APPLICATION OF THE STABLE ISOTOPES OF C, N, S TO EVALUATE THE DEVELOPMENT OF REDUCING CONDITIONS AND ORGANIC MATTER SEQUESTRATION FOLLOWING BRACKISH MARSH CREATION

The carbon, nitrogen, and sulfur stable isotope ratios and organic carbon, total nitrogen, total sulfur, iron, and phosphorus concentrations were measured along sediment cores from an irregularly flooded, 21 year old created tidal marsh to evaluate soil development and quantify organic C and nutrient accumulation. A nearby natural tidal marsh was sampled for a comparison of results. Using d¹³C values of sediment cores, we determined the interface between terrestrial soil and more recent inputs of soil development after wetland creation. Overall, organic C, total N, total S, and phosphorus decreased with depth in the created marsh, while natural marsh sediments varied little with depth. Values of d35S_sulfate, d35S_elemental, and d35S_sulfide in the natural and created marsh were depleted from supplied seawater sulfate suggesting sulfate used in microbial sulfate reduction is recycled by reoxidation via tidal fluctuations. This trend remained with depth in the natural marsh. However, in the created marsh sediments at a depth of 10-30 cm had little to zero fractionation from supplied sulfate. Reduced soil permeability and low carbon concentrations of the upland soil may pose a barrier to tidal flushing, preventing new sulfur inputs and limiting bacterial sulfate reduction. Soil development since marsh construction, has resulted in soil accretion rates of 3.0 - 4.8 mm year^-1, accumulation rates of 88-150 g C m^-2 year^-1 for carbon, 7-25 g N m^-2 year^-1 for nitrogen, 0.45-0.66 g P m^-2 year^-1 for phosphorus, and 13-26 g Fe m^-2 year^-1 for iron. Nutrient accumulation rates since marsh construction are comparable to the natural marsh and to values reported in the literature for Atlantic coast marshes. Vertical accretion rates are similar to the natural marsh and have kept pace with local sea level rise. The development of reducing conditions in the surface layers of the created marsh enhances soil organic matter (SOM) accumulation and nutrient cycling and storage necessary to reach functional equivalency with similar natural coastal marshes. The type of upland soil and its properties may prohibit the development of SOM at depth in constructed marshes, limiting the outwelling nature of coastal wetlands as an important nutrient supply to nearby coastal waters.