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 (OM_sediment) 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 OM_sediment 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 OM_sediment concentration. However, 100 μM of overlying NO₃⁻ did not thoroughly inhibit N₂ fixation rates in sites with intermediate and higher OM_sediment concentrations (189 and 99 μmol N m⁻² h⁻¹, respectively). Both N₂ fixation and denitrification increased with increasing OM_sediment 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.

中文翻译：

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密西西比河三角洲平原新兴的三角洲洪泛平原中的硝酸盐负荷和沉积物有机质对异养氮的固定作用

增加硝酸盐（NO ₃ ^-中的河流由于农业受精涂改装载底栖氮（N）和循环从净源到反应性异养N. N中的净汇位移沿海湿）₂固定，其将Ñ ₂到反应性的，N是常假定在富营养化生态系统中可以忽略不计，而在沿海N预算评估中则排除在外。我们调查Ñ ₂固定和脱氮响应于增加NO ₃ ^-加载（0，10，和100  μ M）和沉积物有机质（OM_沉积物）的浓度在新兴的蜡湖德尔塔。³⁰ N ₂连续流通培养将添加物用于测量N ₂固定。N的变化_2个固定速率从0至437  μ摩尔的N米^-2 ħ ^-1不同NO间₃ ^-和OM_沉淀浓度相当的141-377估计反硝化速率 μ摩尔的N米^-2 ħ ^-1。增加覆盖NO ₃ ^-浓度降低Ñ ₂固定率，并以每OM促进反硝化速率_沉淀浓度。然而，100  μ覆NO的M个₃ ^-不能完全抑制中等和较高OM_沉积物浓度（分别为189和 99μmol N m ^-2 h ^-1）的站点中的N ₂固定率。均为N ₂固定和脱氮随OM增加_沉淀的浓度，但这些处理的相对重要性是由上覆NO大多影响₃ ^-浓度增加NO ₃ ^-切换N中的主导地位₂固定到反硝化底栖氮循环。这项研究强调了异养N ₂的重要性固定在沿海三角洲洪泛区中，并强调了在沿海N预算评估中包括N ₂固定量化的必要性，不仅在贫营养环境中，而且在富营养环境中。