Nitrification inhibitors (NI) are aimed at improving N-fertilizer use efficiency in cropping systems. This study aimed to assess the nitrification inhibition potential and non-target effects of dry leaf powders (botanicals) of ten plant species [neem (Azadirachta indica), lantana (Lantana camara), karanda (Pongamia pinnata), Brachiaria humidicola, cinnamon (Cinnamomum verum), clove (Syzygium aromaticum), wild-sunflower (Tithonia diversifolia), mee (‎Madhuca longifolia), nutmeg (Myristica fragrans) and pepper (Piper nigram)]. The effect of botanicals on the growth of three ammonia oxidizing bacteria isolates (M4, M5 and M7) and NO3- formation in soil were tested in laboratory experiments. In a pot-experiment, botanicals were applied with urea to assess their effect on vegetative-growth of tomato and capsicum. The non-target effects of botanicals on soil bacteria and fungi, and seed germination were assessed separately. Dicyandiamide (DCD) was used as the positive control in all experiments. Only the growth of M7 was significantly suppressed by all botanicals except neem, Brachiaria and pepper. The highest shoot-biomass of tomato was obtained when urea was applied with pepper. Nitrate leaching from pots was significantly reduced (p<0.05) when urea was added with botanicals except for clove and wild-sunflower at basal-dressing. The abundance of culturable fungi and bacteria were not affected significantly by botanicals while seed germination was reduced significantly and consistently by clove only. From the ten botanicals tested leaf powders of nutmeg, lantana, cinnamon, mee, and pepper were identified as materials with better potential to suppress nitrification with minimum non-target effects.

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Nitrification tend to reduce the agronomic fertilizer use efficiency in cropping systems because nitrate, the end product of nitrification, has high potential to loss from soil environment due to denitrification and leaching. The population characteristics of ammonia oxidizing and nitrite oxidizing microorganisms carrying out nitrification in soils may affect the effectiveness of approaches taken to suppress nitrification. A study was conducted to assess the activity and abundance of nitrifiers in soils of intensively vegetable grown regions in Sri Lanka. Soil samples were collected from 72 locations across Nuwara Eliya, Marassana, Kalpitiya and Gannoruwa representing vegetable cultivated fields managed conventionally (n=45) and organically (n=9), and uncultivated areas (n=18). Basic soil characteristics were determined using standard procedures. Potential nitrification rate (PNR) and the abundance of ammonia oxidizers and nitrite oxidizers were estimated. Chemical characteristics of soils varied widely with pH, EC and organic C% ranging from 3.8 to 8.5, 0.04 to 0.94 dS/m and 0.9% to 4.5%, respectively. The PNR of the studied soils ranged from 0.18 to 15.80 NO3 --N/kg/h. The abundance of ammonia oxidizers and nitrite oxidizers ranged from 1.96 to 5.97 log10CFU g -1 and 1.36 to 5.63 log10CFU g -1 , respectively. The potential activity of nitrifiers did not correlate with the abundance of ammonia oxidizers or nitrite oxidizers. Thus, the functional and compositional diversities of nitrifying communities may be different across the soils. PNR values for studied soils are higher compared to reported values in literature. Hence, appropriate measures need to be taken to suppress nitrification as high nitrification rates could lead to reduce fertilizer use efficiency and increase risk of groundwater contamination with nitrate.