is likely replaced by nitrate-rich groundwater when the groundwater recharge exceeds a certain threshold coinciding with late summer. Irrigation is then interrupted and the piezometric head, together with nitrate exchange, decreases. This work suggests that a deep understanding of N dynamics in watersheds with intensive agriculture and animal farming and irrigation via flooding on permeable soils needs the implementation of hydrological studies and the reconstruction of the vertical pathways of nitrate and of river-groundwater interactions. Moreover, the partitioning of annual into seasonal N budgets and their combination with irrigation practices allows the identification of hot moments in N cycling. Agricultural practices minimizing nitrate excess, its mobility and the risk of ground and surface water pollution are suggested for this area.