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Coastal wetlands are large reservoirs of soil carbon (C). However, the annual C accumulation rates contributing to the C storage in these systems have yet to be spatially estimated on a large scale. We synthesized C accumulation rate (CAR) in tidal wetlands of the conterminous United States (US), upscaled the CAR to national scale, and predicted trends based on climate change scenarios. Here, we show that the mean CAR is 161.8 ± 6 g Cm −2  yr −1 , and the conterminous US tidal wetlands sequestrate 4.2–5.0 Tg C yr −1 . Relative sea level rise (RSLR) largely regulates the CAR. The tidal wetland CAR is projected to increase in this century and continue their C sequestration capacity in all climate change scenarios, suggesting a strong resilience to sea level rise. These results serve as a baseline assessment of C accumulation in tidal wetlands of US, and indicate a significant C sink throughout this century. It remains challenging to estimate carbon accumulation rates in tidal wetlands on a scale as large as the conterminous US. Here, the authors find that mean C accumulation rates vary greatly among watershed regions but not among vegetation types, and that tidal wetlands’ C sequestration capability will remain or increase by 2100, suggesting a resilience to sea level rise.

Mangroves can retain both autochthonous and allochthonous marine and/or terrestrial organic carbon (OC) in sediments. Accurate quantification of these OC sources is essential for the proper allocation of blue C credits. Here, we conduct a global-scale analysis of sediments autochthonous and allochthonous OC contributions in estuarine and marine mangroves using stable isotopes. Globally, mangrove-derived autochthonous OC was the main contributor to estuarine and marine mangrove top-meter soil organic carbon (SOC) (49% and 62%, respectively). Less marine allochthonous OC (21%) was deposited than terrestrial allochthonous OC (30%) in estuarine mangrove sediments. Estuarine mangroves accumulated more SOC in sediments than marine mangroves (282 ± 8.1 Mg C ha −1 and 250 ± 5.0 Mg C ha −1 , respectively), primarily due to the additional terrestrial OC inputs. Globally, marine mangroves held 67% of the total mangrove SOC, reaching 3025 ± 345 Tg C, while 1502 ± 154 Tg C was stored in estuarine mangrove sediments. The findings emphasize the substantial influence of coastal environmental settings on OC contributions, underlining the necessity of accurate OC source quantification for the effective allocation of blue carbon credits. Mangrove-derived organic carbon (OC) contributes 49% and 62% to estuarine and marine mangrove soil OC (SOC). Globally, 1502 Tg and 3025 Tg SOC were stored in estuarine and marine mangroves.

With the growing recognition that effective action on climate change will require a combination of emissions reductions and carbon sequestration, protecting, enhancing and restoring natural carbon sinks have become political priorities. Mangrove forests are considered some of the most carbon-dense ecosystems in the world with most of the carbon stored in the soil. In order for mangrove forests to be included in climate mitigation efforts, knowledge of the spatial distribution of mangrove soil carbon stocks are critical. Current global estimates do not capture enough of the finer scale variability that would be required to inform local decisions on siting protection and restoration projects. To close this knowledge gap, we have compiled a large georeferenced database of mangrove soil carbon measurements and developed a novel machine-learning based statistical model of the distribution of carbon density using spatially comprehensive data at a 30 m resolution. This model, which included a prior estimate of soil carbon from the global SoilGrids 250 m model, was able to capture 63% of the vertical and horizontal variability in soil organic carbon density (RMSE of 10.9 kg m−3). Of the local variables, total suspended sediment load and Landsat imagery were the most important variable explaining soil carbon density. Projecting this model across the global mangrove forest distribution for the year 2000 yielded an estimate of 6.4 Pg C for the top meter of soil with an 86-729 Mg C ha−1 range across all pixels. By utilizing remotely-sensed mangrove forest cover change data, loss of soil carbon due to mangrove habitat loss between 2000 and 2015 was 30-122 Tg C with >75% of this loss attributable to Indonesia, Malaysia and Myanmar. The resulting map products from this work are intended to serve nations seeking to include mangrove habitats in payment-for- ecosystem services projects and in designing effective mangrove conservation strategies.

