Evidence is mounting that plant genome size (GS) is associated with species' ecological strategies for water and nutrient utilisation. However, its role in mediating species' responses to ongoing precipitation changes and increasing nitrogen deposition, and its impact on plant community dynamics, remains poorly understood. Here, combining a precipitation gradient experiment with a nitrogen addition experiment in a semi-arid grassland, we show that species with different GS exhibited different responses to these environmental changes. Small-GS species demonstrated great drought tolerance and predominantly influenced community productivity under drought conditions, whereas large-GS species performed better under wetter and nitrogen-enriched conditions, which met their higher water and nutrient demands. These results suggest that plant-GS difference among coexisting species is linked to their varied responses to environmental changes and consequently mediates community dynamics. Therefore, ecosystem feedbacks to global environmental changes can be explained by some relatively stable genetic characters like GS.