TRANSPORT AND PROCESSING OF NITROGEN IN A TIDAL FRESH-WATER WETLAND

We combined field, laboratory, and modeling efforts to construct a process-oriented N budget for a tidal freshwater wetland in eastern Massachusetts. The emergent marsh contained most of the total N in the wetland because of its large area and high N mass per unit area. Total N stored in live marsh plants, dead litter, and inorganic interstitial water were roughly similar and 50-500 x less than total peat N. A simple input-output budget indicated that most of the N entering the wetland annually passes through the ecosystem unaltered; a relatively small amount of N is imported from the river by the marsh. In the short term, mineralization of peat N is sufficient to satisfy N demands within the marsh. Mineralized N is conserved by plant uptake, microbial immobilization, and litter immobilization. The small net N exchange with the river does provide an important subsidy to the marsh over the long term, offsetting losses due to denitrification and burial. Tidal freshwater wetlands may not appear to be influenced by current water quality and yet may respond to long-term cumulative nutrient loading, especially when this loading increases because of incremental watershed development.