Despite overall global declines, mangroves are expanding into and within many subtropical wetlands, leading to heterogeneous cover of marsh-mangrove coastal vegetation communities near the poleward edge of mangroves' ranges. Coastal wetlands are globally important carbon sinks, yet the effects of shifts in mangrove cover on organic carbon (OC) storage remains uncertain. We experimentally maintained black mangrove (Avicennia germinans) or marsh vegetation in patches (n = 1120, 3 × 3 m) along a gradient in mangrove cover (0-100%) within coastal wetland plots (n = 10, 24 × 42 m) and measured changes in OC stocks and fluxes. Within patches, above and belowground biomass (OC) was 1630% and 61% greater for mangroves than for recolonized marshes, and soil OC was 30% greater beneath mangrove than marsh vegetation. At the plot scale, above and belowground biomass increased linearly with mangrove cover but soil OC was highly variable and unrelated to mangrove cover. Root ingrowth was not different in mangrove or marsh patches, nor did it change with mangrove cover. After 11 months, surface OC accretion was negatively related to plot-scale mangrove cover following a high-wrack deposition period. However, after 22 months, accretion was 54% higher in mangrove patches, and there was no relationship to plot-scale mangrove cover. Marsh (Batis maritima) leaf and root litter had 1000% and 35% faster breakdown rates (k) than mangrove (A. germinans) leaf and root litter. Soil temperatures beneath mangroves were 1.4ºC lower, decreasing aboveground k of fast- (cellulose) and slow-decomposing (wood) standard substrates. Wood k in shallow soil (0-15 cm) was higher in mangrove than marsh patches, but vegetation identity did not impact k in deeper soil (15-30 cm). We found that mangrove cover enhanced OC storage by increasing biomass, creating more recalcitrant organic matter and reducing k on the soil surface by altering microclimate, despite increasing wood k belowground and decreasing allochthonous OC subsidies. Our results illustrate the importance of mangroves in maintaining coastal OC storage, but also indicate that the impacts of vegetation change on OC storage may vary based on ecosystem conditions, organic matter sources, and the relative spatio-temporal scales of mangrove vegetation change.

中文翻译：

---

量化变化的红树林覆盖如何影响沿海湿地的生态系统碳储存

尽管全球整体下降，但红树林正在扩展到许多亚热带湿地及其内部，导致红树林范围向极边缘附近的沼泽 - 红树林沿海植被群落的异质覆盖。沿海湿地是全球重要的碳汇，但红树林覆盖变化对有机碳 (OC) 储存的影响仍不确定。我们在沿海湿地样地 (n = 10, 24 × 42 m) 内沿着红树林覆盖 (0-100%) 的梯度，以实验方式维持黑红树林 (Avicennia Germinans) 或沼泽植被 (n = 1120, 3 × 3 m)并测量 OC 库和通量的变化。在斑块内，红树林的地上和地下生物量 (OC) 比重新定植的沼泽地高 1630% 和 61%，红树林下的土壤有机碳比沼泽植被高 30%。在情节尺度上，地上和地下生物量随红树林覆盖率线性增加，但土壤有机碳变化很大且与红树林覆盖率无关。根向内生长在红树林或沼泽斑块中没有什么不同，也没有随着红树林覆盖而改变。11 个月后，在高残骸沉积期后，地表 OC 的增加与地块规模的红树林覆盖率呈负相关。然而，22 个月后，红树林斑块的增生增加了 54%，并且与地块规模的红树林覆盖率无关。沼泽 (Batis maritima) 叶和根凋落物的分解率 (k) 比红树林 (A.germinans) 叶和根凋落物快 1000% 和 35%。红树林下的土壤温度降低了 1.4ºC，降低了快速（纤维素）和缓慢分解（木材）标准基质的地上 k。红树林中浅层土壤（0-15 厘米）中的木材 k 值高于沼泽斑块，但植被特性对深层土壤（15-30 厘米）中的 k 值没有影响。我们发现红树林覆盖通过增加生物量、产生更多顽固的有机物质和通过改变小气候减少土壤表面的 k 来增强 OC 储存，尽管增加了地下木材 k 并减少了外来 OC 补贴。我们的结果说明了红树林在维持沿海 OC 储存方面的重要性，但也表明植被变化对 OC 储存的影响可能因生态系统条件、有机物质来源和红树林植被变化的相对时空尺度而异。尽管增加了地下木材的钾含量并减少了外来有机碳补贴，但仍会通过改变小气候来创造更多顽固的有机质并减少土壤表面的钾含量。我们的结果说明了红树林在维持沿海 OC 储存方面的重要性，但也表明植被变化对 OC 储存的影响可能因生态系统条件、有机物质来源和红树林植被变化的相对时空尺度而异。尽管增加了地下木材的钾含量并减少了外来有机碳补贴，但仍会通过改变小气候来创造更多顽固的有机质并减少土壤表面的钾含量。我们的结果说明了红树林在维持沿海 OC 储存方面的重要性，但也表明植被变化对 OC 储存的影响可能因生态系统条件、有机物质来源和红树林植被变化的相对时空尺度而异。