The fate of fixed nitrogen in Santa Barbara Basin sediments during seasonal anoxia

Abstract

Abstract. Despite long-standing interest in the biogeochemistry of the Santa Barbara Basin (SBB), there are no direct rate measurements of different nitrogen transformation processes. We investigated benthic nitrogen cycling using in situ incubations with 15NO3- addition and quantified the rates of total nitrate (NO3-) uptake, denitrification, anaerobic ammonia oxidation (anammox), N2O production, and dissimilatory nitrate reduction to ammonia (DNRA). Denitrification was the dominant NO3- reduction process, while anammox contributed 0 %–27 % to total NO3- reduction. DNRA accounted for less than half of NO3- reduction except at the deepest station at the center of the SBB where NO3- concentration was lowest. NO3- availability and sediment total organic carbon content appeared to be two key controls on the relative importance of DNRA. The increasing importance of fixed N retention via DNRA relative to fixed N loss as NO3- deficit intensifies suggests a negative feedback loop that potentially contributes to stabilizing the fixed N budget in the SBB. Nitrous oxide (N2O) production as a fraction of total NO3- reduction ranged from 0.2 % to 1.5 %, which was higher than previous reports from nearby borderland basins. A large fraction of NO3- uptake was unaccounted for by NO3- reduction processes, suggesting that intracellular storage may play an important role. Our results indicate that the SBB acts as a strong sink for fixed nitrogen and potentially a net source of N2O to the water column.