Mangrove wetlands exist in the transition zone between terrestrial and marine environments and as such were historically overlooked in discussions of terrestrial and marine carbon cycling. In recent decades, mangroves have increasingly been credited with producing and burying large quantities of organic carbon (OC). The amount of available data regarding OC burial in mangrove soils has more than doubled since the last primary literature review (2003). This includes data from some of the largest, most developed mangrove forests in the world, providing an opportunity to strengthen the global estimate. First‐time representation is now included for mangroves in Brazil, Colombia, Malaysia, Indonesia, China, Japan, Vietnam, and Thailand, along with additional data from Mexico and the United States. Our objective is to recalculate the centennial‐scale burial rate of OC at both the local and global scales. Quantification of this rate enables better understanding of the current carbon sink capacity of mangroves as well as helps to quantify and/or validate the other aspects of the mangrove carbon budget such as import, export, and remineralization. Statistical analysis of the data supports use of the geometric mean as the most reliable central tendency measurement. Our estimate is that mangrove systems bury 163 (+40; −31) g OC m−2 yr−1 (95% C.I.). Globally, the 95% confidence interval for the annual burial rate is 26.1 (+6.3; −5.1) Tg OC. This equates to a burial fraction that is 42% larger than that of the most recent mangrove carbon budget (2008), and represents 10–15% of estimated annual mangrove production. This global rate supports previous conclusions that, on a centennial time scale, 8–15% of all OC burial in marine settings occurs in mangrove systems. Key Points Global mangrove sediments accumulate 26.1 Tg of organic carbon annually This burial equates to twelve percent of annual mangrove primary production The 95 percent confidence interval is from 131 to 203 g OC per square meter per year

The Legal Amazon of Brazil holds vast mangrove forests, but a lack of awareness of their value has prevented their inclusion into results-based payments established by the United Nations Framework Convention on Climate Change. Based on an inventory from over 190 forest plots in Amazon mangroves, we estimate total ecosystem carbon stocks of 468 ± 67 Megagrams (Mg) ha −1 ; which are significantly higher than Brazilian upland biomes currently included into national carbon offset financing. Conversion of mangroves results in potential emissions of 1228 Mg CO 2 e ha −1 , which are 3-fold higher than land use emissions from conversion of the Amazon rainforest. Our work provides the foundation for the inclusion of mangroves in Brazil’s intended Nationally Determined Contribution, and here we show that halting mangrove deforestation in the Legal Amazon would generate avoided emissions of 0.9 ± 0.3 Teragrams (Tg) CO 2 e yr −1 ; which is equivalent to the annual carbon accumulation in 82,400 ha of secondary forests. A new study shows that deforestation of Amazon mangroves releases up to four times more carbon dioxide when compared to emissions arising from terrestrial biomes. This study set a foundation for the use of mangroves in Brazil’s international policy agreements.

A significant proportion of carbon (C) captured by terrestrial primary production is buried in lacustrine ecosystems, which have been substantially affected by anthropogenic activities globally. However, there is a scarcity of sedimentary organic carbon (OC) accumulation information for lakes surrounded by highly productive rainforests at warm tropical latitudes, or in response to land cover and climate change. Here, we combine new data from intensive campaigns spanning 13 lakes across remote Amazonian regions with a broad literature compilation, to produce the first spatially-weighted global analysis of recent OC burial in lakes (over ~50-100-years) that integrates both biome type and forest cover. We find that humid tropical forest lake sediments are a disproportionately important global OC sink of 7.4 Tg C yr −1 with implications for climate change. Further, we demonstrate that temperature and forest conservation are key factors in maintaining massive organic carbon pools in tropical lacustrine sediments. Tropical forest lake sediments are global carbon sinks, representing an important implication for climate change, of which both temperature and forest conservation are key factors in maintaining the carbon burial mechanism in lacustrine ecosystems.

Widespread anthropogenic activities pollute groundwater that eventually seeps out to the coastal ocean. Here, we resolve nutrient transformations and fluxes in 11 sandy subterranean estuaries (STEs) with contrasting nutrient sources and development trajectories. Coastal groundwater nitrogen pollution stems from sewage discharge and land use change. Anthropogenically derived groundwater nutrient fluxes with high N/P ratios (∼170) accounted for 22%–61% of riverine inputs into China's coastal waters, providing an additional source of nutrients that can fuel coastal eutrophication and algal blooms. Sandy STEs remarkably attenuated ∼84% of nitrogen pollution, minimizing the impact of submarine groundwater discharge (SGD) on coastal water quality. Hence, STEs deliver an overlooked ecosystem service that is particularly important in highly polluted coastal aquifers. Protecting STEs and recognizing the integrated nature of groundwater and seawater is thus important in coastal water quality management initiatives. Plain Language Summary The extensive pollution of coastal groundwater is a risk to coastal ecosystems. Subterranean estuaries filter groundwater nutrient pollution, yet their function remains poorly quantified. Our study highlights the influence of anthropogenic activities such as sewage discharge and urbanization in driving coastal groundwater nitrate pollution. Anthropogenically derived nutrients from groundwater contribute significantly to budgets in China's coastal waters. Nevertheless, sandy STEs attenuate up to 84% of nitrogen pollution, effectively mitigating the impact of SGD on coastal water quality. Accordingly, it is imperative to implement targeted measures that go beyond current legislation and initiatives focused on surface water only to ensure the protection of coastal groundwater. Key Points A 10% increase in economic output enhanced coastal groundwater nitrate by ∼24% subterranean estuaries (STEs) attenuated 45%–85% of nutrients, yet groundwater remains a crucial nutrient source for coastal waters Integrated management is vital to safeguard coastal water quality against polluted groundwater seepage

