Pergamon Soil Bid. Biochem. Vol. 21, No. 415. pp. 431-439, 1995 Copyright 0 1995 Elsevier Science Ltd 0038-0717(94)00188-x Printed in Great Britain. All rights resewed 003%0717/95 59.50+0.00 EFFICIENT MANAGEMENT OF SOIL AND BIOLOGICALLY FIXED N2 IN INTENSIVELY-CULTIVATED RICE FIELDS D. K. KUNDU and J. K. LADHA* Soil and Water Sciences Division, International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines Summazy-A decline in productivity in wetland rice has been detected in some intensively-cultivated experimental farms in Asia since the early 1980s. Increased doses of fertilizer N are being used in both experimental and farmers’ fields to maintain the original yield levels. Little attention has been paid to judicious management of native soil N, which is the principal N source for rice, and to biological N2 fixation (BNF), which largely replenishes the soil N concentration. We review here various effects of long-term flooding and puddling associated with intensive cultivation of wetland rice on soil N availability and BNF. Some strategies are suggested to efficiently manage these two N sources to sustain high productivity of the ricelands. INTRODUCIION Rice is the most important food crop of the developing world. It is the staple food for more than 2 x lo9 people in Asia and for many millions in Africa and Latin America. To feed the increasing global population, the world’s annual rice production must increase from the present 520 x lo6 t to 760 x lo6 t by the year 2020. Because increased rice production must come from the same or even less land area, productivity (yield ha-‘) must be enhanced. Yield stagnation-or even decline-has been observed in some rice-growing areas of Asia since the early 1980s. Such disturbing trends have been clearly detected on experimental farms where 2-3 crops of wetland rice are grown annually without adequate drying and thorough preparation of the fields (Flinn and De Datta, 1984; Cassman and Pingali, 1994). Cassman and Pingali (1994) have discussed the extrapolation of these experimental farm observations to farmers’ fields. They observed that yield declines are commonly associated with decreased crop N uptake and that increasing doses of fertilizer N must be applied to maintain original yield levels. The management of native soil N, which is the principal N source for rice, and bio- logical N2 fixation (BNF) may play an important role in sustaining the soil N pool under intensive cropping. Reviews have dealt with the N-supplying capacity of flooded soils (Sahrawat, 1983; Zhu, 1989) and the roles of BNF in rice production (Roger and Watanabe, 1986; Ladha et al., 1993). The effects of long-term flooding and puddling associated with intensive cultivation of wetland rice on the availability of soil N *Author for correspondence. and on BNF have not been considered. We address these issues in this paper. CONTRIBUTION OF SOIL N TO WETLANDRICE The total N content of almost 80% of rice growing soils surveyed in Asia ranged from 0.08 to 0.15% with 1.8-7.0% of the total soil N mineralized during the cropping season and made available to the rice crop (Table 1). The amount of N mineralized in rice soils during a cropping season ranged from 57 to 80 kg ha-’ in the Philippines (Shiga and Ventura, 1976) to 52-107 kg ha-’ in China (Lu, 1981). N supplied by the soil is sufficient to produce rice yields of 2-4 t grain ha-’ in most situations. In South and Southeast Asia, about 10% of 152 test sites yielded more than 5 t ha-’ without fertilizer N (Bouldin, 1986). In highly-pro- ductive rice fields, the soil N supply pattern matches the crop N uptake pattern. The yield benefits from high native soil N fertility are difficult to replace with increased fertilizer N. Cassman et al. (1994) reported that hybrid rice yields in China on a soil with low native N fertility, where 245 kg fertilizer N ha-’ was applied, was 2.2 t ha-’ less than that obtained on a soil of high native N fertility where only 54 kg fertilizer N ha-’ was used. Applying N fertilizer generally enhances the N mineralization rate in the soil (Broadbent, 1979; Bouldin, 1986) and soil N uptake by wetland rice [Table 2; see also De Datta and Broadbent (1990)]. Data obtained from several field experiments under diverse soil and climatic conditions showed that a wetland rice crop supplied with adequate fertilizer N took up 45-l 13 kg soil N ha-‘, constituting 5693% of total crop N uptake (Broadbent, 1984). A study in Japan indicated that the rice crop depends totally on the soil to meet its N requirements from heading to maturity (Table 2).