Long-term nitrogen burial exceeds denitrification in global fjords

Abstract

Abstract Nitrogen (N) availability regulates primary productivity and hence directly affects global oceanic carbon sequestration. Global fjords account for up to 11% of marine carbon burial. However, N loss via sediment burial remains largely unquantified. Here, we show that global fjords are hotspots of N burial, accounting for up to 18% of oceanic N burial despite only covering 0.1% of the ocean area. Burial is the dominant N loss mechanism, exceeding microbial N loss via denitrification and anammox, which are generally considered the major N loss mechanisms. Microbial N loss dominates in anoxic fjords and appears to be a function of temperature and nutrient availability. Overall, fjords efficiently sequester excess N in sediments over long time scales. Accelerated warming will promote both N burial from increased primary production and microbial N loss from warmer temperatures, affecting N budgets in fjords and in the ocean in general.