Denitrification in anoxic sediments supported by biological nitrate transport

Biologically available nitrogen (fixed N) is removed from the oceans by metabolic conversion of inorganic N forms (nitrate and ammonium) to N-2 gas. Much of this removal occurs in marine sediments, where reaction rates are thought to be limited by diffusion. We measured the concentration and isotopic composition of major dissolved nitrogen species in anoxic sediments off the coast of California. At depths below the diffusive penetration of nitrate, we found evidence of a large nitrate pool transported into the sediments by motile microorganisms. A similar to 20 parts per thousand enrichment in N-15 and O-18 of this biologically transported nitrate over bottom water values and elevated [N-2] and delta N-15-N-2 at depth indicate that this nitrate is consumed by enzymatic redox reactions with the production of N-2 as the end product. Elevated N2O concentrations in pore waters below the nitrate diffusion depth confirm that these reactions include the denitrification pathway. A data-constrained model shows that at least 31% of the total N-2 production in anoxic sediments is linked to nitrate bio-transport. Under suboxic/anoxic regimes, this nitrate bio-transport augments diffusive transport, thus increasing benthic fixed nitrogen losses and the reducing burial efficiency of sedimentary organic matter. (C) 2011 Elsevier Ltd. All rights reserved.