Heterotrophic nitrogen fixation in response to nitrate loading and sediment organic matter in an emerging coastal deltaic floodplain within the Mississippi River Delta plain

Increasing nitrate (NO 3 − ) loading in rivers due to agricultural fertilization alters benthic nitrogen (N) cycling and shifts coastal wetlands from being a net source to net sink of reactive N. Heterotrophic N 2 fixation that converts N 2 to reactive N is often assumed negligible in eutrophic ecosystems and excluded in coastal N budget evaluations. We investigated N 2 fixation and denitrification in response to increasing NO 3 − loading (0, 10, and 100  μ M) and sediment organic matter (OM sediment ) concentrations in the emerging Wax Lake Delta. Continuous flow‐through incubations with 30 N 2 addition was applied to measure N 2 fixation. The variation of N 2 fixation rates from 0 to 437  μ mol N m −2 h −1 among different NO 3 − and OM sediment concentrations were comparable to the estimated denitrification rates of 141–377  μ mol N m −2 h −1 . Increasing overlying NO 3 − concentrations reduced N 2 fixation rates and facilitated denitrification rates at each OM sediment concentration. However, 100  μ M of overlying NO 3 − did not thoroughly inhibit N 2 fixation rates in sites with intermediate and higher OM sediment concentrations (189 and 99  μ mol N m −2 h −1 , respectively). Both N 2 fixation and denitrification increased with increasing OM sediment concentrations, but the relative importance of these processes was impacted mostly by overlying NO 3 − concentration as increasing NO 3 − switched the dominance of N 2 fixation to denitrification in benthic N cycling. This study highlights the importance of heterotrophic N 2 fixation in coastal deltaic floodplains and emphasizes the necessity of including N 2 fixation quantification in coastal N budget evaluation, not only in oligotrophic environment but also in eutrophic environment.