Long term nitrogen burial exceeds denitrification in global fjords

Nitrogen (N) availability regulates primary productivity and hence directly affects the global oceanic carbon cycle. Marine carbon sequestration is strongly influenced by the balance between N uptake and loss. Global fjord systems account for up to 11% of total marine C sequestration. However, N cycling – particularly N loss via sediment burial – remains largely unquantified in fjords worldwide. Here, we show that global fjords are hotspots of N burial accounting for up to 18% of global oceanic N burial despite only covering 0.1% of the ocean area. Burial is the dominant N loss mechanism in fjords, exceeding microbial N loss via denitrification and anammox that are often considered the major marine N loss mechanisms. Microbial N loss was dominant in fjords with an anoxic water column. In addition to fjord redox condition, microbial N loss appears to be a function of temperature and nutrient availability. Overall, fjords provide an important service efficiently sequestering N excess over long time scales. Accelerated warming in polar regions will promote both burial and microbial N loss, affecting the N budgets in fjords and in the ocean in general.