Does Excess Nitrogen Cause a Marsh to Build Up or Collapse? Team to Resolve Contradictions

The cascade of reactive nitrogen (N) from watersheds to the coastal zone is well documented, as are the  consequences of that nutrient loading in coastal waters. Salt marshes provide an important line of defense against the harmful impacts of excess nitrogen by intercepting watershed nitrogen before it can induce eutrophication (nutrient over-enrichment, leading to algal blooms and oxygen depletion) in coastal waters.

However, extensive research on salt marsh response to anthropogenic N loading has resulted in contrasting results. Addition of N can increase plant biomass and enhance sediment trapping, thus increasing the rate at which the marsh accretes, or builds up; or, increased anthropogenic N can decrease marsh organic matter accumulation and soil strength, promoting marsh collapse.

To investigate these seemingly contradictory findings, MBL Senior Scientist Anne Giblin has received a grant from the National Science Foundation with collaborators Jennifer Bowen and Randall Hughes of Northeastern University and James Morris of University of South Carolina. The group will investigate three possible reasons for the previous findings:

• First, marshes that do not display the expected response of increased above-ground plant biomass with N additions could be systems that are close to nitrogen saturation, so they will examine nutrient availability.
• Second, the form of reactive N, whether oxidized as nitrate or reduced as ammonium, could affect the competitive interactions between plants and sediment microbes. This will be tested with fertilization experiments. 
• Third, population-level variation in the response of marsh vegetation to different forms of N could further complicate these competitive interactions, so the group will carry out experiments using different genotypes.

The group will carry out parallel experiments at the Plum Island Ecosystems Long-Term Ecological Research site in northern Massachusetts and in North Inlet, South Carolina.

The group suspects that when marshes receive nitrate, microbes can outcompete marsh primary producers, promoting nitrate respiration and accelerating decomposition of marsh organic matter. This response may be further modified by the history of N availability that may have induced genetic- and/or environment-based changes in plant traits. 

Determining whether the form of N supports plant growth, carbon sequestration, and marsh accretion or stimulates microbial decomposition is essential for predicting the long-term persistence of salt marshes in the face of sea-level rise.