High nitrogen uptake and utilization contribute to the dominance of invasive Spartina alterniflora over native Phragmites australis.
A field experiment was conducted to investigate nitrogen (N)-form uptake preference and N utilization efficiency of an invasive plant Spartina alterniflora and its co-occurring native plant Phragmites australis in the Yellow River Delta, China, using a stable isotope-labeling technique. Both species preferred ammonium-N to nitrate-N and glycine-N under both intraspecific and interspecific competition, but the competition type altered the magnitude of this preference. Spartina alterniflora took up more glycine and nitrate but less ammonium under interspecific competition than under intraspecific competition, resulting in a relative preference shift toward organic-N and nitrate. By contrast, P.australis assimilated much less ammonium and nitrate under interspecific competition than under intraspecific competition, but a similar amount of glycine, resulting in a relative preference shift toward organic-N. Biomass, N uptake rate, and N utilization efficiency were all much greater under intraspecific competition than under interspecific competition in P.australis, but smaller in S.alterniflora, suggesting that the invasive species is competitively superior to the native species in terms of growth, N uptake, and N utilization efficiency. We conclude that both species exhibited N-form preference, and competition type could shift such a preference. The competitive advantages in terms of growth, N uptake, and N utilization efficiency might have enabled S.alterniflora to successfully invade communities previously dominated by P.australis in the Yellow River Delta.