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 N2O production and efflux into the atmosphere, with a potentially global impact. Seawater samples from the OMZ core were used to determine how different O2 levels and electron donor/acceptor availability affect N2O cycling. N2O production by denitrification (N2Opd; acetylene treatment) and nitrification (N2Opn; allylthiourea [ATU] treatment), and N2O consumption by denitrification (N2Ocd), were determined with in situ O2, anoxic (0 μM O2), hypoxic (∼22.3 μM O 2), and potential (added substrate) experimental conditions. Under in situ O2 levels (∼4.6 μM), total N2O production (N2Opd + N2Opn) was sim;2.62 μM d-1. Denitrification was responsible for over 92% of the total N2Opd and nitrification for less than 8%. Nearly 100% of the N2O produced was, however, consumed by denitrification. NO3- was reduced twice as rapidly as NO2-. Under anoxia, N2Opd and N2Ocd rates decreased by over 90%. The NO3- reduction was similar to that observed with in situ O2, whereas a high rate of NO2- accumulation was observed. Conversely, increasing O2 levels (∼22.3 μM) doubled N2Opd. Consequently, N2Opd or NO2- reduction seems to be the process most sensitive to O2 fluctuations. Adding organic carbon and NO3- increased N2Opd and N 2Ocd slightly, whereas additional N2O increased N 2Ocd abruptly. The fate of reduced NO3- in the OMZ core was controlled mainly by O2 concentrations and indirectly by available organic carbon. Both variables are susceptible to the changes experienced in the eastern South Pacific during the El Nin̄o Southern Oscillation cycle.