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

Abstract

Increasing nitrate (NO3−) 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 N2 fixation that converts N2 to reactive N is often assumed negligible in eutrophic ecosystems and excluded in coastal N budget evaluations. We investigated N2 fixation and denitrification in response to increasing NO3− loading (0, 10, and 100 μM) and sediment organic matter (OMsediment) concentrations in the emerging Wax Lake Delta. Continuous flow-through incubations with 30N2 addition was applied to measure N2 fixation. The variation of N2 fixation rates from 0 to 437 μmol N m−2 h−1 among different NO3− and OMsediment concentrations were comparable to the estimated denitrification rates of 141–377 μmol N m−2 h−1. Increasing overlying NO3− concentrations reduced N2 fixation rates and facilitated denitrification rates at each OMsediment concentration. However, 100 μM of overlying NO3− did not thoroughly inhibit N2 fixation rates in sites with intermediate and higher OMsediment concentrations (189 and 99 μmol N m−2 h−1, respectively). Both N2 fixation and denitrification increased with increasing OMsediment concentrations, but the relative importance of these processes was impacted mostly by overlying NO3− concentration as increasing NO3− switched the dominance of N2 fixation to denitrification in benthic N cycling. This study highlights the importance of heterotrophic N2 fixation in coastal deltaic floodplains and emphasizes the necessity of including N2 fixation quantification in coastal N budget evaluation, not only in oligotrophic environment but also in eutrophic environment.