Subsurface biogeochemical cycling of nitrogen in the actively serpentinizing Samail Ophiolite, Oman.

Nitrogen (N) is an essential element for life. N compounds such as ammonium ( ) may act as electron donors, while nitrate ( ) and nitrite ( ) may serve as electron acceptors to support energy metabolism. However, little is known regarding the availability and forms of N in subsurface ecosystems, particularly in serpentinite-hosted settings where hydrogen (H) generated through water-rock reactions promotes habitable conditions for microbial life. Here, we analyzed N and oxygen (O) isotope composition to investigate the source, abundance, and cycling of N species within the Samail Ophiolite of Oman. The dominant dissolved N species was dependent on the fluid type, with Mg- type fluids comprised mostly of , and Ca-OH fluids comprised primarily of ammonia (NH). We infer that fixed N is introduced to the serpentinite aquifer as . High concentrations of (>100 μM) with a relict meteoric oxygen isotopic composition (δO ~ 22‰, ΔO ~ 6‰) were observed in shallow aquifer fluids, indicative of sourced from atmospheric deposition (rainwater : δO of 53.7‰, ΔO of 16.8‰) mixed with produced through nitrification (estimated endmember δO and ΔO of ~0‰). Conversely, highly reacted hyperalkaline fluids had high concentrations of NH (>100 μM) with little detectable. We interpret that NH in hyperalkaline fluids is a product of reduction. The proportionality of the O and N isotope fractionation (ε / ε) measured in Samail Ophiolite was close to unity (ε / ε ~ 1), which is consistent with dissimilatory reduction with a membrane-bound reductase (NarG); however, abiotic reduction processes may also be occurring. The presence of genes commonly involved in N reduction processes () in the metagenomes of biomass sourced from aquifer fluids supports potential biological involvement in the consumption of . Production of as the end-product of reduction dissimilatory nitrate reduction to ammonium (DNRA) could retain N in the subsurface and fuel nitrification in the oxygenated near surface. Elevated bioavailable N in all sampled fluids indicates that N is not likely limiting as a nutrient in serpentinites of the Samail Ophiolite.