NET IMPACT OF A PLANT INVASION ON NITROGEN‐CYCLING PROCESSES WITHIN A BRACKISH TIDAL MARSH

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 ∼60% 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 (input–output) 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.