INTERACTIONS OF MANGANESE WITH THE NITROGEN-CYCLE - ALTERNATIVE PATHWAYS TO DINITROGEN

The conversion of combined nitrogen (ammonia, nitrate, organic nitroge n) to dinitrogen (N-2) in marine sediments, an important link in the g lobal nitrogen cycle, is traditionally assumed to take place only via the coupled bacterial nitrification-denitrification process. We provid e field and laboratory evidence that N-2 can also be produced by the o xidation of NH3 and organic-N with MnO2 in air. The reduced manganese formed in this reaction readily reacts with O-2, generating reactive M n(III, TV) species to continue the oxidation of NH3 and organic-N to N -2. Free energy calculations indicate that these two reactions are mor e favorable as a couple than the oxidation of organic matter by O-2 al one. We also provide field evidence consistent with the reduction of N O3- to N-2 by dissolved Mn2+. These two reactions involving nitrogen a nd manganese species can take place in the presence and absence of O-2 , respectively. Our field evidence suggests that the oxidation of NH3 and organic-N to N-2 by MnO2 in the presence of O-2 can outcompete the oxidation of NH3 to NO3- in Mn-rich continental margin sediments and thereby short-circuit the nitrification/denitrification process. The M nO2 catalyzed reaction may account for up to 90% of the N-2 formation in continental margin sediments, the most important N-2 producing envi ronments in the marine N cycle. The oxidation of NH3 and organic-N by MnO2 in the presence of O-2 can explain why N-2 can form in oxic sedim ents; it can also explain why denitrification rates measured by acetyl ene inhibition and labeled tracers can give lower estimates than direc t measurements of N-2 production. Copyright (C) 1997 Elsevier Science Ltd.