Net Impact of a Plant Invasion on Nitrogen-Cycling Processes Within a Brackish Tidal Marsh

Abstract

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.

The biology of Canadian weeds. 129. Phragmites australis (Cav.) Trin. ex Steud.

“…The mean concentrations of tissue nutrients of P. australis in Quebec were: N, 12.330; P, 1.515; K, 13.920; Na, 0.710; Ca, 2.780; Mg, 0.670; Cu, 0.003; Fe, 0.171; Mn, 0.125; Zn, 0.112 mg g -1 (Auclair 1979). The nitrogen concentration in aboveground tissues in August was 0.92 ± 0.12% (mean ± 2 SE) in P. australis stands in brackish tidal marshes in New Jersey, USA (Windham and Ehrenfeld 2003). Key nutrients such as N, P, and K fluctuated seasonally as well.…”

Nitrogen Uptake by Native and Invasive Temperate Coastal Macrophytes: Importance of Dissolved Organic Nitrogen

“…In addition, these plants could take up significantly more N (both DON and DIN) than S. alterniflora, which is evidenced by our field experiment and greater N assimilation by introduced Phragmites. Numerous studies have shown that Phragmites demands in introduced Phragmites and S. alterniflora marsh zones Estuaries and Coasts (2010) 33:784-797>50% more N than the species it displaces in salt, brackish, and tidal fresh marshes (Templer et al 1998, Windham 2001, Windham and Ehrenfeld 2003, Windham and Meyerson 2003. The high affinity and greater uptake of N could reduce N availability in sediment porewater for the other species.…”

“…Over the past century, the common reed, Phragmites australis, (hereafter Phragmites) has become pervasive in Atlantic coast tidal marshes, displacing Spartina spp. and other intertidal species; its unprecedented expansion is a concern for wetland ecologists working in both tidal and non-tidal wetlands due to changes in faunal use (Meyerson et al 2000a, Angradi et al 2001, Buchsbaum et al 2006, biogeochemical cycles (Windham and Lathrop 1999, Meyerson et al 2000b, Windham and Ehrenfeld 2003, and reductions in species richness and biodiversity (Chambers et al 1999, Bertness et al 2002. Phragmites is thought to be one of the world's most widely distributed angiosperms (Holm et al 1977, Mal andNarine 2004).…”

“…This is particularly important since introduced Phragmites demands up to 50% more N than the species it is replacing (Windham and Ehrenfeld 2003), and inorganic N availability is typically limited by microbial activity. This suggests that Phragmites may be getting N from sources other than ammonium (NH 4 + ) and nitrate (NO 3 − ).…”

Analyzing the Long-Term Phenological Trends of Salt Marsh Ecosystem across Coastal LOUISIANA

“…Under elevated levels of CO 2 , the C 3 pathway will probably become dominant in Phragmites, and this would allow the plant to dominate the coastal salt marsh ecosystem, especially if the ecosystem experiences nitrogen enrichment through runoff [109]. If Phragmites displaces Spartina as the dominant species in the coastal marsh landscape, there might be far-reaching ecological consequences such as changes in plant species abundance [110] and trophic interactions [111,112], herbivore population crashes [113], and changes in soil biogeochemistry [114]. In this research, we don't claim that such a phenomenon is already underway, as it is beyond the scope of this study, but supporting evidence from our analysis suggests that there is a risk of such a cascading effect in future.…”