Isotopic Records of C, N Storage and Processing in Natural and Restored Typha Wetlands

Wetland soils and vegetation store significant amounts of carbon; however, organic matter production, decomposition, and storage are affected by plant species, temperature, and hydrologic processes, all of which are affected by climate. In this study we conducted decomposition experiments and examined soil characteristics in 7 Typha-dominated freshwater wetland sites in 3 wetlands (constructed tidal, constructed non-tidal, and natural tidal). The sites are within 30 km of each other; they experience similar air temperatures and precipitation regimes, but they have different land use histories, water chemistry, and inundation patterns. At these seven sites, we monitored water levels, air and water temperatures, and aboveground biomass. At each site, we conducted decomposition experiments to obtain rates of carbon loss and changes in C and N isotopic composition. Core samples were obtained from each site and analyzed for bulk density, carbon content, nitrogen content, and C, N isotopic compositions. Typha decomposition rates were similar among the sites. Typha leaf pack experiments documented carbon loss and a decrease in C/N ratios, but little systematic change in isotopic composition over the duration of the experiments. Core carbon content is higher in the natural wetlands, but it decreases with depth at all sites. Core carbon isotopic composition is similar to initial Typha leaf values and generally exhibits little change with core depth, except near the base of the 2 constructed wetlands. Nitrogen concentration and isotopic composition of surface soils varies among the sites, possibly reflecting differences in N sources. Both N concentration and 15N values decrease with depth at all sites. These carbon isotopic data suggest some C loss in the cores without C fractionation (e.g., as dissolved fluxes), but the nitrogen isotopic data suggest microbial N processing, particularly after incorporation into wetland soils.