Coastal marshes mitigate allochthonous nitrogen (N) inputs to adjacent marine habitat; however, their extent is declining rapidly. As a result, marsh restoration and construction have become a major foci of wetland management. Constructed marshes can quickly reach similar plant biomass to natural marshes, but biogeochemical functions like N removal and retention can take decades to reach functional equivalency, often due to lags in organic matter (OM) pools development in newly constructed marshes. We compared denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates in a 32 year-old constructed marsh and adjacent reference marsh in the Northern Gulf of Mexico. Marsh sediments packed into 3 mm “thin discs” were subjected to three OM quality treatments (no OM addition, labile OM, or recalcitrant OM) and two N treatments (ambient nitrate or elevated nitrate) during a 13 day incubation. We found that OM addition, rather than marsh type or nitrate treatment, was the most important driver of nitrate reduction, increasing both denitrification and DNRA and promoting DNRA over denitrification in both marshes. Fungal and bacterial biomass were higher in the natural marsh across treatments, but recalcitrant OM increased fungal biomass in the constructed marsh, suggesting OM-limitation of fungal growth. We found that constructed marshes are capable of similar denitrification and DNRA as natural marshes after 30 years, and that labile OM addition promotes N retention in both natural and constructed marshes.

Graphical Abstract

Conceptual figure highlighting the findings of this experiment. Under control treatment with no C addition (bottom panel), constructed and natural marshes have similar rates of both DNRA and denitrification. The natural marsh has higher fungal and bacterial biomass, while fungal biomass is not detectable in the constructed marsh. Under labile OM additions (upper left panel), rates of both DNRA and denitrification are increased and DNRA becomes favored over denitrification in both marshes. Recalcitrant OM additions (upper right) increase denitrification, but do not affect DNRA or % denitrification. The addition of recalcitrant OM also increases the detectability of fungal biomass in the constructed marsh.

中文翻译：

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不稳定有机物促进天然和人工墨西哥湾沿岸沼泽中的氮保留

沿海沼泽减少了对邻近海洋栖息地的外来氮（N）输入；然而，它们的范围正在迅速缩小。因此，沼泽的恢复和建设已成为湿地管理的重点。人工沼泽可以快速达到与天然沼泽相似的植物生物量，但氮去除和保留等生物地球化学功能可能需要数十年才能达到功能等效性，这通常是由于新建沼泽中有机质（OM）池发展滞后所致。我们比较了墨西哥湾北部一个有 32 年历史的人工沼泽和邻近参考沼泽的反硝化和异化硝酸盐还原为铵 (DNRA) 的速率。在 13 天的培养过程中，将沼泽沉积物装入 3 毫米“薄盘”中，进行三种 OM 质量处理（不添加 OM、不稳定 OM 或顽固 OM）和两种 N 处理（常温硝酸盐或升高硝酸盐）。我们发现，添加 OM，而不是沼泽类型或硝酸盐处理，是硝酸盐减少的最重要驱动因素，增加了两个沼泽中的反硝化和 DNRA，并促进 DNRA 超过反硝化。在不同处理中，天然沼泽中的真菌和细菌生物量较高，但顽固的 OM 增加了人工沼泽中的真菌生物量，表明 OM 对真菌生长的限制。我们发现，30 年后，人工沼泽能够进行与天然沼泽类似的反硝化和 DNRA，并且不稳定的 OM 添加可促进天然沼泽和人工沼泽中的氮保留。

图形概要

概念图强调了该实验的结果。在不添加 C 的对照处理下（下图），人工沼泽和天然沼泽的 DNRA 和反硝化率相似。天然沼泽具有较高的真菌和细菌生物量，而在人工沼泽中检测不到真菌生物量。在不稳定的 OM 添加下（左上图），DNRA 和反硝化的速率都会增加，并且在两个沼泽中 DNRA 都比反硝化更受青睐。顽固的 OM 添加（右上）会增加反硝化作用，但不会影响 DNRA 或反硝化百分比。顽固性 OM 的添加还增加了人工沼泽中真菌生物量的可检测性。