Abstract: Effects of dicyandiamide (DCD) inhibiting nitrification and soil acidification caused by nitrogen fertilizer in two Ultisol samples collected separately from Langxi of Anhui Province and Yingtan of Jiangxi Province were investigated through indoor incubation.Results show that when no nitrogen fertilizer was applied, DCD suppressed nitrification of residual ammonium nitrogen in the soils and subsequent soil acidification,leading to rise in pH by 0.37 and 0.40 in the soils from Langxi and from Yingtan,respectively as compared with the controls at the end of the incubation. When urea or ammonium bicarbonate was added, DCD inhibited nitrification of ammonium released from the nitrogen fertilizers and subsequent soil acidification. In addition to hydrolysis of  urea and consumption of protons by the association of bicarbonate ions with H⁺,the addition of DCD also offset to a certain extent the effect of the two fertilizers on red soil acidity.At the end of incubation, Treatments urea+DCD and ammonium bicarbonate+DCD was 0.62 and 0.46 higher in pH than their counterparts without DCD in the soil from Langxi, and 0.82 and 0.69 higher in the soil from Yingtan. The treatments with DCD were also significantly lower in content of exchangeable acids than their couterparts without DCD.It is, therefore,concluded that combined application of nitrogen fertilizer and DCD can effectively alleviate red soil acidification caused by nitrogen fertilizers.

Key words: dicyandiamide, red soil, soil acidity, amelioration, nitrification inhibition