Effects of the nitrification inhibitor dicyandiamide (DCD) on gross N transformation rates and mitigating N2O emission in paddy soils

The nitrification inhibitor dicyandiamide (DCD, C2H4N4) is effective in suppressing nitrification and N2O emission in many agro-ecosystems. However, whether DCD affects other nitrogen (N) transformation processes, and the mechanisms of its inhibitory effects on N2O emission, are not fully understood. In the present study, we conducted a 15N dilution incubation experiment to investigate the effects of DCD on gross N transformation rates and N2O emissions in two paddy soils under an intensive irrigated rice-upland crop rotation system. The duration of microbial inhibition by DCD was also evaluated. We used the numerical model FLUAZ, based on the 15N tracing technique, to calculate gross N transformation rates. The results showed that DCD could effectively inhibit gross N nitrification in the tested soils; however, no influence on N mineralization and immobilization was detected. Lower N2O emissions from both soils were observed in the presence of DCD, due to the dual functions of DCD on both nitrification rates and the fraction of N2O in nitrification products. While, further study to confirm our results by direct measurement of DCD in reducing the fraction of N2O in nitrification products should be conducted. The inhibitory effect of DCD on gross N nitrification and N2O emission decreased over time, and was not statistically significant four weeks after the addition of this inhibitor to the soils.