Nitrification inhibition and dose-dependent effect of dicyandiamide on sandy, loamy and clayey soils

Ammonium and nitrate are the main forms of available nitrogen (N) for plant. Nitrate is the dominant form of N in upland soils in arid regions. However, nitrate can easily leach through soil profiles. Furthermore, N can easily be lost in the form of NOx via (de)nitrification. Inhibiting the processes of nitrification in soil through nitrification inhibitors is therefore critical for optimizing soil NH₄⁺/NO₃^- ratio and improving the efficiency of N fertilizers. As one of the most common nitrification inhibitors, dicyandiamide (DCD) was applied at different rates (0%, 1.0%, 2.0%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 6.0% and 7.0% of applied N) in this study. And incubation experiment was conducted in sandy, loamy and clayey desert soils at 25 oC to investigate the inhibition effect, dose-dependent effects of DCD on soil nitrification in arid regions. The study may further enhance our existing knowledge on the mechanisms of DCD inhibited nitrification. The results showed that during 30 days of incubation, DCD exhibited significant inhibition effects on nitrification in all the tested soil types. Nitrification inhibition rates were 96.5%~99.4% in sandy soil, 66.9%~85.6% in clayey soil and 49.3%~79.4% in loamy soil. For the three soil types, DCD nitrification inhibition efficacy was in the following order: sandy soil > clayey soil > loamy soil. When DCD application rate increased from 1.0% to 7.0% of applied nitrogen, nitrate concentration in sandy soil merely increased from 1.9 to 10.7 mg·kg^-1. This suggested that DCD dose did not significantly affect nitrification in sandy soils. However, soil nitrate concentration in loamy and clayey soils decreased sharply with increasing rates of DCD, indicating significant dose effect on soil nitrification. In conclusion, DCD significantly inhibited soil nitrification in calcareous desert soils. The recommended optimum application rates of DCD in sandy, clayey and loamy soils were 6.0%, 7.0% and 7.0% of applied nitrogen, respectively.