Nitrogen dynamics and natural attenuation in the unsaturated zone: Insights from a high-resolution study in the Arborea NVZ (Italy)

The intensification of agricultural and livestock activities has historically led to the surplus application of organic and chemical fertilizers, challenging the sustainability of agroecosystems. Despite regulatory efforts like the EC Nitrate Directive, nitrogen (N) losses remain a critical issue in Nitrates Vulnerable Zones (NVZs). Central to this challenge is the unsaturated zone, a key but often neglected that regulates water movement and the leaching of N-species from the soil to the aquifer. This study explores nitrogen dynamics and natural attenuation mechanisms within the soil-water-rock continuum to identify strategies for minimizing environmental impacts while sustaining productivity. Research was conducted at a pilot site in the Arborea Plain (Sardinia, Italy), an NVZ characterized by intensive dairy farming. We implemented a high-resolution monitoring system—including TDR probes for volumetric water content and suction cups for pore-water sampling—to track N-transformations through the soil profile. By integrating chemical analysis with nitrate isotopes, we evaluated the biogeochemical processes and natural attenuation mechanisms occurring during transport through the unsaturated zone. Our findings highlight how seasonality and agricultural practices influence nitrogen dynamics, with deeper soil layers often accumulating higher nitrate concentrations due to complex, overlapping reactions. This research bridges the gap between fundamental process understanding and practical applications, providing essential data to optimize nitrogen management. The results offer a scientific basis for developing targeted mitigation strategies to safeguard groundwater resources while sustaining agricultural productivity.