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Nitrification and Nitrous Oxide Production in the Offshore Waters of the Eastern Tropical South Pacific
Author(s) -
Santoro Alyson E.,
Buchwald Carolyn,
Knapp Angela N.,
Berelson William M.,
Capone Douglas G.,
Casciotti Karen L.
Publication year - 2021
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2020gb006716
Subject(s) - nitrification , nitrite , nitrate , environmental chemistry , nitrous oxide , ammonia , biogeochemistry , photic zone , nitrogen cycle , ammonium , new production , reactive nitrogen , nitrogen , environmental science , oceanography , chemistry , nutrient , geology , organic chemistry , phytoplankton
Marine oxygen deficient zones are dynamic areas of microbial nitrogen cycling. Nitrification, the microbial oxidation of ammonia to nitrate, plays multiple roles in the biogeochemistry of these regions, including production of the greenhouse gas nitrous oxide (N 2 O). We present here the results of two oceanographic cruises investigating nitrification, nitrifying microorganisms, and N 2 O production and distribution from the offshore waters of the Eastern Tropical South Pacific. On each cruise, high‐resolution measurements of ammonium ([NH 4 + ]), nitrite ([NO 2 − ]), and N 2 O were combined with 15 N tracer‐based determination of ammonia oxidation, nitrite oxidation, nitrate reduction, and N 2 O production rates. Depth‐integrated inventories of NH 4 + and NO 2 − were positively correlated with one another and with depth‐integrated primary production. Depth‐integrated ammonia oxidation rates were correlated with sinking particulate organic nitrogen flux but not with primary production; ammonia oxidation rates were undetectable in trap‐collected sinking particulate material. Nitrite oxidation rates exceeded ammonia oxidation rates at most mesopelagic depths. We found positive correlations between archaeal amoA genes and ammonia oxidation rates and between Nitrospina ‐like 16S rRNA genes and nitrite oxidation rates. N 2 O concentrations in the upper oxycline reached values of >140 nM, even at the western extent of the cruise track, supporting air‐sea fluxes of up to 1.71 μmol m −2 day −1 . Our results suggest that a source of NO 2 − other than ammonia oxidation may fuel high rates of nitrite oxidation in the offshore Eastern Tropical South Pacific and that air‐sea fluxes of N 2 O from this region may be higher than previously estimated.