Potential of pongam oil and karanjin as natural nitrification inhibitors: mechanistic effects on nitrifying microorganisms and soil nitrogen transformation.

Nitrification inhibitors delay the microbial oxidation of ammonium to nitrate, thereby improving nitrogen use efficiency and reducing nitrate losses and nitrous oxide emissions from fertilized soils. Although synthetic inhibitors such as dicyandiamide (DCD) are effective, plant-derived alternatives are increasingly sought because of their environmental compatibility and renewable origin. Karanjin, a furanoflavonoid present in Pongamia pinnata I (L.) seed oil, has long been recognized as a potential nitrification inhibitor, but its contribution within crude pongam oil and its influence on nitrifying microorganisms and soil nitrogen transformation remain poorly understood. This study evaluated crude pongam oil and purified karanjin using in vitro assays with Nitrosomonas europaea and Nitrobacter winogradskyi, together with a complementary soil microcosm experiment. Crude pongam oil contained 3.24% karanjin and 0.45% pongamol and was compared with neem oil and DCD. Both pongam oil and karanjin inhibited nitrifier growth in a concentration-dependent manner and preferentially suppressed ammonia monooxygenase- and hydroxylamine oxidation-associated activities over nitrite oxidoreductase activity, with inhibitory efficacy ranked as DCD > karanjin > pongam oil > neem oil. In urea-amended soil, these treatments enhanced NH₄⁺ retention while reducing NO₂⁻ and NO₃⁻ accumulation, demonstrating consistent alterations in nitrogen transformation dynamics. Multivariate analyses, including correlation, principal component analysis, hierarchical clustering, and relative efficacy assessment, identified karanjin as the most effective biological treatment. Pongam oil and karanjin also reduced quorum-sensing-related activity, suggesting that disruption of microbial communication may contribute to nitrification inhibition, although this mechanism requires further validation in nitrifying microorganisms. Overall, the findings support pongam oil as a promising plant-derived nitrification inhibitor and provide a basis for developing pongam-based coated urea and other bio-based nitrogen stabilizers for sustainable nutrient management.