Denitrification Capacity of a Natural and a Restored Marsh in the Northern Gulf of Mexico

Evidence of nitrogen inputs affecting soil nitrogen purification by mediating root exudates of salt marsh plants

Salt marsh has an important 'purification' role in coastal ecosystems by removing excess nitrogen that could otherwise harm aquatic life and reduce water quality. Recent studies suggest that salt marsh root exudates might be the 'control centre' for nitrogen transformation, but empirical evidence is lacking. Here we sought to estimate the direction and magnitude of nitrogen purification by salt marsh root exudates and gain a mechanistic understanding of the biogeochemical transformation pathway(s). To achieve this, we used a laboratory incubation to quantify both the root exudates and soil nitrogen purification rates, in addition to the enzyme activities and functional genes under Phragmites australis populations with different nitrogen forms addition (NO3-, NH4+ and urea). We found that NO3- and urea addition significantly stimulate P. australis root exudation of total acids, amino acids, total sugars and total organic carbon, while NH4+ addition only significantly increased total acids, amino acids and total phenol exudation. High total sugars, amino acids and total organic carbon concentrations enlarged nitrogen purification potential by stimulating the nitrogen purifying bacterial activities (including enzyme activities and related genes expression). Potential denitrification rates were not significantly elevated under NH4+ addition in comparison to NO3- and urea addition, which should be ascribed to total phenol self-toxicity and selective inhibition. Further, urea addition stimulated urease and protease activities with providing more NH4+ and NO2- substrates for elevated anaerobic ammonium oxidation rates among the nitrogen addition treatments. Overall, this study revealed that exogenous nitrogen could increase the nitrogen purification-associated bacterial activity through accelerating the root exudate release, which could stimulate the activity of nitrogen transformation, and then improve the nitrogen removal capacity in salt marsh.

Vegetation zones as indicators of denitrification potential in salt marshes

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.

Contrasting effects of the aboveground litter of native Phragmites australis and invasive Spartina alterniflora on nitrification and denitrification

Aboveground litter inputs from plants are among the most important pathways for carbon and nutrient fluxes to the soil. Previous studies on the effects of aboveground litter from invasive plants on ecosystem processes have primarily focused on biogeochemical cycling processes such as C and N mineralization, whereas the effects of aboveground litter from invasive plants on nitrogen removal processes are not well understood. In this study, the effects of the aboveground litter of native Phragmites australis and exotic Spartina alterniflora on soil nitrification and denitrification were compared. Results showed that the removal of the aboveground litter of both species had no effect on nitrification or denitrification in the early growth phase. However, after aboveground litter removal in the late growth phase, nitrification and denitrification in the P. australis stands decreased by 41.18% and 25.11%, respectively, whereas no such changes were observed in the S. alterniflora stands. These results indicate that the impacts of aboveground litter on nitrification and denitrification are species-specific. The aboveground litter from indigenous P. australis affected the SOC content and then indirectly affected nitrification or denitrification, and these effects were clearer in the late growth phase. Although other studies have reported that the invasive S. alterniflora have strong impacts on nitrogen removal processes, our study showed that the aboveground litter from S. alterniflora did not alter nitrification or denitrification, which indicates that other pathways may play important roles in nitrogen removal processes than its aboveground litter does.