Abstract

As a key element in fundamental biomolecules, the cycling and availability of nitrogen is a central factor governing the extent of ecosystems across the Earth. In the organic-lean sediment porewaters underlying the oligotrophic ocean, where low levels of microbial activity persist despite exceedingly energy-depleted conditions, the extent and modes of nitrogen transformations have not been widely investigated. We used the N and O isotopes of porewater nitrate from the North Pond site in the oligotrophic North Atlantic (North Pond) to provide constraints on the extent of both nitrate production (via nitrification) and consumption (via denitrification). Nitrate accumulates far above bottom seawater concentrations (~21µM) throughout the sediment column (up to almost ~50µM) and persists down to the oceanic basement as deep as 90 mbsf, indicating the predominance of aerobic nitrification and remineralization in these sediments. However, large changes in the δ¹⁵N and δ¹⁸O of nitrate also reveal variable influence of nitrate respiration across the three boreholes. Combining the N and O isotopes and using an inverse porewater diffusion-reaction model, we estimate rates of nitrification and denitrification throughout the sediment column. Results indicate a range of rates across the three boreholes and are generally consistent with variations observed in profiles of dissolved oxygen at this site. The model also estimates values of the N isotopic composition of newly produced nitrate, which were generally lower than measurements of sinking particulate nitrogen in this region. We suggest that this must be the result of sedimentary-hosted nitrogen fixation supplementing the relatively small organic matter pool derived from the overlying euphotic zone. These findings indicate that the production of organic matter by in situ autotrophy (by both nitrification and nitrogen fixation) must supply a large fraction of the biomass and organic substrate for heterotrophy in these sediments. This work sheds new light on an active nitrogen cycle operating, despite exceedingly low carbon inputs, in the deep sedimentary biosphere.