Heterotrophic nitrogen fixation in response to nitrate loading and sediment organic matter in an emerging coastal deltaic floodplain within the Mississippi River Delta plain. Issue 5 (13th April 2021)

• Record Type: Journal Article
• Authors: Li, Song; Twilley, Robert R.; Hou, Aixin
• Is Part Of: Limnology and oceanography. Volume 66:Issue 5(2021)
• Page Start: 1961
• Page End: 1978
• Publication Date: 2021-04-13
• Subjects: Limnology -- Periodicals; Oceanography -- Periodicals; Océanographie; Limnologie; Limnology; Oceanography; Computer network resources; Périodique électronique (Descripteur de forme); Ressource Internet (Descripteur de forme); Periodicals; 551.4805
• DOI: 10.1002/lno.11737
• Languages: English
• ISSNs: 0024-3590

Abstract

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 μ M) and sediment organic matter (OMsediment ) concentrations in the emerging Wax Lake Delta. Continuous flow‐through incubations with 30 N2 addition was applied to measure N2 fixation. The variation of N2 fixation rates from 0 to 437 μ mol N m −2 h −1 among different NO3 − and OMsediment concentrations were comparable to the estimated denitrification rates of 141–377 μ mol N m −2 h −1 . Increasing overlying NO3 − concentrations reduced N2 fixation rates and facilitated denitrification rates at each OMsediment concentration. However, 100 μ M of overlying NO3 − did not thoroughly inhibit N2 fixation rates in sites with intermediate and higher OMsediment concentrations (189 and 99 μ mol 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.