N2O cycling at the core of the oxygen minimum zone off northern Chile

Abstract

Northern Chile's oxygen minimum zone (OMZ) is considered to be an important site of N 2 O production and efflux into the atmosphere, with a potentially global impact. Seawater samples from the OMZ core were used to determine how different O 2 levels and electron donor/acceptor availability affect N 2 O cycling. N 2 O production by denitrification (N 2 Opd; acetylene treatment) and nitrification (N 2 Opn; allylthiourea [ATU] treatment), and N 2 O consumption by denitrification (N 2 Ocd), were determined with in situ O 2 , anoxic (0 M O 2 ), hypoxic (~22.3 M O 2 ), and potential (added substrate) experimental conditions. Under in situ O 2 levels (~4.6 M), total N 2 O production (N 2 Opd + N 2 Opn) was ~2.62 M d -1 . Denitrification was responsible for over 92% of the total N 2 Opd and nitrification for less than 8%. Nearly 100% of the N 2 O produced was, however, consumed by denitrification. NO 3 -was reduced twice as rapidly as NO 2 -. Under anoxia, N 2 Opd and N 2 Ocd rates decreased by over 90%. The NO 3 -reduction was similar to that observed with in situ O 2 , whereas a high rate of NO 2 -accumulation was observed. Conversely, increasing O 2 levels (~22.3 M) doubled N 2 Opd. Consequently, N 2 Opd or NO 2 -reduction seems to be the process most sensitive to O 2 fluctuations. Adding organic carbon and NO 3 -increased N 2 Opd and N 2 Ocd slightly, whereas additional N 2 O increased N 2 Ocd abruptly. The fate of reduced NO 3 -in the OMZ core was controlled mainly by O 2 concentrations and indirectly by available organic carbon. Both variables are susceptible to the changes experienced in the eastern South Pacific during the El Nio Southern Oscillation cycle.