Is denitrification kinetically-limited or transport-limited in tidal freshwater marshes?

Abstract

Abstract Tidal freshwater wetlands are highly complex ecosystems that are important sites for nutrient retention. Although marsh surfaces have been identified as important sites for N processing, temporal and spatial variability is not well understood. Previous research has focused on measuring denitrification rates in sediment cores, sampled along marsh vegetation or salinity gradients, through laboratory experiments or plot studies. Extending these results to the ecosystem scale requires that we know whether ecosystem denitrification is kinetically-limited or transport-limited. The purpose of this study is to examine this question in freshwater tidal marshes. A multi-scale, mass balance approach was used to measure tidal NO 3 - retention and loss in freshwater tidal wetlands. Geomorphic characteristics (marsh surface area and channel inlet characteristics) were examined within the freshwater tidal ecosystem of the Patuxent River, Maryland. From the measured range of geomorphic characteristics in the ecosystem, individual marshes were selected for mass balance studies of hydrologic fluxes and NO 3 - retention. The data indicated that net NO 3 - retention per water volume is constant for marshes of different sizes, whereas NO 3 - retention per marsh surface area decreases with an increase in marsh size. These data suggest that NO 3 - retention is transport, not kinetically-limited in this freshwater tidal marsh ecosystem. This hydrologic limitation is likely due to the effects of macrophyte vegetation on flow resistance in tidal channels and on marsh surfaces, which produces non-synchronous flooding of marsh surfaces of equal elevation, particularly in large marshes. The reported measurements are for high tidal stages in autumn, but transport limitations on denitrification rates should be significant throughout the growing season when macrophytes are present.