Nitrite isotope dynamics in coastal sediments: An intricate link between nitrogen and oxygen cycling

Marine sediments often exhibit strong redox gradients, hosting a range of important nitrogen transformation processes. While the interplay among these microbially catalyzed nitrogen transformations has been well studied in the water column, the sharp redox transition in sediments often makes it far more difficult to unravel the complexity underpinning the cycling of nitrogen. Although often low in concentration, nitrite represents an important ‘crossroad’ in the nitrogen cycle as a reactive intermediate of both reductive and oxidative N transformations, including nitrification, dissimilatory nitrate reduction to ammonium and denitrification. Here we focus on the dual isotopic composition of nitrite (δ¹⁵N and δ¹⁸O), in concert with nitrate and ammonium data, as a means for constraining the sedimentary N cycling.

Intact flow-through core incubations were performed on sediments collected from intertidal flats on the island of Sylt, Germany. Three types of substrate (i.e., sand, silt, and clay) were collected and subjected to different oxygen (i.e., ambient vs depleted) and nitrogen (i.e., ambient vs highly loaded) regimes. In addition to the measurement of natural abundance N and O stable isotopes, we also amended cores with nitrate having a positive ∆¹⁷O in our high nitrogen treatment, which offers yet an additional tracer to further constrain these transformations.

While the concentration and isotopic composition (δ¹⁵N and δ¹⁸O) of nitrite act to integrate the influence of major N redox reactions, the N and O isotope systematics are decoupled. Although nitrogen atoms are generally conserved among these transformations, oxygen isotopes of nitrite are subject to a different set of processes. For example, the loss of an oxygen atom during the reductive processes of NO₃^- and NO₂^- reduction, the gain of oxygen atoms from O₂ and water during nitrification, and oxygen isotopic equilibration between nitrite and water are all reflected in the δ¹⁸O of NO₂^-. Thus, the δ¹⁵N, δ¹⁸O and Δ¹⁷O can be used together to help elucidate the interconnections between the cycling of nitrogen and oxygen. While these sediments were a net source of NO₂^- to the overlying water, our NO₂^- isotope data revealed the integrated influence of denitrification, nitrification and oxygen isotope exchange in these dynamic systems.