 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com J. Plant Nutr. Soil Sci. 2009, 172, 651–660 DOI: 10.1002/jpln.200800197 651 Effects of urease and nitrification inhibitors added to urea on nitrous oxide emissions from a loess soil Mohammad I. Khalil 1,2 *, Reinhard Gutser 1 , and Urs Schmidhalter 1 1 Institute of Plant Nutrition, Technische Universität München, Department of Plant Sciences, 85350 Freising, Germany 2 Teagasc Environment Research Centre, Johnstown Castle, Wexford, Ireland Abstract Urea fertilizer-induced N 2 O emissions from soils might be reduced by the addition of urease and nitrification inhibitors. Here, we investigated the effect of urea granule (2–3 mm) added with a new urease inhibitor, a nitrification inhibitor, and with a combined urease inhibitor and nitrification inhibitor on N 2 O emissions. For comparison, the urea granules supplied with or without inhibitors were also used to prepare corresponding supergranules. The pot experiments without vegeta- tion were conducted with a loess soil at (20 ± 2)°C and 67% water-filled pore space. Urea was added at a dose of 86 kg N ha –1 by surface application, by soil mixing of prills (<1 mm) and gran- ules, and by point-placement of supergranules (10 mm) at 5 cm soil depth. A second experiment was conducted with spring wheat grown for 70 d in a greenhouse. The second experiment included the application of urea prills and granules mixed with soil, the point-placement of super- granules and the addition of the urease inhibitor, and the combined urease plus nitrification inhi- bitors at 88 kg N ha –1 . In both experiments, maximum emissions of N 2 O appeared within 2 weeks after fertilization. In the pot experiments, N 2 O emissions after surface application of urea were less (0.45% to 0.48% of total fertilization) than from the application followed by mixing of the soil (0.54% to 1.14%). The N 2 O emissions from the point-placed-supergranule treatment amounted to 0.64% of total fertilization. In the pot experiment, the addition of the combined urease plus nitrification inhibitors, nitrification inhibitor, and urease inhibitor reduced N 2 O emis- sions by 79% to 87%, 81% to 83%, and 15% to 46%, respectively, at any size of urea applica- tion. Also, the N 2 O emissions from the surface application of the urease-inhibitor treatment exceeded those of the granules mixed with soil and the point-placed-supergranule treatments receiving no inhibitors by 32% to 40%. In the wheat growth experiment, the N 2 O losses were generally smaller, ranging from 0.16% to 0.27% of the total fertilization, than in the pot experi- ment, and the application of the urease inhibitor and the combined urease plus nitrification inhibi- tors decreased N 2 O emissions by 23% to 59%. The point-placed urea supergranule without inhi- bitors delayed N 2 O emissions up to 7 weeks but resulted in slightly higher emissions than appli- cation of the urease inhibitor and the urease plus nitrification inhibitors under cropped conditions. Our results imply that the application of urea fertilizer added with the combined urease and nitrification inhibitors can substantially reduce N 2 O emissions. Key words: N 2 O emissions / urease and nitrification inhibitors / substituted phosphoric acid triamide / dicyandiamide plus 1H-1,2,4-triazole / cambisol Accepted January 16, 2009 1 Introduction Urea is the cheapest and most consumed N fertilizer for agri- cultural production (>50%) and seems to be responsible for a majority of nitrous oxide (N 2 O) emissions (8.59%) over other N fertilizers such as ammonium sulfate (3.48%) (Khalil et al., 2002). Nitrous oxide (N 2 O) has a large potential for global warming and stratospheric-ozone-layer depletion. Application of urea to agricultural soils leads to a temporal increase of soil pH favoring nitrification and subsequent denitrification, depending on soil type and moisture conditions (Eichner , 1990; Mulvaney et al., 1997; Khalil et al., 2002). Broadcast- ing, top-dressing, or side-dressing during/before crop growth are the common practices for the application of prilled or small-granular urea (1–2 mm) onto the upland systems, which are prone to ammonia (NH 3 ) losses during hydrolysis. By contrast, mixing urea with soils, band or deep placement of urea, immediate rainfall, or rapid drying of the surface soil after application of urea might reduce NH 3 losses (Bouw- meester et al., 1985), depending on the soil type and its water-transmission characteristics. A unilateral and independent target of the European Union to reduce greenhouse-gas (GHG) emissions, including N 2 O and NH 3 emissions, by 20% to 1990 levels by 2020 has been adopted in compliance with the Kyoto and Bali protocols/ declarations (CEC, 2008). Use of urea supergranule for upland systems may be an effective option with regard to * Correspondence: Dr. M. I. Khalil; e-mail: ibrahim.khalil@teagasc.ie; khalil_ibrahim@yahoo.com