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Heterotrophic nitrogen fixation in response to nitrate loading and sediment organic matter in an emerging coastal deltaic floodplain within the Mississippi River Delta plain
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2021
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Source: Limnology and Oceanography, 66(5), 1961-1978
[PDF-2.24 MB]
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Journal Title:Limnology and Oceanography
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Description: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.
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Source:Limnology and Oceanography, 66(5), 1961-1978
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DOI:
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ISSN:0024-3590;1939-5590;
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Rights Information:Accepted Manuscript
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Compliance:Submitted
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