Effects of common reed (Phragmites australis) expansions on nitrogen dynamics of tidal marshes of the northeastern US

Several recent studies indicate that the replacement of extant species with Phragmites australis can alter the size of nitrogen (N) pools and fluxes within tidal marshes. Some common effects of P. australis expansion are increased standing stocks of N, greater differentiation of N concentrations between plant tissues (high N leaves and low N stems), and slower whole-plant decay rates than competing species (e.g., Spartina, Typha spp.). Some of the greater differences between marsh types involve R australis effects on extractable and porewater pools of dissolved inorganic nitrogen (DIN) and N mineralization rates. Brackish and salt marshes show higher concentrations of DIN in porewater beneath Spartina spp. relative to R australis, but this is not observed in freshwater tidal marshes when P australis is compared with Typha spp. or mixed plant assemblages. With few studies of concurrent N fluxes, the net effect of P australis on marsh N budgets is difficult to quantify for single sites and even more so between sites. The magnitude and direction of impacts of R australis on N cycles appears to be system-specific, driven more by the system and species being invaded than by P australis itself. Where P. australis is found to affect N pools and fluxes, we suggest these alterations result from increased biomass (both aboveground and belowground) and increased allocation of that biomass to recalcitrant stems. Because N pools are commonly greater in P australis than in most other communities (due to plant and litter uptake), one of the most critical questions remaining is From where is the extra N in R australis communities coming? It is important to determine if the source of the new N is imported (e.g., anthropogenic) or internally produced (e.g., fixed, remineralized organic matter). In order to estimate net impacts of P australis on marsh N budgets, we suggest that further research be focused on the N source that supports high standing stocks of N in P australis biomass (external input versus internal cycling) and the relative rates of N loss from different marshes (burial versus subsurface flow versus denitrification).