『Abstract
　Biologically available nitrogen (fixed N) is removed from the oceans by metabolic conversion of inorganic N forms (nitrate and ammonium) to N2 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 〜20‰ enrichment in ¹⁵N and ¹⁸O of this biologically transported nitrate over bottom water values and elevated [N2] and δ¹⁵N-N2 at depth indicate that this nitrate is consumed by enzymatic redox reactions with the production of N2 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 N2 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.』

1. Introduction
2. methods
　2.1. Core incubations for benthic N2 flux determination in San Pedro basin
　2.2. Dissolved N2 and δ¹⁵N-N2 analysis using IRMS
　2.3. Dissolved [N2] and [N2O] analysis by gas chromatography
　2.4. Dissolved Fe²⁺, nitrate, nitrite, ammonium and δ¹⁵N analysis
3. Results: Subsurface nitrogen cycling
　3.1. Nitrate distribution in the subsurface controlled by diffusion and by biological nitrate transport
　3.2. Pore water N2, δ¹⁵N-N2 and N2O
　3.3. Pore water ammonium and δ¹⁵N-NH4⁺
　3.4. N2 flux
4. Model
　4.1. Model description
　4.2. Running the model
　4.3. Model sensitivity analysis
　4.4. Model results - contribution of nitrate bio-transport to benthic nitrogen losses
5. Discussion
　5.1. Potential electron donors for denitrification of transported nitrate and organisms involved
　5.2. Biological nitrate transport, carbon oxidation and implications for N cycle
6. Conclusions
Acknowledgements
Appendix A
　Predicted. ΔN2 in pore water based on diffusion supported denitrification and Anammox
Appendix B. Model
　Decomposition of organic nitrogen (Norg)
　Modeling of ¹⁵N isotopes
References