Invasive plant species and microbial processes in a tidal freshwater marsh

Vegetation has a strong influence on N retention in wetlands via direct upt ake and by indirect effects on microbial N uptake and denitrification . Veg etation change in freshwater tidal marshes is dynamic due to both natural a nd anthropogenic factors and these changes may influence the water quality maintenance value (i.e., the ability to absorb exogenous N) of these marshe s. We measured sediment microbial biomass and activity and plant N content and height in stands of Lythrum salicaria (L.), Phragmites australis [(Cav. ) Trin, Ex Steud], and Typha angustifolia (L.) in 1995 and 1996 in Tivoli N orth Bag,, a tidal freshwater marsh along the Hudson River in Dutchess Coun try, New York. Lythrum and Phragmites are invasive plants that have displac ed Typha in significant areas of this marsh over the past felv decades. A f ertilizer (two 5 g N m(-2) additions of urea) response study was conducted in 1996. There were few differences in microbial biomass and activity and r esponse to N addition between the different plants despite marked differenc es in plant biomass and N content and significant annual variation in sever al variables. Although plant change has been dramatic in this marsh, and th e new plants appear to cycle N differently than the native plant, microbial ly-based water quality functions mag not have been affected by this change.