Restoration enhances denitrification and DNRA in subsurface sediments of Zostera marina seagrass meadows

Seagrasses exude oxygen and labile carbon into the sediment, which can stimulate microbial activity. However, it is not clear how seagrasses impact competing nitrate reduction processes, including nitrogen removal through denitrification and nitrogen retention through dissimilatory nitrate reduction to ammonium (DNRA). Using an in situ push-pull incubation method, we measured denitrification and DNRA rates in the root zone of a restored Zostera marina meadow, in adjacent unvegetated sediments, and in experimentally cleared plots within the meadow. Denitrification and DNRA rates in the meadow sediments were highly variable and contained 'hotspots' where maximum rates exceeded median rates by more than an order of magnitude. Hotspots were not observed in bare sediments, leading to average rates 4x greater in vegetated sediments than in bare sediments. In the meadow sediments, denitrification dominated over DNRA except in fall, during seagrass senescence, and after the experimental removal of seagrass. Extrapolated rates of annual nitrate removal via denitrification were greater in the vegetated sediments compared to bare sediments (0.62 compared to 0.16 g N m(-2) yr(-1)) and accounted for 44 % of annual N loading to the system. Similarly, annual DNRA rates were greater in the vegetated compared to bare sediments (0.45 and 0.12 g N m(-2) yr(-1), respectively). The restoration of the seagrass meadow thus increased both nitrogen removal and recycling, but removal via denitrification was the dominant process. The dominance of denitrification demonstrates how seagrass restoration can enhance the filter function of shallow coastal systems.