Ecological Restoration of a Cultivated Wetland to Enhance Nitrogen Retention in a Rapidly Developing Watershed

Abstract

For much of the coastal United States, water quality continues to worsen due to excess nitrogen (N) from agricultural production and urban development. Restoration of freshwater wetlands, especially those connected to streams (i.e., fluvial wetlands), is a promising management option for curtailing watershed inputs of N to coastal embayments, with regional models assuming 77% retention of N by natural wetlands. In this study, we combine modeling, field measurements, and mass‐balance analysis to quantify N inputs and retention in a 56‐ha, 7‐year‐old restored fluvial wetland in southeastern Massachusetts, United States. Rapid watershed land‐use change, including development of ∼30% of forested land between 2001 and 2019, contributed to wastewater and turfgrass fertilizers accounting for the majority (80%) of watershed inputs of N to the restored wetland compared with the atmosphere (13%), a landfill (7%), and agricultural fertilizers (<1%). Field measurements showed that in‐stream processes retained 28% of surface water and groundwater inputs of nitrate (NO ₃ ⁻ ), which was slightly lower than N retention for restored wetlands and about half the N retention in natural wetlands. NO ₃ ⁻ retention of 1,029 kg N yr ⁻¹ was not strongly correlated with stream residence time or NO ₃ ⁻ inputs, but rather to stream temperature, pointing to environmental effects on in‐stream processes such as uptake and denitrification. Although NO ₃ ⁻ retention may increase as the wetland matures, the current N removal rate suggests that regional models may overestimate N removal in restored wetlands and that wetland restoration is an unlikely substitute for centralized sewering or other forms of enhanced wastewater management.