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We investigated if the success of the invasive common reed Phragmites australis could be attributed to a competitive ability to use dissolved organic nitrogen (DON) when compared to the dominant macrophyte Spartina alterniflora in tidal wetlands. Short-term nutrient uptake experiments were performed in the laboratory on two genetic lineages of Phragmites (native and introduced to North America) and S. alterniflora. Our results provide the first evidence for direct assimilation of DON by temperate marsh plants and indicate that amino acids are assimilated intact by all plant types at similar rates. Both Phragmites lineages had significantly greater urea-N assimilation rates than S. alterniflora, and the affinity for dissolved inorganic nitrogen (DIN) species was the greatest in native Phragmites > introduced Phragmites > S. alterniflora. Field studies demonstrated uptake of both DON and DIN in similar proportion as those determined in the laboratory experiments. Based on these uptake rates, we estimate that DON has the potential to account for up to 47% of N demand for Phragmites plants, and up to 24% for S. alterniflora plants. Additionally, we suggest that differences in N uptake between native and introduced Phragmites lineages explain one mechanism for the success of the introduced type under increasingly eutrophic conditions. [ABSTRACT FROM AUTHOR]