Asian Journal of Soil Science and Plant Nutrition

Rapid nitrification under warm humid climate in the tropics leads to high nitrogen losses from soil. Understanding the factors deciding the performance of dicyandiamide (DCD) as a nitrification inhibitor (NI) is important for its applications in tropical agriculture. This study investigated the effect of soil type, temperature, and nitrogen (N) source on performance of DCD (at 10 mg kg^-1 rate) using tropical soils in laboratory-scale experiments. The percent inhibition of potential nitrification rate (PNR) by DCD was evaluated using eight soil types with contrasting properties. Then two soils (Rhodustalfs and Typic Quartzipsamments) were treated with DCD and urea, and NO₃^- produced during first 48 h after incubating at 21±1, 24±0 and 29±1 °C was investigated. Further NO₃^- released from a Typic Quartzipsamments over nine-days after applying urea, compost and poultry-manure, with and without DCD was determined. PNR was significantly different (P = 0.05) among soil types, and DCD inhibited nitrification to different levels (14-70%). The highest suppression of PNR by DCD when urea was not applied was registered at 21 °C and 29 °C for Rhodustalfs (55%) and Typic Quartzipsamments (80%), respectively. But DCD could not hold the same level of suppression at these temperatures when urea was applied. Although DCD effectively suppress nitrification when applied with urea, it was not effective as a NI when applied with compost or poultry manure. Results confirmed that the effectiveness of DCD is not consistent in tropical soils, and varied with soil type, temperature and N-source applied to soil.  The findings have implications in developing strategies targeting to improve nitrogen use efficiency in crop cultivation in the tropics under changing climate.