Interactions of manganese with the nitrogen cycle: Alternative pathways to dinitrogen

Abstract The conversion of combined nitrogen (ammonia, nitrate, organic nitrogen) to dinitrogen (N 2 ) in marine sediments, an important link in the global nitrogen cycle, is traditionally assumed to take place only via the coupled bacterial nitrification-denitrification process. We provide field and laboratory evidence that N 2 can also be produced by the oxidation of NH 3 and organic-N With MnO 2 in air. The reduced manganese formed in this reaction readily reacts With O 2 , generating reactive Mn(III, IV) species to continue the oxidation of NH 3 and organic-N to N 2 . Free energy calculations indicate that these two reactions are more favorable as a couple than the oxidation of organic matter by O 2 alone. We also provide field evidence consistent With the reduction of NO 3 − to N 2 by dissolved Mn 2+ . These two reactions involving nitrogen and manganese species can take place in the presence and absence of O 2 , respectively. Our field evidence suggests that the oxidation of NH 3 and organic-N to N 2 by MnO 2 in the presence of O 2 can outcompete the oxidation of NH 3 to NO 3 in Mn-rich continental margin sediments and thereby short-circuit the nitrification/ denitrification process. The MnO 2 catalyzed reaction may account for up to 90% of the N 2 formation in continental margin sediments, the most important N 2 producing environments in the marine N cycle. The oxidation of NH 3 and organic-N by MnO 2 in the presence of O 2 can explain Why N 2 can form in oxic sediments; it can also explain Why denitrification rates measured by acetylene inhibition and labeled tracers can give lower estimates than direct measurements of N 2 production.