Nitrogen (N) loss via nitrate (NO₃^-) leaching from soil is one of the major contributors to environmental degradation. Vegetable soil is characterized by intensive fertilization with low nitrogen usage efficiency as a result of NO₃^- leaching. Nitrification and de-nitrification are the most important processes in NO₃^- formation and transformation, and are driven by functional microorganism in soil. The use of nitrification inhibitors, such as DCD, may play a key role in mitigating N loss. Investigations were conducted on the inhibitory effect of DCD to the functional microorganisms among the processes of nitrification and de-nitrification, and the relationship between DCD and nitrogen transformation. In this research, two kinds of vegetable soils were selected from Huangxing country, Changsha, reflecting long-term (about 20 years) and short-term vegetable cropping (1 year). Each sample was a mixture of 5 cores(0-20cm top soil) randomly taken in the same field. Two treatments for each soil sample were conducted: CK treatment (urea at 0.2g N/kg), and DCD treatment (urea at 0.2g N/kg with DCD 0.02g/g), each treatment with 3 replicates. The pots were incubated for 64 days at 25℃, and systematically measured NH₄⁺ and NO₃^- concentrations. According to the dynamics of NH₄⁺ transformation, three soil samples were taken at the 3^rd, 7^th and 64^th days, which represented the increasing, peak and decreasing status of NH₄⁺ concentration. The abundances of nitrifying gene amoA and denitrifying gene nirK were analyzed by Real-time PCR,.
Results indicated that NH₄⁺ concentrations of DCD treatments increased 248% and 225% compared to the CK treatment in the long-term and short term vegetable soils, respectively. NO₃^- concentrations of DCD treatments decreased 31% and 20% compared to the CK treatment in the long-term and short-term vegetable soils separately. Furthermore, NH₄⁺ concentration of DCD treated short term soil decreased slightly while long term soil remained the same level, suggesting that DCD had a better effect on long-term vegetable soil. The abundances of 16S rRNA gene and nirK gene had no significant difference between two treatments. However, the amoA gene abundance of DCD treatment for the long-term vegetable soil and the short-term vegetable soil had significantly decreased compared to the CK treatment, 61% and 48% respectively on the 3^rd day, 71% and 54% respectively on the 7^th day, but no differences were observed between two treatments after 64 days incubation. There was extremely significant correlation between NO₃^- concentration and the amoA gene abundance.
The results showed that the process of nitrification could be inhibited by DCD through restraining the growth of nitrifying bacteria. In addition, intensive application of inorganic fertilizer accelerated N element cycle in soil through the shift of composition and abundance of nitrogen-transformation functional microorganism. Thus, it might be better to use DCD together with suitable amount of fertilizer to improve nitrogen fertilizer usage efficiency and reduce nitrate loss.