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Denitrification and nitrous oxide cycling within the upper oxycline of the eastern tropical South Pacific oxygen minimum zone
Author(s) -
Farías Laura,
Castro-González Maribeb,
Cornejo Marcela,
Charpentier José,
Faúndez Juan,
Boonta Narin,
Yoshida Naohiro
Publication year - 2009
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.2009.54.1.0132
Subject(s) - denitrification , oxygen minimum zone , denitrifying bacteria , nitrous oxide , cycling , nutrient cycle , environmental chemistry , chemistry , nitrogen , oxygen , ecology , nutrient , biology , organic chemistry , history , archaeology
One of the shallowest, most intense oxygen minimum zones (OMZs) is found in the eastern tropical South Pacific, off northern Chile and southern Peru. It has a strong oxygen gradient (upper oxycline) and high N 2 O accumulation. N 2 O cycling by heterotrophic denitrification along the upper oxycline was studied by measuring N 2 O production and consumption rates using an improved acetylene blockage method. Dissolved N 2 O and its isotope ( 15 N: 14 N ratio in N 2 O or δ 15 N) and isotopomer composition (intramolecular distribution of 15 N in the N 2 O or δ 15 N α and δ 15 N β ), dissolved O 2 , nutrients, and other oceanographic variables were also measured. Strong N 2 O accumulation (up to 86 nmol L −1 ) was observed in the upper oxycline followed by a decline (around 8‐12 nmol L −1 ) toward the OMZ core. N 2 O production rates by denitrification (NO 2 − reduction to N 2 O) were 2.25 to 50.0 nmol L −1 d −1 , whereas N 2 O consumption rates (N 2 O reduction to N 2 ) were 2.73 and 70.8 nmol L −1 d −1 . δ 15 N in N 2 O increased from 8.57% in the middle oxycline (50‐m depth) to 14.87% toward the OMZ core (100‐m depth), indicating the progressive use of N 2 O as an electron acceptor by denitrifying organisms. Isotopomer signals of N 2 O (δ 15 N α and δ 15 N β ) showed an abrupt change at the middle oxycline, indicating different mechanisms of N 2 O production and consumption in this layer. Thus, partial denitrification along with aerobic ammonium oxidation appears to be responsible for N 2 O accumulation in the upper oxycline, where O 2 levels fluctuate widely; N 2 O reduction, on the other hand, is an important pathway for N 2 production. As a result, the proportion of N 2 O consumption relative to its production increased as O 2 decreased toward the OMZ core. A N 2 O mass balance in the subsurface layer indicates that only a small amount of the gas could be effluxed into the atmosphere (12.7‐30.7 µmol m −2 d −1 ) and that most N 2 O is used as an electron acceptor during denitrification (107‐468 µmol m −2 d −1 ).