The Role Geomorphic Origin and Agricultural Legacy on Plant Communities and Surface Soil Organic Matter in Salt Marshes

Abstract Saltmarsh habitats provide numerous ecosystem services, including carbon storage, but are eroding at increasingly rapid rates due to ongoing sea level rise. Yet, where space and topography permit, saltmarshes are transgressing into adjacent uplands and counterbalancing the lost area. In these areas, older portions of marshes have accumulated more soil organic matter (SOM) over time than more recently formed marshes, though processes driving this pattern can be complex. Sediment deposition dynamics and plant community can influence SOM with both playing important but interacting roles. We used structural equation modeling to explore how surface SOM and plant communities vary across saltmarshes in the Chesapeake Bay region of Virginia, USA. We evaluated the influence of geomorphic origin (young vs. old), agricultural legacy, and two principal components, PC1, driven by hydrological conditions, and PC2, driven by salinity gradients on surface SOM and plant responses. We found that species-specific plant cover and tree counts were each affected by different combinations of these factors. Surface SOM was greatest in marshes formed prior to 1937, with an agricultural legacy, higher PC1 scores, lower PC2 scores, greater nearby Phragmites australis cover, and more nearby snags. We also found important indirect effects, whereby landscape history and environmental predictors influenced surface SOM accumulation through their effects on plants. We hypothesize that elevated surface SOM in areas with an agricultural legacy may result from restricted tidal flooding and reduced inorganic sediment input, conditions potentially created by the presence of berms. However, further investigation is necessary to conclusively address this issue.