Meta-analysis of vegetation responses to experimental nitrogen enrichments done in salt marshes under different sea level rise regimes reveal interaction of N supply and sea level rise

Agricultural-style fertilization studies to assess responses of salt marsh vegetation to increased nitrogen supply have been done in many coastal regions of the world. Initial enrichment experiments first revealed the role of nitrogen (N) supply as a key control of vegetation responses. A review of results from 43 N-fertilization studies carried out in a variety of coasts worldwide found increased responses by above-ground salt marsh vegetation in 94.5 % of the enrichments. The responses, although variable, increased significantly as N supply increased, confirming the limiting function of N supply. Enrichments with urea, a mix of N forms, or ammonium were more effective than enrichment with nitrate. Synthesis of compiled data suggested an interaction of N supply with sea level rise may be important. Sea level rise >3.4 mm yr^-1 damped the ability of above-ground salt marsh vegetation to respond to increased N supply, and it is possible that critical thresholds (about 15 mm yr^-1) for above-ground marsh vegetation may be reached by 2100. Below-ground vegetation responses were not greatly affected by N enrichment in the data compiled, but as sea level increases in the future, data trendlines suggest a possible future threshold (perhaps about 9 mm yr^-1) in submergence that would impair below-ground vegetation responses. These findings suggest that sea level rise effects may overwhelm the positive effects of increased N supply, an interaction that may impair salt marsh vegetation function, leading to increasing future submergence of these valuable habitats. These results emphasize the urgency for new mitigation strategies.