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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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Nitrogen derived from symbiotic and mineral sources by a legume is determined by the interactions between mineral N supply, plant N demand, and N assimilation traits. These interactions need to be understood to maximize legume N2, fixation and yield, and to identify plant traits supporting high N2‐fixation. These interactions were examined in inoculated soybean [Glycine max Merr. (L.)] and common bean (Phaseolus vulgaris L.) by varying N supply (9, 120, and 900 kg N ha−1) at two field sites. Nitrogen fixation was measured by 15N dilution method. Plants were sampled at full bloom (R2), 21 to 25 d from R2, and physiological maturity (R7). Total N of both legumes at R7 was 25% greater with 900 than 9 kg N ha−1. With 900 kg N ha−1, soybean N accumulation at R7 (271 kg N ha−1) was 42% more than common bean but was 22% less at R2 (78 kg N ha−1). Applied N had the largest impact on N accumulation rates before R2 and from the second sampling to R7, both periods of minimal N2 fixation. Rate of N accumulation by common bean (0.19 g N m−2 d−1) was more uniform over the growth cycle than by soybean (0.26 g N m−2 d−1) which peaked (0.58 g N m−2 d−1) between R2 and the second sampling. This peak also coincided with maximum N2 fixation rate. Our 15N uptake and extractable soil N data indicate that common bean derived more N from the mineral source than soybean because of more efficient uptake. Greenhouse data indicated greater root weight and uptake of mineral N per unit root weight for common bean than soybean. Maximizing both N2 fixation and yield might entail timing the mineral N supply during early vegetative and late reproductive phases. The limited N2 fixation capacity of common bean might be due to N assimilation traits favoring mineral N uptake.

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Opinions vary on fertilization strategies in part because of uncertainties in methods assessing P supply across sites. We quantified the fate and extractability of fertilizer P after two to four crops with four to five P levels applied to upland rice (Oryza sativa L.)–soybean [Glycine max (L.) Merr.] rotations in three experiments in Asia. Soil P pools were measured by Mehlich‐1 extractant, a modified Hedley fractionation and by mixed‐bed resin capsules after 1 and 14 d (resin adsorption quantity, RAQ‐P1 and RAQ‐P14). Without P addition, 84% of the total P was in the NaOH‐Po and residual‐P fractions across sites. Phosphorus fertilization increased Mehlich‐1 P, resin‐P, NaOH‐Pi, H2SO4‐P, RAQ‐P1, and RAQ‐P14 across sites, whereas NaOH‐Po and residual‐P were unchanged. The sum of resin‐P and NaOH‐Pi increased from 10% to between 20 and 30% of the total soil P. Mehlich‐1 P and resin P increased similarly across sites and fitted quadratic models: the increase in Mehlich‐1 P (mg kg−1 per kg P ha−1) ranged from 0.050 at low P rates to >0.125 at >400 kg P ha−1 The increases per unit P of RAQ‐P, NaOH‐Pi, and H2SO4‐P varied among sites. Oxalate‐extractable Fe accounted for most of the variation in NaOH‐Pi and RAQ‐P. Changes in soil P pools in tropical upland Oxisols and Ultisols following P addition are likely better reflected by NaOH‐Pi and RAQ‐P than Mehlich‐1 P and resin P. Improvements in soil P tests are needed to better discriminate the changes in P pools from fertilization across soils.