In situ plant production is often assumed to be the major contributor to organic matter (OM) accumulation and vertical accretion in tidal marshes. Here, we evaluate the contribution of mud‐associated OM in salt and brackish marshes in Louisiana. Based on 14 soil cores, the OM content of the mud fraction—i.e., any material smaller than 64 μm—was 17% ± 7% for the salt marshes and 28% ± 14% for the brackish marshes. This remains nearly uniform over the top 35 cm depth, suggesting that this material is deposited contemporaneously with the mud. The dry bulk density of the mud (300–450 kg m⁻³) is also much lower than what was estimated using a previously proposed two‐constituent mixing model (1990 kg m⁻³). To reconcile this discrepancy, we developed a modified mixing model that includes mud OM and differentiates sand as a separate constituent with its high dry bulk density. The model estimates that mud contributes to ~ 60% of the total marsh vertical accretion in Louisiana, considerably higher than the ~ 14% estimated with the two‐constituent mixing model. The result, which is a direct consequence of the relatively high porosity of mud, highlights that mud deposition is crucial for the accretion of microtidal marshes. Further, the model estimates that the mud OM constitutes ~ 60% of the total soil OM, emphasizing that in situ plant production is not the only—and, in minerogenic marshes, not the major—contributor to OM accumulation.

中文翻译：

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与泥浆相关的有机质及其在沼泽有机质积累和垂直积聚中的直接和间接作用

人们通常认为原位植物生产是潮汐沼泽中有机质（OM）积累和垂直增生的主要因素。在这里，我们评估了与泥浆相关的OM在路易斯安那州盐和咸淡沼泽中的贡献。以14个土壤核心为基础，盐分（即小于64μm的任何物质）的OM含量 对于盐沼而言为17％±7％，对于微咸沼泽而言为28％±14％。这在顶部35厘米深度上几乎保持均匀，表明该材料与泥浆同时沉积。泥浆的干堆积密度（300–450 kg m ^-3）也大大低于先前提出的两成分混合模型（1990 kg m ^-3）的估计值。）。为了消除这种差异，我们开发了一种改良的混合模型，其中包括泥浆OM，并以高干容密度将沙子作为单独的成分进行区分。该模型估计，泥浆占路易斯安那州沼泽总垂直积聚的60％左右，大大高于两成分混合模型估计的约14％。该结果是泥浆相对较高的孔隙度的直接结果，突出表明泥浆沉积对于潮汐沼泽的堆积至关重要。此外，该模型估计泥浆OM约占土壤总OM的60％，强调原位植物生产不是OM积累的唯一因素，而且在成矿沼泽中不是主要因素。