Study On Regulation Rules Of A Nitrification Inhibitor, Dicyandiamide (DCD) On Fertilizer Nitrogen Losses From A Greenhouse Vegetable Soil

Aiming at the serious waste of resources and environmental negative effect problems of low utilization efficiency of nitrogen and nitrogen losses via NO₃^--N leaching, N₂O emission and NH₃ volatilation in the greenhouse vegetable production systems, resulting from the excessive application of nitrogen fertilizers and unreasonable irrigation, the objectives of this study were to explore the inhibition efficiency of DCD and its affected mechanism in a greenhouse vegetable soil, and then select the optimum and effective nitrogen and irrigation management method and application rate of DCD for the greenhouse tomato production, and fertilizer type for greenhouse celery production, which would provided theoretic basis and technical instructions for the DCD regulation of fertilizer nitrogen losses in greenhouse vegetable production systems. Main results of this study were summarized as follows:1. The remaining time of DCD in the soil decreased with the increasing temperature and its inhibition effect was strongly enhanced with increasing application rate of DCD. Thus the effective inhibition effects could also be achieved by increasing the application rate of DCD under high temperature. Therefore, under the greenhouse vegetable production systems, DCD applied at 10% of the fertilizer nitrogen rate under 20℃³0℃, would remain effective in the soil for 29 days, and would thus be effective in inhibiting nitrification.2. The combination of recommended nitrogen and irrigation with the addition of DCD can significantly inhibit the decrease of the NH₄⁺-N contents in the soil, and reduce nitrogen loss as NO₃^--N. Comparing with the traditional management of nitrogen and irrigation, the combination of recommended nitrogen and irrigation with the addition of DCD can improve the utilization efficiency of nitrogen from 13.84% to 22.45%. In addition, the average NO₃^--N contents in the top soil depth (0¹0 cm) were decreased by 49.34%⁵5.54%, the NO₃^--N contents in the 0³0 cm and 30¹20 cm soil profiles were also reduced by 35.21%⁶4.88% and 61.08%⁷2.00%, respectively.3. Under different management of nitrogen and irrigation, the flux peak of N₂O emission and NH₃ volatilization generally appeared 3 days and 3⁵ days after the application of nitrogen and irrigation, respectively. Comparing with the traditional management of nitrogen and irrigation, the combination of recommended nitrogen and irrigation with the addition of DCD can reduce N₂O emission factors by 84.07%⁹6.19%, and decrease the total NH₃ volatilization by 20.61%⁴1.51%.4. The combination of recommended nitrogen and irrigation with the addition of DCD can significantly decrease the NO₃^--N contents in tomato fruit, and improve the quality of tomato fruit. The NO₃^--N contents in tomato fruit were decreased by 51.94%⁶2.82%, and the Vc, soluble sugar and soluble protein contents were also improved, but the free amino acid and organic acid contents of tomato were not significantly affected by the combination of recommended nitrogen and irrigation with the addition of DCD.5. The three different types of fertilizers (Urea, Dung, and 1:1Urea + Dung) coupled with DCD can significantly affect the conversion of nitrogen, and consequently increase the nitrogen uptakes of celery and improve the celery quality. Comparing with the fertilizers applied alone, three different fertilizers coupled with DCD increased the NH₄⁺-N contents in the soil by 17.70%⁹2.97%, decreased the NO₃^--N contents in the soil by 12.28%⁸1.90%. The nitrogen uptakes of the shoot and leaf of celery were increased by 29.24%⁸0.20% and 30.89%⁶6.33%, respectively. The NO₃^--N contents in the shoot and leaf of celery were decreased by 17.18%³7.05% and 7.63%²5.21%, respectively. In addition, the contents of soluble sugar, soluble protein, free amino acid and Vc were also improved by the addition of DCD.6. The combination of the nitrogen rate of 168 kg/hm² and optimum management irrigation with DCD applied at 10% of nitrogen rate offers the optimum combination of reduced nitrogen losses and increased utilization efficiency of nitrogen for the greenhouse tomato production. Urea coupled with DCD was the optimum combination in reducing N losses for the greenhouse celery production.