Coastal wetlands are among the most productive habitats on Earth and sequester globally significant amounts of atmospheric carbon (C). Extreme rates of soil C accumulation are widely assumed to reflect efficient C storage. Yet the fraction of wetland C lost via hydrologic export has not been directly quantified, since comprehensive budgets including direct estimates of lateral C loss are lacking. We present a complete net ecosystem C budget (NECB), demonstrating that lateral losses of C are a major component of the NECB for the largest stable brackish tidal marsh on the U.S. Pacific coast. Mean annual net ecosystem exchange of CO₂ with the atmosphere (NEE = −185 g C m² year⁻¹, negative NEE denoting ecosystem uptake) was compared to long‐term soil C burial (87–110 g C m² year⁻¹), suggesting only 47–59% of fixed atmospheric C accumulates in soils. Consistently, direct monitoring in 2017–2018 showed NEE of −255 g C m⁻² year⁻¹, and hydrologic export of 105 g C m⁻² year⁻¹ (59% of NEE remaining on site). Despite their high C sequestration capacity, lateral losses from coastal wetlands are typically a larger fraction of the NECB when compared to other terrestrial ecosystems. Loss of inorganic C (the least measured NECB term) was 91% of hydrologic export and may be the most important term limiting C sequestration. The high productivity of coastal wetlands thus serves a dual function of C burial and estuarine export, and the multiple fates of fixed C must be considered when evaluating wetland capacity for C sequestration.

中文翻译：

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水文出口是沿海湿地碳预算的主要组成部分

沿海湿地是地球上生产力最高的栖息地之一，在全球范围内螯合了大量的大气碳（C）。普遍认为土壤碳累积的极端速率反映了有效的碳储存。然而，由于缺乏直接估算横向C损失的综合预算，因此尚未直接量化因水文出口而损失的湿地C的比例。我们提供了完整的净生态系统碳预算（NECB），这表明C的侧向损失是NECB的主要组成部分，是美国太平洋沿岸最大的稳定咸淡潮沼泽地。大气中CO _2的年平均净生态系统交换量（NEE = −185 g C m ²年^-1，将NEE的负值表示生态系统的吸收）与长期土壤C埋葬（87-110 g C m ²年^-1）进行了比较，表明固定土壤中仅47-59％的土壤C积累。一致地，2017-2018年的直接监测显示，NEE为-255 g C m - ² 年^-1，水文出口为105 g C m - ² 年^-1（59％的NEE仍在现场）。尽管它们的固碳能力很高，但与其他陆地生态系统相比，沿海湿地的侧向损失通常是NECB的较大部分。无机碳的损失（最少测量的NECB术语）占水文出口量的91％，可能是限制碳固存的最重要术语。因此，沿海湿地的高生产力起到了掩埋碳和河口的双重作用，在评估湿地的固碳能力时，必须考虑固定碳的多重命运。