Suppression of nitrification and N2O emission by karanjin––a nitrification inhibitor prepared from karanja (Pongamia glabra Vent.)

A laboratory incubation study was undertaken to study nitirification and N2O emission in an alluvial, sandy loam soil (typic ustochrept), fertilized with urea and urea combined with different levels of two nitrification inhibitors viz. karanjin and dicyandiamide (DCD). Karanjin [a furanoflavonoid, obtained from karanja (Pongamia glabra Vent.) seeds] and DCD were incorporated at the rate of 5%, 10%, 15%, 20% and 25% of applied urea-N (100 mg kg−1 soil), to the soil (100 g) adjusted to field capacity moisture content. Mean N2O flux was appreciably reduced on addition of the inhibitors with urea. Amounts of nitrified N (i.e. (NO3−+NO2−)–N) in total inorganic N (i.e. (NO3−+NO2−+NH4+)–N) in soil were found to be much lower on the addition of karanjin with urea (2–8%) as compared to urea plus DCD (14–66%) during incubation, indicating that karanjin was much more potent nitrification inhibitor than DCD. Nitrification inhibition was appreciable on the application of different levels of karanjin (62–75%) and DCD (9–42%). Cumulative N2O–N loss was found to be in the range of 0.5–80% of the nitrified N at different stages of incubation. Application of karanjin resulted in higher mitigation of total N2O–N emission (92–96%) when compared with DCD (60–71%).