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Weeds are a major constraint to upland rice production in west Africa. The objectives of this investigation were to study dierences in weed suppression ability among upland rice cultivars and to determine the morphological traits involved. Twelve contrasting cultivars, including West African indigenous Oryza glaberrima (Steudel) lines and traditional and improved O. sativa L., were cultivated under natural weed competition and low-input conditions in Coà te d'Ivoire in two seasons. Signi®cant dierences between cultivars were observed in weed biomass at 100 days after seeding (DAS) in 1994 and 1995, indicating dierences in their competitive ability. Weed biomass was negatively correlated with rice root growth at early growth stages and with rice shoot and root growth at later growth stages. Across cultivars, grain yields with a single handweeding were 60% in 1994 and 49% in 1995 of those in plots that were weeded regularly. High grain yields in weedy plots were associated with low weed biomass. IG 10, an O. glaberrima cultivar, was the most competitive against weeds.

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-The study of sustainable land use is complex and long-term experiments are required for a better understanding of the processes of carbon stabilization. Objectives were (i) to describe for four long-term experiments the effects of fertilization and soil management on crop yields and the dynamics of soil organic carbon (SOC) and total N, and (ii) to discuss the usefulness of models for a better understanding of the underlying processes. Data of soil organic carbon and total N of four long-term experiments in Germany and China which studied the effect of fertilization (Bad Lauchstädt, Darmstadt) and tillage (Göttingen, Quzhou) were evaluated and soil organic carbon fractionation was carried out. The Rothamsted Carbon Model was used for a description and prediction of soil organic carbon dynamics as affected by fertilization and tillage in Bad Lauchstädt and Quzhou. The type of fertilizer added at common rates -either mineral N or farmyard manure -affected the crop yields only slightly, with slightly lower yields after manure application compared with mineral N fertilization. For both fertilization trials, manure applications at common rates had beneficial effects on soil organic carbon stocks in the labile pool (turnover time estimated as <10 years) and to a greater extent in the intermediate pool (turnover time estimated to be in the range of 10 to 100 years). A comparison of the effects of conventional tillage, reduced tillage and no-tillage carried out in Göttingen and Quzhou indicated only small differences in crop yields. Reduced tillage in Göttingen resulted in an increased C storage in the surface soil and C was mainly located in the mineral-associated organic matter fraction and in water-stable macro-aggregates (>0.25 mm). For Quzhou, no-tillage and conventional tillage had similar effects on total C stocks, with a greater spatial variability in soil organic carbon stocks in the no-tillage plots. Modeling required site-specific calibrations for the stock of inert organic matter for each of the sites, indicating that not all carbon stabilization processes are included in the model and that application of a model to a new site may also need site-specific adjustments before it can be used for predictions. After site-specific calibration, however, model predictions for the remaining treatments were generally accurate for the fertilization and tillage trials, which emphasizes the importance of temperature, moisture, soil cover and clay content on the decomposition dynamics of soil organic carbon and the significance of amounts and quality of carbon inputs in the soil for maintaining or increasing soil organic carbon stocks in arable soils.soil organic matter / C dynamics / Rothamsted carbon model / tillage / fertilization / soil organic carbon (SOC)