Application of dual isotope analysis of nitrate to trace the sources of nitrogen compounds in the Lomellina rice paddy area

The management of nitrogen-based fertilizers is a key agronomic challenge, particularly in Mediterranean paddy environments such as the Lomellina area (Northern Italy), within the main rice-producing district in Italy. Despite the importance of the topic, few studies have investigated nitrate sources and transformations in Mediterranean paddy environments. This study systematically applies dual δ¹⁵N-δ¹⁸O isotope analysis of nitrate in the Lomellina phreatic aquifer to trace the origin and fate of nitrogen-bearing compounds within this rice agroecosystem. Thirteen monitoring campaigns were carried out at 25 monitoring points tapping the phreatic aquifer. Isotopic data were integrated with nitrate concentration measurements, land use information, and records of fertilizers applied at field scale. Results show that the highest nitrate concentrations (up to 164.5 mg/L) are mainly associated with NH₄⁺-based inorganic fertilizers, whereas concentrations between 5 and 15 mg/L can be attributed to a combination of fertilizer inputs, soil organic nitrogen and atmospheric deposition. The study’s findings provide a scientific basis for improving groundwater protection strategies in the Lomellina district and in comparable Mediterranean paddy contexts. This study was carried out in the context of the PROMEDRICE project (https://promedrice.org/; PRIMA Section2–2022) funded, for the Italian partners, by MUR (Italian Ministry of University and Research).