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Nitrogen transformations in a through‐flow wetland revealed using whole‐ecosystem pulsed 15 N additions
Author(s) -
O'Brien Jonathan M.,
Hamilton Stephen K.,
Kinsman-Costello Lauren E.,
Len Jay T.,
Ostrom Nathaniel E.
Publication year - 2012
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2012.57.1.0221
Subject(s) - ammonium , nitrate , environmental chemistry , nitrification , diel vertical migration , chemistry , nitrogen , denitrification , constructed wetland , nitrogen cycle , water column , wetland , ecology , biology , organic chemistry
We used pulsed and continuous additions of 15 N together with whole‐ecosystem metabolism measurements to elucidate the mechanisms of nitrogen (N) transformations in a small (1170 m 2 ) through‐flow wetland situated along a stream. From measurements of the wetland inflow and outflow, we observed a consistent decrease in nitrate (by 10% of inflow concentrations), while ammonium increased by an order of magnitude. Outflow ammonium concentrations oscillated in a diel cycle, inverse to the concentration of dissolved oxygen (i.e., greater ammonium export at night). The pulsed 15 N additions showed little uptake of nitrate over time in the wetland and rapid daytime uptake of ammonium from the water column (rate constant, k t = 0.11 h −1 ). A steady‐state 15 N‐ammonium addition demonstrated a similar rate of ammonium uptake (k t = 0.067 h −1 ), no detectable nitrification, and highlighted the spatial pattern of ammonium and nitrate uptake within the wetland. Porewater concentration profiles suggest high rates of net ammonium diffusion from the sediments. Ecosystem metabolism measurements indicate that release was attenuated during the day by autotrophic uptake, resulting in lower ammonium export during the day. Denitrification rates were modeled from dissolved N 2 : Ar ratios, but they were not sufficient to account for the observed loss in nitrate. Nitrate was removed near the pond inflow but not actively cycled throughout the pond, while the balance between sediment release and subsequent uptake of ammonium from the water‐column dominated N cycling in this wetland.