Nitrogen cycling in the deep sedimentary biosphere: nitrate isotopes in porewaters underlying the oligotrophic North Atlantic

摘要

Nitrogen (N) is a key component of fundamental biomolecules. Hence, its cycling and availability are central factors governing the extent ofecosystems across the Earth. In the organic-lean sediment porewatersunderlying the oligotrophic ocean, where low levels of microbial activitypersist despite limited organic matter delivery from overlying water, theextent and modes of nitrogen transformations have not been widelyinvestigated. Here we use the N and oxygen (O) isotopic composition ofporewater nitrate (NO₃−) from a site in the oligotrophic NorthAtlantic (Integrated Ocean Drilling Program – IODP) to determine the extent and magnitude of microbial nitrateproduction (via nitrification) and consumption (via denitrification). We findthat NO₃- accumulates far above bottom seawater concentrations(~ 21 μM) throughout the sediment column (up to~ 50 μM) down to the oceanic basement as deep as 90 m b.s.f. (below sea floor),reflecting the predominance of aerobic nitrification/remineralization withinthe deep marine sediments. Large changes in the δ15N andδ18O of nitrate, however, reveal variable influence of nitraterespiration across the three sites. We use an inverse porewaterdiffusion–reaction model, constrained by the N and O isotope systematics ofnitrification and denitrification and the porewater NO₃- isotopiccomposition, to estimate rates of nitrification and denitrificationthroughout the sediment column. Results indicate variability of reactionrates across and within the three boreholes that are generally consistentwith the differential distribution of dissolved oxygen at this site, thoughnot necessarily with the canonical view of how redox thresholds separatenitrate regeneration from dissimilative consumption spatially. That is, weprovide stable isotopic evidence for expanded zones of co-occurringnitrification and denitrification. The isotope biogeochemical modeling alsoyielded estimates for the δ15N and δ18O of newly producednitrate (δ15N_NTR (NTR, referring to nitrification) and δ18O_NTR), as well as theisotope effect for denitrification (15ϵ_DNF) (DNF, referring to denitrification), parameterswith high relevance to global ocean models of N cycling. Estimated values ofδ15N_NTR were generally lower than previously reportedδ15N values for sinking particulate organic nitrogen in this region. We suggest that thesevalues may be, in part, related to sedimentary N₂ fixation andremineralization of the newly fixed organic N. Values ofδ18O_NTR generally ranged between ?2.8 and 0.0 ‰,consistent with recent estimates based on lab cultures of nitrifyingbacteria. Notably, some δ18O_NTR values were elevated,suggesting incorporation of 18O-enriched dissolved oxygen duringnitrification, and possibly indicating a tight coupling of NH₄+ andNO₂− oxidation in this metabolically sluggish environment. Ourfindings indicate that the production of organic matter by in situ autotrophy(e.g., nitrification, nitrogen fixation) supplies a large fraction of thebiomass and organic substrate for heterotrophy in these sediments,supplementing the small organic-matter pool derived from the overlyingeuphotic zone. This work sheds new light on an active nitrogen cycleoperating, despite exceedingly low carbon inputs, in the deep sedimentarybiosphere.