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

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 O-2 levels and electron donor/acceptor availability affect N2O cycling. N2O production by denitrification (N(2)Opd; acetylene treatment) and nitrification (N(2)Opn; allylthiourea [ATU] treatment), and N2O consumption by denitrification (N(2)Ocd), were determined with in situ O-2, anoxic (0 muM O-2), hypoxic (similar to22.3 muM O-2), and potential (added substrate) experimental conditions. Under in situ O-2 levels (similar to4.6 muM), total N2O production (N(2)Opd + N(2)Opn) was similar to2.62 muM d(-1). Denitrification was responsible for over 92% of the total N(2)Opd 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, N(2)Opd and N(2)Ocd rates decreased by over 90%. The NO3 reduction was similar to that observed with in situ O-2, whereas a high rate of NO2- accumulation was observed. Conversely, increasing O-2 levels (similar to22.3 muM) doubled N(2)Opd. Consequently, N(2)Opd or NO2- reduction seems to be the process most sensitive to O-2 fluctuations. Adding organic carbon and NO3 increased N(2)Opd and N(2)Ocd slightly, whereas additional N2O increased N(2)Ocd abruptly. The fate of reduced NO3- 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 Niho Southern Oscillation cycle.