Effects of dicyandiamide on N₂O and CO₂ emissions and the underlying driving factors in greenhouse vegetable fields

Excessive nitrogen fertilizer inputs in greenhouse vegetable field intensify soil carbon and nitrogen migration and transformation, and drive the production and emissions of greenhouse gases nitrous oxide (N₂O) and carbon dioxide (CO₂), thus contributing to global climate change. Nitrification inhibitors (such as dicyandiamide, DCD) are widely utilized as a crucial measure to alleviate nitrogen losses and greenhouse gas emissions. Herein, a experiment was conducted out at the Changsha Agricultural and Environmental Monitoring Station in Hunan Province to explore the impacts of DCD on N₂O and CO₂ emissions and their key environmental drivers in greenhouse vegetable field located at the Changsha Agricultural and Environmental Monitoring Station in Hunan Province. Two treatments were established, conventional chemical nitrogen fertilization (CK) and chemical nitrogen fertilization combined with dicyandiamide (DCD), Throughout the entire growth period of vegetables, the emission fluxes of N₂O and CO₂, as well as soil physicochemical properties, were monitored regularly. The results showed that compared with the CK treatment, DCD application significantly reduced N₂O flux during the early stage of vegetable planting (0–53 days)(P<0.05). but there was no significant difference between the two treatments during the later stage (54–163 days). The overall cumulative N₂O emission during the whole growth period under the DCD treatment was 0.94 kg (N)·hm⁻², which was significantly 21.67% lower than that under the CK treatment (P < 0.05). Different from N₂O emission, DCD application had no significant effect on the CO₂ fluxes and its cumulative emission the cumulative emissions were 1,778.79 and 1,731.02 kg(C)∙hm⁻² under the DCD and CK treatment, respectively. Throughout the entire growth period, the DCD application did not significantly alter soil temperature and water-filled pore space (WFPS) in greenhouse vegetable fields, but significantly increased soil ammonium nitrogen (NH₄⁺-N) content (P < 0.05) and decreasing soil nitrate nitrogen (NO₃⁻-N) content (P < 0.05) . Random forest results showed that soil WFPS, NH₄⁺-N, NO₃⁻-N and temperature were the main controlling factors for the seasonal fluctuation of N₂O fluxes in greenhouse vegetable fields, explaining 27% of its variation. The seasonal dynamics of CO₂ emissions had a significant positive correlation with soil temperature (R²=0.51,P < 0.05) .Regression analysis also indicated that the N₂O fluxes were significantly positively correlated with both NH₄⁺-N and NO₃⁻-N contents (P < 0.05) , suggesting that DCD application reduced N₂O emissions by inhibiting the nitrification process and thus decreasing NO₃⁻-N accumulation This study clarified that DCD reduced N₂O emissions in the early stage of application by decreasing NO₃⁻-N content, while having no significant effect on the CO₂ fluxes. DCD mitigates N₂O emission by regulating nitrogen forms in the early stage of application, providing a theoretical basis for greenhouse gas emission reduction in facility agriculture.