International policy frameworks recognize the net drawdown and storage of atmospheric greenhouse gases through management interventions on blue carbon ecosystems (saltmarshes, mangroves, seagrasses) as potential emissions offset strategies. However, key questions remain around the “additionality” of the carbon sequestered by these ecosystems, and whether some fraction of the organic carbon (OC) that does not derive from in situ production (allochthonous) should be included in carbon budgets. This study compares the radiocarbon (¹⁴C) contents of saltmarsh soils and CO₂ evolved from aerobic laboratory incubations to show that young OC is preferentially respired over aged OC, and that the latter is also vulnerable to remineralization under oxic conditions. This highlights that management interventions which reduce the exposure of saltmarsh soils to oxic conditions support the inclusion of some portion of allochthonous OC in carbon budgets. Elevated temperature incubations provide preliminary evidence that the predominant source of respired OC will not change under predicted future warmer conditions. Saltmarsh typology also influences the ¹⁴C content of both the bulk soil and respired CO₂, highlighting the importance of site selection for optimized blue carbon additionality.

中文翻译：

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蓝碳额外性：盐沼土壤放射性碳含量及其呼吸二氧化碳的新见解

国际政策框架承认通过对蓝碳生态系统（盐沼、红树林、海草）进行管理干预来减少和储存大气温室气体，作为潜在的排放抵消策略。然而，关键问题仍然围绕着这些生态系统封存的碳的“额外性”，以及并非来自原位生产（异地）的有机碳（OC）的某些部分是否应包含在碳预算中。本研究比较了盐沼土壤中的放射性碳 ( ¹⁴ C) 含量和有氧实验室培养过程中产生的 CO ₂，​​结果表明，年轻的 OC 比老化的 OC 更容易呼吸，而且后者在含氧条件下也容易发生再矿化。这凸显了减少盐沼土壤暴露于含氧条件的管理干预措施支持将部分外来有机碳纳入碳预算。高温孵化提供了初步证据，表明在预测的未来温暖条件下，呼吸 OC 的主要来源不会改变。盐沼类型也影响大块土壤和呼吸 CO _2的¹⁴ C 含量，凸显了选址对于优化蓝碳额外性的重要性。