Vegetation zones as indicators of denitrification potential in salt marshes.

ABSTRACT

Salt marsh vegetation zones shift in response to large-scale environmental changes such as sea-level rise (SLR) and restoration activities, but it is unclear if they are good indicators of soil nitrogen removal. Our goal was to characterize the relationship between denitrification potential and salt marsh vegetation zones in tidally restored and tidally unrestricted coastal marshes, and to use vegetation zones to extrapolate how SLR may influence high marsh denitrification at the landscape scale. We conducted denitrification enzyme activity assays on sediment collected from three vegetation zones expected to shift in distribution due to SLR and tidal flow restoration across 20 salt marshes in Connecticut, USA (n = 60 sampling plots) during the summer of 2017. We found lower denitrification potential in short-form Spartina alterniflora zones (mean, 95% CI 4, 3-6 mg N h-1  m-2 ) than in S. patens (25, 15-36 mg N h-1  m-2 ) and Phragmites australis (56, 16-96 mg N h-1  m-2 ) zones. Vegetation zone was the single best predictor and explained 52% of the variation in denitrification potential; incorporating restoration status and soil characteristics (soil salinity, moisture, and ammonium) did not improve model fit. Because denitrification potential did not differ between tidally restored and unrestricted marshes, we suggest landscape-scale changes in denitrification after tidal restoration are likely to be associated with shifts in vegetation, rather than differences driven by restoration status. Sea-level-rise-induced hydrologic changes are widely observed to shift high marsh dominated by S. patens to short-form S. alterniflora. To explore the implications of this shift in dominant high marsh vegetation, we paired our measured mean denitrification potential rates with projections of high marsh loss from SLR. We found that, under low and medium SLR scenarios, predicted losses of denitrification potential due to replacement of S. patens by short-form S. alterniflora were substantially larger than losses due to reduced high marsh land area alone. Our results suggest that changes in vegetation zones can serve as landscape-scale predictors of the response of denitrification rates to rapid changes occurring in salt marshes.