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 (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 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 NO3- loading (0, 10, and 100 mu M) and sediment organic matter (OMsediment) concentrations in the emerging Wax Lake Delta. Continuous flow-through incubations with N-30(2) addition was applied to measure N-2 fixation. The variation of N-2 fixation rates from 0 to 437 mu mol N m(-2) h(-1) among different NO3- and OMsediment concentrations were comparable to the estimated denitrification rates of 141-377 mu mol N m(-2) h(-1). Increasing overlying NO3- concentrations reduced N-2 fixation rates and facilitated denitrification rates at each OMsediment concentration. However, 100 mu M of overlying NO3- did not thoroughly inhibit N-2 fixation rates in sites with intermediate and higher OMsediment concentrations (189 and 99 mu mol N m(-2) h(-1), respectively). Both N-2 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 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.