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In this study, two sediment cores (*70 cm) were collected from separate mangrove forests straddling the Ba Lat Estuary, Red River of northern Vietnam, to examine the origins of sedimentary organic carbon (SOC) and reconstruct the paleoenvironment. In addition, mangrove leaves and particulate organic matter were collected and measured for d 13 C to trace the origins of SOC. The cores were analyzed by high-resolution sections for d 13 C, TOC, C/N ratios, sediment grain size, water content, and porosity, with values of d 13 C, TOC, and C/N ratios ranging from -28.19 to -22.5%, 2.14-30.94 mg/g, and 10.29-18.32, respectively. The d 13 C and TOC relationship indicated that there were some small residual effects of diagenetic processes on TOC and d 13 C values in mangrove sediments. However, the shifts of d 13 C and C/N ratios from the bottom to the surface sediment of the cores explained the change in organic matter sources, with values of C/N [ 12 and d 13 C \ -25%, and C/N \ 12 and d 13 C [ -25% indicated terrestrial (e.g., mangrove litter) and marine phytoplankton sources, respectively. The covarying d 13 C, C/N ratios, and sediment grain sizes during the past 100 years in sediment cores showed that the paleoenvironment may be reconstructed into three environments (subtidal, tidal flat, and intertidal mangrove). General trends in d 13 C and C/N followed a gradual increase in the C/N ratio and a concomitant decrease in d 13 C from the subtidal, through to tidal flat, and to the intertidal mangrove. d 13 C and C/N ratios are therefore effective in measuring the continuum of environmental change in mangrove ecosystem.

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Mangroves are among the world's most carbon‐dense ecosystems, but they are threatened by rapid climate change and rising sea levels. The accumulation and decomposition of soil organic matter (SOM) are closely tied to mangroves' carbon sink functions and resistance to rising sea levels. However, few studies have investigated the response of mangrove SOM dynamics to likely future environmental conditions. We quantified how mangrove SOM decay is affected by predicted global warming (+4°C), sea level changes (simulated by altering of the inundation duration to 0, 2, and 6 hr/day), and their interaction. Whilst changes in inundation duration between 2 and 6 hr/day did not affect SOM decay, the treatment without inundation led to a 60% increase. A warming of 4°C caused SOM decay to increase by 21%, but longer inundation moderated this temperature‐driven increase. Our results indicate that (a) sea level rise is unlikely to decrease the SOM decay rate, suggesting that previous mangrove elevation gain, which has allowed mangroves to persist in areas of sea level rise, might result from changes in root production and/or mineral sedimentation; (b) sea level fall events, predicted to double in frequency and area, will cause periods of intensified SOM decay; (c) changing tidal regimes in mangroves due to sea level rise might attenuate increases in SOM decay caused by global warming. Our results have important implications for forecasting mangrove carbon dynamics and the persistence of mangroves and other coastal wetlands under future scenarios of climate change.

Mangroves are among the world's most carbon-dense ecosystems, but have suffered extensive deforestation, prompting reforestation projects. The effects of mangrove reforestation on belowground carbon dynamics are poorly understood. In particular, we do not know how fine root production develops following mangrove reforestation, despite fine root production being a major carbon sink and an important control of mangrove soil accretion.Using minirhizotrons, we investigated fine root production and its depth variation along a chronosequence of mature Vietnamese mangroves.Our results showed that fine root production decreases strongly with stand age in the uppermost 32 cm of our soil profiles. In younger mangrove stands, fine root production declines with depth, possibly due to a vertical gradient in soil nutrient availability; while root production in the oldest stand is low at all depths and exhibits no clear vertical pattern. A major fraction of fine root production occurs deeper than 30 cm, depths that are commonly omitted from calculations of mangrove carbon budgets.Younger mangroves may accrue shallow soil organic matter faster than older mangroves. Therefore, root productivity and forest stand age should be accounted for when forecasting mangrove carbon budgets and resistance to sea-level rise.