Mangrove loss has reduced its carbon (C) sink function and ecosystem services. To effectively allocate climate finance for mangrove restoration, a thorough assessment of restoration potential is necessary. Here we show a net loss of ecosystem service value (ESV) of 29.2 billion USD ($) due to land changes in mangroves from 1996 to 2019. The estimated mangrove ESV in 2019 amounts to $894 billion yr −1 , mainly provided by regulating and provisioning services (57.4% and 19.7%). Over the next two decades, we project that the restoration of mangroves would necessitate an investment of $40.0–52.1 billion, yielding net gains in ESV of $231–725 billion. The global benefit-cost ratio (BCR) of mangrove restoration ranges from 6.35 to 15.0, demonstrating that such projects are highly cost-effective. Furthermore, an estimated of 19.4 Tg C can be sequestrated in mangrove soils based on a 20-year mangrove restoration program, which can generate $68.6–$236 million via blue C trading. Our findings highlight the significant opportunities for blue C restoration projects to mitigate climate change and support livelihoods. Potential mangrove restoration would necessitate an investment of $40.0–52.1 billion, yielding net gains in ESV of $231–725 billion. An estimated of 19.4 Tg C can be sequestrated in mangrove soils, generating $68.6–$236 million via blue C trading.

There is concern that accelerating sea-level rise will exceed the vertical growth capacity of coastal-wetland substrates in many regions by the end of this century. Vertical vulnerability estimates rely on measurements of accretion and/or surface-elevation-change derived from soil cores and/or surface elevation tables (SETs). To date there has not been a broad examination of whether the multiple timescales represented by the processes of accretion and elevation change are equally well-suited for quantifying the trajectories of wetland vertical change in coming decades and centuries. To examine the potential for timescale bias in assessments of vertical change, we compared rates of accretion and surface elevation change using data derived from a review of the literature. In the first approach, average rates of elevation change were compared with timescale-averaged accretion rates from six regions around the world where sub-decadal, decadal, centennial, and millennial timescales were represented. Second, to isolate spatial variability, temporal comparisons were made for regionally unique environmental categories within each region. Last, comparisons were made of records from sites where SET-MH stations and radiometric measurements were co-located in close proximity. We find that rates vary significantly as a function of measurement timescale and that the pattern and magnitude of variation between timescales are location-specific. Failure to identify and account for temporal variability in rates will produce biased assessments of the vertical change capacity of coastal wetlands. Robust vulnerability assessments should combine accretion rates from multiple timescales with the longest available SET record to provide long-term context for ongoing monitoring observations and projections.

which incorrectly read. 'We find that humid tropical forest lake sediments are a disproportionately important global OC sink of 80 Tg C yr -1 with implications for climate change.'The correct version replaces this sentence with 'We find that humid tropical forest lake sediments are a disproportionately important global OC sink of 7.4 Tg C yr -1 with implications for climate change.'The original version of this Article contained an error in the 'Tropical drivers of OC accumulation in lakes' section, which incorrectly read.'Applying our recent OC burial rates to global lake area 13 resulted in an estimated global sink of 79 Tg C yr -1 , which is equivalent to 27% of estimated global carbon dioxide (CO 2 ) emissions from lake waters to the atmosphere (i.e., 292 Tg C yr -1 ) 20 'The correct version replaces this sentence with 'Applying our recent OC burial rates to global lake area 13 resulted in an estimated global sink of 80 Tg C yr -1 , which is equivalent to 27% of estimated global carbon dioxide (CO 2 ) emissions from lake waters to the atmosphere (i.e., 292 Tg C yr -1 ) 20 'This has been corrected in both the PDF and HTML versions of the article.