Dicyandiamide (DCD) is one of the quality and inexpensive, environmentally friendly nitrification inhibitors, but long-term field experiments were not completed. In order to evaluate the environmental effects of DCD on farmland, researches on long-term application of DCD is extremely required. Application of DCD could increase soil NH₄⁺-N, and decrease soil NO₃^--N contents. However, the changes of NH₄⁺-N and NO₃^--N in soil and the nitrate in vegetable when DCD single fertilized is consistent with those in multiple application of DCD are not clearly known. Therefore, three field trials were adopted to comprehensively investigate the changes of soil nitrogen types and nitrate accumulation in vegetables, comprising two treatments and each treatment was replicated three times. Two trials with single application of DCD were located in Putian City(latitude 25°25'06″ N, longitude 119°01'54″ E), the long-term leafy cabbage and short-term leafy spinach were planted from 28 November 2009 to 27 March 2010, and 21 June to 24 July 2011, respectively. The experiment with successive cropping of vegetables was carried out at the Experimental Station of Fujian Farmland Conservation, Ministry of Agriculture, in Baisha Town, Fuzhou City(latitude 26°12'33″ N, longitude 119°04'52″ E), the vegetables such as cabbage, spinach, water spinach, radish, Chinese cabbage were cultivated in sequence from 4 December 2008 to 27 April 2011. In comparison to chemical fertilizer application alone (control), in the growth process of cabbage, single application of DCD, soil NH₄⁺-N increased from 21.3 to 339.4%, NO₃^--N in soils and in vegetables decreased from 5.4 to 80.2% and from 4.4 to 58.3%, respectively. In the harvest time of spinach, when DCD was single applied, soil NH₄⁺-N increased about 299.4%, soil NO₃^--N decreased about 26.2%, NO₃^--N in vegetables decreased about 31.7%, respectively. However, in the "cabbage-spinach-water spinach-radish-Chinese cabbage" successive cropping system, soil NH₄⁺-N and NO₃^--N, and vegetable NO₃^--N contents were accumulated in the multiple DCD application (once for each vegetable season). Soil NH₄⁺-N was found from slightly high (44.0%) to significantly higher (392.5%,P<0.01) than chemical fertilizer applied alone. While soil NO₃^--N ranged from extremely low (-68.2%,P<0.01) to high (146.6%,P < 0.05), and the range of vegetable NO₃^--N was also from low (-30.2%,P<0.05) to significantly high (40.4%,P<0.01). Why DCD single fertilized could significantly decrease NO₃^--N contents in soil and vegetable, showing cumulative trends with multiple application of DCD? With multiple application of DCD, the surplus soil NH₄⁺-N in season further converted to NO₃^--N form in soil, and it should be absorbed by the next planting season vegetables. Thus, the nitrification inhibition of DCD was immediate, nitrification inhibition diminished and accompanied by the degradation of DCD. For the practice of agricultural production, the multiple application of DCD should be avoided. After 1or 2 seasons of DCD application, application of nitrogen rate and DCD dosage should be reduced appropriately, to avoid high levels of NO₃^--N in soil and the risk of vegetable nitrate enrichment.