Effect of the nitrification inhibitors 1-amidino-2-thiourea and dicyandiamide in combination with urea and ammonium sulfate

The efficiency of the nitrification inhibitors dicyandiamide (DCD) and 1-amidino-2-thiourea (or guanylthiourea = GTU) in reducing losses from N fertilizers was investigated in two greenhouse experiments where leaching of nitrate-N was induced by percolation at 3 and 5 weeks after fertilization.At an application rate of 10% by weight of fertilizer-N (e.g. 10 kg GTU/ha), GTU in combination with ammonium sulfate (AS) had effects similar to those of DCD (e.g. 15 kg DCD/ha) with regard to nitrate leaching, plant yields and nitrogen uptake. However, in combination with urea (U), GTU was more effective than DCD when applied at the same ratio except with a humic sandy clay soil (pH 7.3, 4.4% organic C), where GTU did not perform as effectively. Nitrate leaching was reduced by as much as 50% using U/GTU instead of U/DCD, and plant yield increased by 30%.At temperatures between 17 and 25°C, the combination U/GTU could protect a high percentage of the nitrogen from being nitrified and leached over a 3 to 5 weeks period. The superiority of GTU over DCD was demonstrated especially in the treatments with 5 weeks of preincubation, despite the considerably lower application rate.