Groundwater nitrogen processing in Northern Gulf of Mexico restored marshes

Groundwater nitrogen processing was examined in a restored black needlerush (Juncus roemerianus) marsh to assess its potential for removing land-derived nitrogen pollution. Two restoration designs, one initially planted at 50% cover (half density plots) and the other one at 100% cover (full density plots), were compared with non-vegetated controls:111e introduction via groundwater of a NO3- solution with a conservative tracer (Br-) and labeled isotopically (N-15) allowed calculation of nitrogen removal in the plots following two methods. The first method used changes in the ratio [NOx]:[Br-] as the groundwater plume traveled through the plot, and the second method relied on balancing N-15 input with N-15 export. Both methods showed approximate to 97% of the N from the simulated groundwater plume was removed (i.e. not delivered to the open waters of the adjacent estuary) in vegetated plots and approximate to 86% was removed in nonvegetated controls. The most dominkit routes of N removal from the introduced solution were N-2 production and assimilation into macrophyte biomass, which were similar in magnitude for the vegetated plots, whereas N-2 production dominated in the unvegetated plots. The majority of N removed from the introduced solution occurred in the first 30 cm the solution traveled in the vegetated treatments. In addition, ambient porewater concentrations of dissolved inorganic nitrogen (DIN) were similar between full and half density plots, but lower than the non-vegetated control (approximate to 8.5 x and 7.5x), suggesting full and half density plots removed more DIN than non-vegetated plots. These results suggest that restoring marshes by planting 50% of the area may be a more cost-effective restoration design in terms of mitigating land-derived nutrient pollution than planting 100% of the area since it requires less effort and cost while removing similar quantities of N. (C) 2014 Elsevier Ltd. All rights reserved.