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Differential N 2 O dynamics in two oxygen‐deficient lake basins revealed by stable isotope and isotopomer distributions
Author(s) -
Wenk Christine B.,
Frame Caitlin H.,
Koba Keisuke,
Casciotti Karen L.,
Veronesi Mauro,
Niemann Helge,
Schubert Carsten J.,
Yoshida Naohiro,
Toyoda Sakae,
Makabe Akiko,
Zopfi Jakob,
Lehmann Moritz F.
Publication year - 2016
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 - 10.1002/lno.10329
Subject(s) - denitrification , environmental chemistry , biogeochemical cycle , nitrification , stable isotope ratio , isotopes of nitrogen , nitrous oxide , environmental science , chemistry , ecology , nitrogen , biology , physics , organic chemistry , quantum mechanics
Lakes are a nitrous oxide (N 2 O) source to the atmosphere, but the biogeochemical controls and microbial pathways of N 2 O production are not well understood. To trace microbial N 2 O production (denitrification, nitrifier denitrification, and nitrification) and consumption (denitrification) in two basins of Lake Lugano, we measured the concentrations and N and O isotope compositions of N 2 O, as well as the intramolecular 15 N distribution, i.e., site preference (SP). Our results revealed differential N 2 O dynamics in the two lake basins, with N 2 O concentrations between 12 nmol L −1 and > 900 nmol L −1 in the holomictic South Basin, and significantly lower concentrations in the meromictic North Basin (<13 nmol L −1 ). In the South Basin, the isotope signatures reflected a complex combination of N 2 O production by nitrifying bacteria through hydroxylamine (NH 2 OH) oxidation, N 2 O production through incomplete denitrification, and N 2 O reduction to N 2 , all occurring in close vicinity within the redox transition zone (RTZ). In the North Basin, in contrast, the N 2 O isotopomer signatures suggested that nitrifier denitrification was the main N 2 O source. The pronounced decrease in N 2 O concentrations to undetectable levels within the RTZ, in tandem with an increase in δ 15 N‐N 2 O, δ 18 O‐N 2 O, and SP indicated quantitative N 2 O consumption by microbial denitrification. In the northern basin this was primarily sulfide‐dependent. The apparent N and O isotope enrichment factors associated with net N 2 O consumption were 15 ε ≈ 3.2‰ and 18 ε ≈ 8.6‰, respectively. The according 18 O to 15 N enrichment ratio ( 18 ε : 15 ε ≈ 2.5) is consistent with previous reports for microbial N 2 O reduction, underscoring its robust nature across environments.