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Leymus chinensis (Trin.) Tzvelev and Phragmites communis Trin. are co-dominant grasses in both nutrient-poor and rich steppes in the Songnen Plains of northeastern China. A replacement series experiment was conducted along a five-level nutrient gradient to test for changes in the competitive relationships of the species with nutrient availability. There were significant effects of nutrient level and species proportion on mean plant height, biomass and rhizome length. Facilitation was found in these plant attributes for both species when grown in mixture compared with monoculture, especially at high nutrient levels. For example, L. chinensis grew taller and intercepted more light when in mixture. The aboveground dominance of P. communis did not decrease in mixture, possibly because it benefited from belowground interactions with L. chinensis. Biomass allocation was apparently influenced by both above and belowground competition. For example, L. chinensis allocated much more biomass to shoots and less to roots when neighbor number was increased, suggesting that aboveground competition occurred. Root: shoot ratios of L. chinensis in monoculture decreased gradually with increasing nutrient availability, whereas the ratio in mixture declined rapidly, implying intense competition for light at high levels. Our results support the hypothesis that nutrient competition is more important than light competition when soil nutrients are scarce. Although the interactions between these species benefit both, P. communis is more likely to displace L. chinensis at high nutrient availability. [ABSTRACT FROM AUTHOR]