Nitrogen removal by anammox and denitrification in a subtropical seagrass ecosystem

Abstract:

Anammox is now recognized as a globally important process that may rival denitrification in its removal of inorganic N from aquatic systems. Owing to its fairly recent discovery, however, measurements of anammox in coastal environments are sparse. This study measured the N removal processes, denitrification and anammox, in a seagrass ecosystem in New South Wales, Australia, using a modified version of the isotope pairing technique. This previously published method has yet to be applied to seagrass environments and relies on the measurement of both N₂O and N₂ to calculate rates of anammox and denitrification. Intact sediment cores were incubated under in situ conditions and amended with ¹⁵N-NO₃^- as a tracer. The role of organic C quality in controlling N removal rates was evaluated through addition of seagrass or phytoplankton detritus. In control cores (i.e., no C addition), the total N removal rate was 0.48 ± 0.22 μmol N m^-2 h^-1. These are among the lowest rates measured in seagrasses and other coastal areas. N removal rates were stimulated when seagrass detritus was added (3.3 times higher than control) whereas the addition of phytoplankton detritus did not stimulate N removal. This is surprising, as phytoplankton detritus is generally a more labile C source than seagrass detritus. These results indicate that the microbial community responds more quickly to organic matter they are conditioned to process. Regardless of C treatment, anammox made up a greater proportion of N₂ production (71 ± 16%) than denitrification (24 ± 13%), in contrast to most other studies. The high proportion of anammox-derived N₂ production implies that previous N removal rates measured in other systems without anammox inclusion could be significantly underestimated.