Abstract A pot experiment with a loam soil and spring wheat as test crop showed that an application of dicyan- diamide (DCD), and especially its combination with hy- droquinone (HQ), gave a much larger recovery of soil urea- 15 N than treatments based on the application of urea alone or urea plus HQ. Most of the urea- 15 N applied to soil was present as organic plus chemically fixed 15 N in the DCD and DCD plus HQ treatments. These two treat- ments showed the smallest accumulation of urea-derived (NO 3 – +NO 2 – )- 15 N. Under well-drained conditions, there was a synergistic effect of the nitrification inhibitor DCD and the urease inhibitor HQ on urea- 15 N transformations and the recovery of fertilizer 15 N in soil after the applica- tion of urea. Keywords Urea-nitrogen-15 transformation · Hydroquinone · Dicyandiamide · Synergistic effect Introduction Inhibitors have been used to regulate the transformation processes of fertilizer N in soil so as to improve fertili- zer efficiency and decrease environmental pollution (Bielek and Kudeyarov 1991; Zhao et al. 1992, 1993; Zhu et al. 1997; Chen et al. 1998; Xu et al. 1999, 2000). In the presence of urease inhibitors such as hydroqui- none (HQ), phenyl phosphorodiamidate and N-(n-butyl) phosphorothioic triamide, the urea- 15 N recovery in an alkaline soil was increased by 5–30% of the amount ap- plied as fertilizer 15 N, and the effect depended on the in- hibitor and soil type (Wang et al. 1991). In some incuba- tion experiments, the application of nitrification inhibi- tors such as 4-amino-1,2,4-trizaole (Juma and Paul 1983), dicyandiamide (DCD) (Guiraud et al. 1989; Vils- meier et al. 1991; Xu et al. 1999), Didin (Guiraud et al. 1992) and nitrapyrin (Chalk et al. 1990) resulted in an increased N immobilization of the labelled fertilizer N in soil. The mineralization of the newly immobilized N in the soil was slow, occurring over periods measured in months (Shen et al. 1989). Of course, the concentra- tion of mineral N as well as fertilizer-N efficiency de- pended on the competition between microbial immobili- zation and plant uptake (Machet et al. 1987). As far as we know, less attention has been paid to the synergistic effect of urease and nitrification inhibitors on the immo- bilization-mineralization turnover of fertilizer 15 N in soil. Our previous work with unlabelled urea (Chen et al. 1998; Xu et al. 1999) showed that a combined applica- tion of the urease inhibitor HQ and the nitrification in- hibitor DCD retarded the hydrolysis of soil urea and ni- trification for a long period and decreased the accumula- tion of soil NO 3 – plus NO 2 – . Unfortunately, the applica- tion of unlabelled urea did not allow us to show the im- mobilization of fertilizer N in soil and the mineralization of the newly immobilized N fraction. This paper mainly reports the effects of HQ and DCD on the transformation of urea- 15 N in soil cropped to wheat, with particular attention to N mineralization and immobilization. This investigation complemented that of Xu et al. (2000) who studied N losses and fertilizer-N distribution in wheat by using 15 N-enriched urea under the same experimental conditions. X. Xu ( ✉ ) · Y. Huang State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, People’s Republic of China e-mail: xingkai_xu@hotmail.com Tel.: +86-10-82084278, Fax: +86-10-62041393 X. Xu · L. Zhou · G. Huang Laboratory of Terrestrial Ecological Process, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China O. Van Cleemput Laboratory of Applied Physical Chemistry, Faculty of Agricultural and Applied Biological Sciences, University of Ghent, 9000 Gent, Belgium Present address: X. Xu, LAPC, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, People’s Republic of China Biol Fertil Soils (2001) 34:286–290 DOI 10.1007/s003740100393 ORIGINAL PAPER Xingkai Xu · Yao Huang · Likai Zhou Guohong Huang · Oswald Van Cleemput Effect of dicyandiamide and hydroquinone on the transformation of urea-nitrogen-15 in soil cropped to wheat Received: 24 November 2000 / Published online: 9 August 2001 © Springer-Verlag 2001