Root-zone carbon and nitrogen pools across two chronosequences of coastal marshes formed using different restoration techniques: Dredge sediment versus river sediment diversion

Abstract Coastal marshes have among the highest rates of soil carbon accumulation of any ecosystem on Earth, yet they are increasingly being lost due to high rates of relative sea-level rise, changes to their hydrology, and other natural and anthropogenic factors. To mitigate some of this wetland loss, restoration strategies in Louisiana have been focused on creating marshes using dredge spoil and diverting river flow into deteriorating wetland basins. Here, we aimed to compare these two different restoration approaches in terms of the development of soil carbon and nitrogen within the root zone (top 30 cm) by examining two chronosequences of newly formed marshes that span 32- and 25-year timeframes—one of which was a series of brackish, created marshes in the Chenier Plain (Sabine; marsh creation), and the other, a series of deltaic, tidal, freshwater marshes in the currently prograding Wax Lake Delta (WLD; sediment diversion). We also assessed the vegetation coverage and the relative chemical stability of the soil organic carbon and nitrogen (refractory versus labile) that has accumulated across these restoration sites. We observed that vegetation coverage initially increased more rapidly in the Sabine marshes, but it was similar to that in the WLD marshes by ~30 years post-establishment. Whereas the soil carbon pool reached similar quantities between the two chronosequences, it increased nearly twice as fast on average in WLD versus Sabine. The density of soil nitrogen closely tracked soil carbon in WLD, but it remained uniform across the Sabine marshes. Refractory soil carbon accounted for 59 ± 12% of the total soil carbon, and the refractory carbon pool increased at a rate that was similar between both chronosequences, despite the characteristic differences in salinity, nutrient input, plant species composition, and mineral sedimentation rates. Overall, our results suggest that marshes formed from sediment diversions in Louisiana have the potential to rival or outpace marshes created using dredge sediment in terms of vegetation and soil organic matter development.