Plant and soil responses to sediment deposition and nutrient‐enrichment in healthy, deteriorating, and newly created coastal marshes of the Mississippi River Delta

Abstract

Coastal marshes vulnerable to sea‐level rise may benefit from sediment amendments to increase elevation. However, nutrient‐loading to estuaries may counter elevation gain through reduced root production and/or increased decomposition. Here, we test if belowground plant productivity, root decomposition, and marsh accretion response to nutrient‐loading differs with sediment addition in three marsh types: a healthy intermediate salinity marsh, a deteriorating brackish marsh, and a created marsh in Barataria Basin, Louisiana, United States. In this region, wetland loss is rapid, and a major restoration project is underway to introduce Mississippi River water, sediment, and nutrients to offset marsh loss. Porewater nutrient concentrations, vegetation structure, belowground productivity and decomposition, and accretion rates were measured over 450 days. Experimental nitrate additions yielded high porewater ammonium‐N concentrations in the Deteriorating marsh but had a smaller effect in Healthy and Created marshes. Belowground productivity in the Deteriorating marsh was neutral to negatively affected by nutrient and sediment treatments, whereas positive effects occurred in Healthy and Created marshes. Despite elevation increase from sediment treatments, and substantial effects of nutrient treatments on porewater nutrient concentrations, belowground decomposition and surface accretion rates were not affected by treatments. Sediment deposition increased species richness in the Healthy marsh and added sediment and added nutrients with sediment increased plant height in the Created and Deteriorating marshes, respectively. Over the timescale measured, experimental sedimentation and pulsed nutrient input had few consistent effects on belowground ingrowth, decomposition, or surface accretion. Our findings highlight that response to nutrient pulses are complex and depend on baseline marsh conditions.