Cultivar, nitrogen, and water effects on productivity, and nitrogen-use ef®ciency and balance for rice±wheat sequences of Bangladesh J. Timsina a,* , U. Singh a,1 , M. Badaruddin b,2 , C. Meisner c , M.R. Amin b a Agronomy, Physiology, and Agro-ecology Division, IRRI, P.O. Box 933, Manila, Philippines b Department of Plant Sciences, North Dakota State University, Fargo, ND 58105, USA c CIMMYT Of®ce, House 18, Road 4, Sector 4, Uttara, Dhaka, Bangladesh Accepted 12 June 2001 Abstract Rice Oryza sativa L.) and wheat Triticum aestivum L.) are often grown in sequences under a range of nitrogen N), water W), and planting date in South Asia. Field experiments were conducted from 1994±1995 to 1996±1997 to de®ne the effects of two W and three N regimes on growth and productivity, N uptake and N-use ef®ciencies, and N balance for rice±wheat systems of northern Bangladesh. Mean grain yields of rice and wheat were greatest 4.9 and 3.1 t ha 1 , respectively) during the ®rst and smallest 2.2 and 2.4 t ha 1 , respectively) during the third year. The cultivars of rice and wheat responded to irrigation and to N, with greater response to irrigation in rice, but to N in wheat. Delayed wheat seeding reduced wheat yields in all years. Agronomic N-use ef®ciency kg grain yield per kg N applied), physiological ef®ciency kg grain yield per kg N absorbed), and fertilizer N-recovery ef®ciency kg N absorbed per kg N applied, expressed as %) for rice across treatments ranged from 2.8 to 10.8, 5.2 to 27.5, and 33 to 61, respectively, and all were greater for N application at 90 compared with 135 kg N ha 1 . For wheat, those values ranged from 15 to 27, 33 to 51, and 45 to 63, respectively, and were greater at 120 compared with 180 kg N ha 1 , and under irrigation than rainfed. All those parameters had greater values under irrigation than rainfed. Total soil N increased slightly after 3 years of cropping, while organic carbon and pH decreased slightly in all treatments. There was a net increase of soil ammonium N 80 kg ha 1 ) and a zero balance of N after the ®rst year of cropping under irrigation with high N 135 and 180 kg ha 1 for rice and wheat, respectively), but without N there was a decrease of soil mineral N 70 kg ha 1 ) with a balance of 16 kg ha 1 . Biological N ®xation accounted for N balance in N-omitted as well as N-applied treatments. The results emphasize the need for regular monitoring of weather, crop performance, irrigation water, and soil and plant mineral N for further understanding the growth, productivity, N-use ef®ciencies, and balance in rice±wheat systems. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Cultivar; Nitrogen; Productivity; Rice±wheat; Water Field Crops Research 72 2001) 143±161 Abbreviation: C, cultivar; FL, ¯owering; MT, maximum tillering; N, nitrogen; NUE, nitrogen-use ef®ciency; PD, sowing date; PE, physiological ef®ciency; PI, panicle initiation; PM, physiological maturity; RF, fertilizer-recovery ef®ciency; W, water * Corresponding author. Present address: Department of Crop Production, Institute of Land and Food Resources, University of Melbourne, Melbourne, Vic. 3010, Australia. Tel.: 61-38344-8084; fax: 61-38344-4665. E-mail address: j.timsina@landfood.unimelb.edu.au J. Timsina). 1 Present address: IFDC, P.O. Box 2040, Muscle Shoals, AL 35662, USA. 2 Present address: Minn-Dak Growers Ltd., Highway 81 North, P.O. Box 13276, Grand Forks. 0378-4290/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0378-429001)00171-X