The economic and environmental costs of the heavy use of chemical N fertilizers in agriculture are a global concern. Sustainability considerations mandate that alternatives to N fertilizers must be urgently sought. Biological nitrogen fixation (BNF), a microbiological process which converts atmospheric nitrogen into a plant-usable form, offers this alternative. Nitrogen-fixing systems offer an economically attractive and ecologically sound means of reducing external inputs and improving internal resources. Symbiotic systems such as that of legumes and Rhizobium can be a major source of N in most cropping systems and that of Azolla and Anabaena can be of particular value to flooded rice crop. Nitrogen fixation by associative and free-living microorganisms can also be important. However, scientific and socio-cultural constraints limit the utilization of BNF systems in agriculture. While several environmental factors that affect BNF have been studied, uncertainties still remain on how organisms respond to a given situation. In the case of legumes, ecological models that predict the likelihood and the magnitude of response to rhizobial inoculation are now becoming available. Molecular biology has made it possible to introduce choice attributes into nitrogen-fixing organisms but limited knowledge on how they interact with the environment makes it difficult to tailor organisms to order. The difficulty in detecting introduced organisms in the field is still a major obstacle to assessing the success or failure of
aimed at high yields. The low yield of upland rice, however, is largely a consequence of its production being Upland rice (Oryza sativa L.), commonly considered to be low limited to low harvest index (HI, ratio of grain to total yielding, can be high yielding if the genotype is improved for harvest index (HI) and the crop is grown relatively free from nutrient and biomass) varieties (George et al., 2001) and to infertile drought stresses. We examined whether high and stable rice yields or drought-prone uplands. could be obtained in aerobic soil. In four experiments of 1-to 3-yr Indications are that upland rice can also be high yieldduration, lime, N, and P were inputs for wet-season upland rice ing if the genotype is improved for yield and the crop 'UPLRi-5' in a favorable rainfed Oxisol. In a 3-yr experiment conis not subject to nutrient and drought stresses. With sisting of two crops per year in an irrigated Ultisol, different lowland improved upland rice, yields approaching 7 Mg ha Ϫ1 can and upland varieties were grown in limed and fertilized aerobic soil. be obtained, even in highly acidic upland soils, given First-season rainfed UPLRi-5 yield varied from 1.5 to 7.4 Mg ha Ϫ1 , adequate amounts of lime, N, and P (George et al., with low yields in fields receiving low early-season rainfall. With 2001). In the highly acidic soil of the 240-million-ha irrigation, the lowland hybrid 'Magat' yielded 7.8 Mg ha Ϫ1 vs. 2.1 Mg Cerrado region in Brazil, upland rice is a commercial ha Ϫ1 for traditional upland rice 'Lubang Red'. Magat's high yield was associated with a HI of 0.43 in contrast to 0.31 of improved upland crop in rotational systems (Guimarã es and Yokoyama, rice variety 'Apo' and 0.17 of Lubang Red. Whether the crop was 1998), producing about 5 Mg ha Ϫ1 with fertilizers and rainfed or irrigated, yield loss was rapid following the first season: irrigation (Stone et al., 1997). In China's Huang-Huai-Grain yields decreased by up to 73% for rainfed UPLRi-5 in the Hai River plains, with no soil acidity constraints, yields second to third season. In the irrigated upland, yield loss in the second approaching 7 Mg ha Ϫ1 have been observed for imto fourth season was reflected in a 16 to 79% decline in 10-wk biomass.proved upland rice lines on irrigated dryland (Huaqi Here, the 13-wk biomass in the fifth crop was only half that of the Wang, personal communication, 2000). simultaneously grown first-season crop. We conclude that while prom-
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