Coastal ocean primary productivity is often limited by nitrogen (N) availability, which is determined by the balance between N sources (e.g., N-fixation, groundwater, river inputs, etc.) and sinks (e.g., denitrification, sediment burial, etc.). Historically, heterotrophic N-fixation in sediments was excluded as a significant source of N in estuarine budgets, based on low, indirectly measured rates (e.g., acetylene reduction assay) and because it was unnecessary to achieve mass balance. Many recent studies using net N2 flux measurements have shown that sediment N-fixation can equal or exceed N2 loss. In an effort to quantify N2 production and consumption simultaneously, we measured N-fixation and denitrification directly in sediment cores from a temperate estuary (Waquoit Bay, MA). N-fixation, dissimilatory nitrate reduction to ammonium, and denitrification occurred simultaneously, and the net N2 flux shifted from uptake (N-fixation) to efflux (denitrification) over the 120-h incubation. Evidence for N-fixation included net 28N2 and 30N2 uptake, 15NH4 + production from 30N2 additions, 15Norganic matter production, and nifH expression. N-fixation from 30N2 was up to eight times higher than potential denitrification. However, N-fixation calculated from 15NO3 − was one half of the measured fixation from 30N2, indicating that 15NO3-isotope labeling calculations may underestimate N-fixation. These results highlight the dynamic nature of sediment N cycling and suggest that quantifying individual processes allows a greater understanding of what net N2 fluxes signify and how that balance varies over time.
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We thank Chris Weidman and the staff at WB-NERR for boat time and Sarah Foster, Hollie Emery, Kristin Yoshimura, and Kaitlyn Pritchard for assistance in the field and in the lab. We also thank Allen Hunt for his help with the diffusion calculations. SEN was funded by an NSF Postdoctoral Fellowship in Biology (1103692). This work was supported in part by funding from the Woods Hole Sea Grant to RWF, as well as the Sloan Foundation fellowship to RWF.
Communicated by David Reide Corbett
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Newell, S.E., McCarthy, M.J., Gardner, W.S. et al. Sediment Nitrogen Fixation: a Call for Re-evaluating Coastal N Budgets. Estuaries and Coasts 39, 1626–1638 (2016). https://doi.org/10.1007/s12237-016-0116-y