Adaptation of plant populations to nutrient—poor environments may follow two essentially different pathways: maximizing the assimilation of nutrients or minimizing the loss of nutrients. Our hypothesis was that plant species that are adapted to nutrient—poor soils can replace species that are characteristic for relatively nutrient—rich habitats by features that enable them to restrict their nutrient losses. We chose Festuca rubra and Arrhenatherum elatius as species to test this hypothesis. In a long—term field experiment in a hayfield it was observed that Festuca was very common and Arrhenatherum was almost absent in the unfertilized plots, whereas after fertilization Festuca disappeared and Arrhenatherum increased strongly. It was hypothesized that differences in nutrient and carbon losses resulting from mowing were responsible for the different success of the two species in the unfertilized plots. A pot experiment was carried out in a greenhouse with clipped and unclipped and with unfertilized and fertilized treatments in a factorial design with four series. In the fertilized treatments Arrhenatherum replaced Festuica; its heavier seeds resulted in more rapid early growth and its more erect architecture enabled it to overtop Festuca after some time. In the two unfertilized treatments Festuca replaced Arrhenatherum, but much more rapidly in the clipped pots than in the unclipped ones. Losses of nitrogen due to clipping were much greater in Arrhenatherum than in Festuca because of the strongly increased allocation of nitrogen to aboveground parts in Arrhenatherum in response to clipping. It was concluded that differences in nitrogen loss resulting from clipping have an important impact upon the competitive balance between plant species under nutrient—poor conditions.