Reusing water, reshaping nitrogen: Unravelling nitrogen dynamics in soils receiving treated wastewater

Soils are increasingly receiving treated wastewater (TWW) to support crop production and protect natural resources. However, besides water, various inputs, such as soil nutrients (salts), organic carbon, and various contaminants of emerging concern, including antibiotics, antibiotic resistance genes, pharmaceutical compounds, heavy metals, and microplastics, can be delivered. This review aims at (a) an integrated and mechanistic synthesis of current knowledge on potential impacts of TWW irrigation on key soil nitrogen turnover processes, such as nitrogen mineralization, nitrification, denitrification, and ammonia volatilization, and their drivers, and (b) identification of key knowledge and conceptual uncertainties to support future studies. Across the synthesis, no single, uniform TWW effect emerges. In fact, outcomes are consistently conditional on several co-factors, such as treatment level, irrigation duration, soil type, and plant species, with the latter two sometimes exerting effects exceeding those of water quality itself. TWW alters soil conditions through substrate availability, redox dynamics, soil acidity and salinity, and contaminant exposure, but the mechanistic evidence linking specific drivers to specific nitrogen-process outcomes is itself uneven. Identified knowledge and methodological gaps suggest long-term field studies across TWW regimes, soil types, and climates; designs capable of disentangling co-varying drivers at field scale; and laboratory studies addressing contaminant-specific impacts, microbial functional persistence, and potential decoupling between nitrogen and carbon dynamics, an effect already evident, though unexplored as a general phenomenon, within the reviewed literature. Overall, this review offers an integrated, system-level framework to support the evidence-based management of environmentally and climate-vulnerable soils receiving reclaimed or even conventional irrigation water.