Sediment Nitrogen Fixation: a Call for Re-evaluating Coastal N Budgets

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 N-2 flux measurements have shown that sediment N-fixation can equal or exceed N-2 loss. In an effort to quantify N-2 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 N-2 flux shifted from uptake (N-fixation) to efflux (denitrification) over the 120-h incubation. Evidence for N-fixation included net N-28(2) and N-30(2) uptake, (NH4)-N-15 (+) production from N-30(2) additions, N-15(organic matter) production, and nifH expression. N-fixation from N-30(2) was up to eight times higher than potential denitrification. However, N-fixation calculated from (NO3)-N-15 (-) was one half of the measured fixation from N-30(2), indicating that (NO3)-N-15-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 N-2 fluxes signify and how that balance varies over time.