Comparison of nutrient accrual in constructed living shoreline and natural fringing marshes

Living shoreline marshes are coastal wetlands constructed as alternatives to “hardened shorelines” (e.g., bulkheads, riprap) to mitigate erosion and to allow for landward migration of intertidal habitat as sea level rises. Living shorelines are designed to mimic natural fringing marshes and over time should be sinks for carbon and other nutrients. We collected soil cores and aboveground plant material from 13 pairs of natural fringing marshes and living shoreline marshes of different ages and degree of isolation from more extensive marsh shorescapes to compare nutrient pools and accrual. Although the nutrient content of plants was similar within and between marsh types, soil nutrients were variable from both living shorelines aged 2–16 years and long-established natural marshes. Most—but not all—living shoreline marshes had lower soil organic content, higher bulk density, and lower soil % carbon, nitrogen and phosphorus than their natural marsh pair. Variation in soil nutrients from living shorelines was not strongly correlated with either marsh age or degree of isolation in the estuarine shorescape. Assuming constant accrual within individual marshes, we estimated soil nutrient levels in living shorelines would approach those observed in their paired, natural fringing marshes over timescales from less than 10 years to many decades. Living shoreline marshes are on trajectories to match natural marsh function with respect to carbon and nutrient storage in estuarine systems.