No abstract

The impact of salt-water intrusion on microbial organic carbon (C) mineralization in tidal freshwater marsh (TFM) soils was investigated in a year-long laboratory experiment in which intact soils were exposed to a simulated tidal cycle of freshwater or dilute salt-water. Gas fluxes [carbon dioxide (CO 2 ) and methane (CH 4 )], rates of microbial processes (sulfate reduction and methanogenesis), and porewater and solid phase biogeochemistry were measured throughout the experiment. Flux rates of CO 2 and, surprisingly, CH 4 increased significantly following salt-water intrusion, and remained elevated relative to freshwater cores for 6 and 5 months, respectively. Following saltwater intrusion, rates of sulfate reduction increased significantly and remained higher than rates in the freshwater controls throughout the experiment. Rates of acetoclastic methanogenesis were higher than rates of hydrogenotrophic methanogenesis, but the rates did not differ by salinity treatment. Soil organic C content decreased significantly in soils experiencing salt-water intrusion. Estimates of total organic C mineralized in freshwater and salt-water amended soils over the 1-year experiment using gas flux measurements (18.2 and 24.9 mol C m -2 , respectively) were similar to estimates obtained from microbial rates (37.8 and 56.2 mol C m -2 , respectively), and to losses in soil organic C content (0 and 44.1 mol C m -2 , respectively). These findings indicate that salt-water intrusion stimulates microbial decomposition, accelerates the loss of organic C from TFM soils, and may put TFMs at risk of permanent inundation.

Isotopic fractionation of ammonium (NH,+ ) and nitrate (NO,-) during uptake by phytoplankton was examined in batch culture experiments with the diatom SkeZetonema costatum under nitrogen-enriched conditions (5-100 PM). The fractionation factor (E) for NOJ-uptake by Skeletonema was -9.Of0.7%0 and was concentration-independent. For NH,+, E was more variable and dependent on ambient NH,+ concentration. For NH4+ concentration ranges of 100-50, 50-20, and 20-5 PM, E was -24.6k5.5, -27.2-+ 1.6, and -7.8+3.0%.In these cultures, isotopic fractionation by phytoplankton caused variations in 615N of up to 50?& for NH4+, 1 ~%XI for NO,-, and 25% for particulate N. Similar variability in the 615N of both dissolved inorganic and particulate organic N pools should be expected during phytoplankton blooms in nature. As a result, phytoplankton-mediated isotopic variability must be considered when isotopic data are used to examine biogeochemical and physical processing of organic matter in marine ecosystems, particularly when biosynthesis and loss processes are decoupled in either space or time during bloom conditions.

Tidal wetlands produce long-term soil organic carbon (C) stocks. Thus for carbon accounting purposes, we need accurate and precise information on the magnitude and spatial distribution of those stocks. We assembled and analyzed an unprecedented soil core dataset, and tested three strategies for mapping carbon stocks: applying the average value from the synthesis to mapped tidal wetlands, applying models fit using empirical data and applied using soil, vegetation and salinity maps, and relying on independently generated soil carbon maps. Soil carbon stocks were far lower on average and varied less spatially and with depth than stocks calculated from available soils maps. Further, variation in carbon density was not well-predicted based on climate, salinity, vegetation, or soil classes. Instead, the assembled dataset showed that carbon density across the conterminous united states (CONUS) was normally distributed, with a predictable range of observations. We identified the simplest strategy, applying mean carbon density (27.0 kg C m−3), as the best performing strategy, and conservatively estimated that the top meter of CONUS tidal wetland soil contains 0.72 petagrams C. This strategy could provide standardization in CONUS tidal carbon accounting until such a time as modeling and mapping advancements can quantitatively improve accuracy and precision.