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Abstract:
The global marine nitrogen cycle is constrained by nitrogen fixation as
a source of reactive nitrogen, and denitrification or anammox on the
sink side. These processes with their respective isotope effects set the
marine nitrate N-15-isotope value (delta N-15) to a relatively constant
average of 5 parts per thousand. This value can be used to better assess
the magnitude of these sources and sink terms, but the underlying
assumption is that sedimentary denitrification and anammox, processes
responsible for approximately one-third of global nitrogen removal, have
little to no isotope effect on nitrate in the water column.
We investigated the isotope fractionation in sediment incubations,
measuring net denitrification and nitrogen and oxygen stable isotope
fractionation in surface sediments from the coastal Baltic Sea (Boknis
Eck, northern Germany), a site with seasonal hypoxia and dynamic
nitrogen turnover.
Sediment denitrification was fast, and regardless of current paradigms
assuming little fractionation during sediment denitrification, we
measured fractionation factors of 18.9 parts per thousand for nitrogen
and 15.8 parts per thousand for oxygen in nitrate. While the input of
nitrate to the water column remains speculative, these results challenge
the current view of fractionation during sedimentary denitrification and
imply that nitrogen budget calculations may need to consider this
variability, as both preferential uptake of light nitrate and release of
the remaining heavy fraction can significantly alter water column
nitrate isotope values at the sediment-water interface.
