Organic Inhibitor-Based Smart Urea Formulations Modulate Soil Nitrogen Dynamics and Reduce Ammonia Volatilization Across Contrasting Soils

Smart Urea Formulations (SUFs) incorporating urease and nitrification inhibitors represent a promising strategy to reduce environmental N losses while improving fertilizer efficiency. This study evaluated the effects of two novel SUFs amended with gallic acid (GA) or methylene blue (BM) on soil N transformation processes. A 21-day microcosm incubation was conducted on three contrasting soils. Soil inorganic N dynamics (NH4+ and NO3−), NH3 and CO2 emissions were monitored, while urease and protease activities were measured as indicators of key biochemical processes involved in N cycling. Ecotoxicological assays (seed germination and root elongation) were also performed. GA and BM modified the temporal dynamics of soil N transformations compared with conventional urea, delaying early NH4+ accumulation and altering NO3− formation patterns across soils. Urease activity changed in response to fertilizers during the incubation across contrasting soils. SUFs also increased CO2 emission and protease activity, which may reflect changes in microbial processes involved in soil C and N cycling. SUFs effectively modulated soil N cycling with a moderate phytotoxic effect comparable with a commercial inhibitor (i.e., NBPT). These findings highlight the potential of the two novel SUFs to regulate soil biochemical processes and N transformations, supporting their further evaluation under field conditions.