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Using comparative analysis of the rates of key processes, we have documented the net effect of a shift in plant species composition on nitrogen cycles with the example of the rapid expansion of Phragmites australis (common reed) and its replacement of short grasses (e.g., Spartina patens) in coastal marshes of the eastern United States. In this study, we measured nitrogen (N) uptake by marsh plants, N adsorption from the water column by litter, changes in N content of litter, sediment N mineralization, nitrification, and nitrate consumption in adjacent plots dominated either by P. australis or by historically dominant S. patens. Rates of individual processes were generally greater in P. australis than in S. patens, but the magnitude of difference varied greatly among processes. Seasonal measurements of standing stock nitrogen in plant tissue indicate that P. australis took up ϳ60% more N than did S. patens, and annual rates of N immobilization were nearly 300% greater in P. australis litter than in S. patens litter. The greater demand for N in P. australis plots, however, was apparently compensated for by increased rates of N supply; mineralization rates in P. australis sediments were nearly 300% greater than those in sediments with S. patens. Rates of nitrate reduction (dissimilatory and assimilatory) were 300% greater in P. australis sediments. Whereas P. australis clearly sequestered more N in live and dead biomass than did S. patens, the presence of P. australis also stimulated the microbial production of inorganic N. This compensation of increased N demand with increased N supply suggests that the net nitrogen budget (inputϪoutput) of brackish tidal marshes is not immediately altered by the replacement of S. patens with P. australis. However, the greater magnitude of internal N cycling in P. australis communities is likely to influence the mobility of N pools, thereby altering pathways of N export.

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