In basins with carbon added, interspecific competition reduced both mean Phalaris biomass and mean Carex biomass (F2,32 = 39·26, P < 0·001; Fig. 2). Mean Phalaris biomass was reduced more by interspecific competition when NH4-N addition rates were low (C-0·00 and C-0·025) than when NH4-N addition rates were higher (by 82% compared with 42%) (species × competition × N, F6,32 = 3·47, P = 0·009; competition × N, F6,32 = 3·92, P = 0·005), while NH4-N addition did not significantly alter the influence of interspecific competition on mean Carex biomass (competition × N, F6,32 = 0·81, P = 0·566). Consequently, in C-0·00 and C-0·025 basins, mean Phalaris biomass was reduced significantly more by interspecific competition than was mean Carex biomass (species × competition, F2,14 = 10·74, P = 0·002). In C-0·00 and C-0·025 basins, averaged across competition treatments, mean Phalaris biomass (170·0 ± 24·6 g m−2) was less than one-third mean Carex biomass (617·7 ± 60·6 g m−2) (F1,14 = 131·46, P < 0·001), whereas in C-1·25 basins (species × N, F3,32 = 50·66, P < 0·001) mean Phalaris biomass (5645·6 ± 622·7 g m−2) was three times mean Carex biomass (1882·3 ± 399·7 g m−2) (F1,5 = 29·64, P = 0·003). Carex made up 92% of the biomass in Carex/Phalaris mixtures in C-0·00 and C-0·025 basins, but only 17% of the biomass in Carex/Phalaris mixtures in C-1·25 basins.