Ariane Arias-Ortiz

Crucial for coastal carbon sequestration, global seagrass organic carbon (OC) burial rates remain uncertain as early estimates were derived from mixed methodologies across geographically and taxonomically biased datasets. Here, we present a data-driven reassessment from a synthesis of 332 sediment cores spanning the last century. Results show that seagrasses bury OC at 25 ± 3 g OC m ⁻² yr ⁻¹ (geometric mean), with an area-weighted global average of 33 ± 9 g OC m ⁻² yr ⁻¹ . Rates reflect a balance between sediment OC content and sediment accumulation, which share a negative exponential relationship driven by mineral dilution. Based on a revised mapped seagrass extent (247,700–366,400 km ² ), global burial scales to 6–15 Tg C yr ⁻¹ . Although one-fourth of earlier benchmarks, seagrasses account for 3–4% to 3–6% of total oceanic OC burial despite covering <0.1% of the seafloor. Combined with mangroves and tidal marshes, vegetated coastal ecosystems contribute 8–13% of oceanic OC burial.

Tidal wetlands sequester carbon (C) at much higher rates per area than other ecosystems, helping to offset C emissions. The burial of organic C in tidal wetland soils, “blue C”, is tightly linked to the cycling of nitrogen (N), which is a key pollutant and limiting nutrient for many ecosystems. The large fluxes of N in and out of tidal wetlands can have strong impacts on surrounding water quality. However, the global burial rate of “blue nitrogen” and its potential response to future sea‐level rise (SLR) and wetland extent changes have not been quantified. Here, we assembled a global database of 8012 soil N measurements from 255 tidal wetland sites and found that the relationship between soil C and N concentrations was consistent but differed between the two dominant types of emergent tidal wetlands—marshes and mangroves. Leveraging extensive knowledge of blue C accumulation, we estimated that tidal wetlands, which cover less than 0.1% of the Earth's surface, bury 3.2 Tg N yr ⁻¹ (2.5 Tg N yr ⁻¹ in mangroves, and 0.7 Tg N yr ⁻¹ in marshes), representing 13%–15% of marine N burial. This rate could more than double globally by 2,100 if wetland elevation increases with accelerating SLR, enhancing wetland burial of coastal N prior to release into estuarine bays and oceans. Alternatively, if wetlands submerge and soil erodes, N sinks could reverse and become sources, exacerbating coastal pollution.

Blue carbon ecosystems, classically defined as mangroves, tidal marshes and seagrasses, but increasingly expanded to include ecosystems such as tidal flats, macroalgal forests and shelf sediments, contribute to climate change mitigation and biodiversity support. Here, seven years after the last global assessment of research priorities, we conducted a priority-setting exercise to identify persistent knowledge and implementation gaps, and the strategic priorities that must be addressed to enable scalable, high-integrity and equitable management of blue carbon ecosystems in a rapidly evolving policy and finance landscape. The highest priority focuses on managing blue carbon ecosystems to support coastal communities while integrating traditional ecological knowledge, emphasizing the essential role of social legitimacy and equity in enabling scalable, long-lasting outcomes. Additional priorities focus on developing cost-effective restoration methods, improving the accuracy of greenhouse gas flux estimates, quantifying the impacts of human activities on carbon cycling and integrating co-benefits such as biodiversity and coastal protection into natural capital frameworks. Emerging technologies like remote sensing, machine learning and data-sharing platforms are also highlighted as transformative tools to fill knowledge gaps and scale solutions. Collectively, these priorities highlight the complexity of blue carbon science and the need for inclusive interdisciplinary approaches that support the resilience and livelihoods of coastal communities.

Different aquatic ecosystems may have opposing effects on global climate acting as sources and/or sinks of greenhouse gases. The conversion of sediment organic carbon to the powerful greenhouse gas methane is a key indicator of the potential climate impact of a given ecosystem. Here, we assess the lifetime climatic role of aquatic ecosystems by contrasting methane (CH ₄ ) emissions with long-term organic carbon burial. Global compilations revealed that the ecosystem-specific ratio of organic carbon burial (kg CO ₂ ) to CH ₄ emissions (kg CH ₄ ) ranged from 20:1 in lakes to 2500:1 in continental shelves. Using 100-year sustained-flux global warming potential and a radiative balance model, we show that most ecosystems currently exhibit a net negative radiative balance (i.e., net cooling or a CO ₂ -equivalent sink). The lifetime cooling effect per area was strongest in the coastal ocean and fjords. Freshwater ponds, lakes, and inland wetlands had lowest organic carbon burial to CH ₄ emission ratios, requiring ~ 0-700 years after their establishment to shift from a positive to net negative radiative balance. All aquatic ecosystems gradually increase their net climate benefit via long-term enhancement of the ratio of carbon sequestration to CH ₄ emissions. Most aquatic ecosystems will thus achieve negative radiative balance over their lifetime.

The proliferation of invasive alien species (IAS) is one of the main threats to the conservation of estuarine habitats, including saltmarshes. Differences between IAS and their native counterparts in structural traits (e.g. plant size, biomass allocation, shoot stiffness) and chemical composition (e.g. nutrient and lignin content) can affect the accumulation and long-term storage of organic carbon (OC) in saltmarsh sediments. However, the impact of IAS colonization on sedimentary carbon sinks in saltmarshes remain largely unexplored, particularly in Europe. Existing studies are scarce and focus primarily on the herbaceous species Spartina alterniflora, while no research has yet assessed the impact of the spread of woody shrub species such as Baccharis halimifolia, one of the main IAS in European estuaries. This study examines organic carbon (OC) stocks, 210Pb-derived accumulation rates and the molecular composition of the organic matter (i.e. through pyrolysis techniques) in 12 sediment cores sampled across native saltmarsh (i.e. Juncus maritimus and Spartina maritima) and invasive saltmarsh communities (i.e. Spartina alterniflora, Spartina anglica and Baccharis halimifolia) in the Gulf of Biscay. The results of this study serve as a basis for the implementation of conservation and restoration actions in saltmarsh environments that address both biodiversity and climate change mitigation goals.

Globally, wetlands can sequester and store large amounts of soil carbon over the long term due to high primary productivity and slow decomposition. Yet centuries of drainage for agriculture and development have turned many of these carbon sinks into greenhouse gas (GHG) sources. Restoring degraded wetlands, particularly in peat‐rich landscapes, is increasingly promoted as a nature‐based solution for climate change mitigation. However, the trajectory and timing of recovery remain uncertain, especially given the complex interplay among vegetation dynamics, hydrology, and GHG fluxes. In this study, we analyzed 44 site‐years of continuous eddy covariance measurements of carbon dioxide (CO ₂ ) and methane (CH ₄ ) fluxes from restored wetlands in California's Sacramento‐San Joaquin Delta. Our findings reveal substantial interannual variability in GHG exchange across sites, driven by differences in restoration design, water management, and vegetation establishment. While rapid vegetation growth, especially dense stands of macrophytes, can enhance CO ₂ uptake, it often elevates CH ₄ emissions and complicates predictions of when wetlands become net GHG sinks. Crucially, wetlands with delayed vegetation establishment due to high or inconsistent water levels (e.g., significant drawdown) remained persistent GHG sources, even years after restoration. Conversely, sites with tailored planting or natural and rapid recolonization exhibited earlier transitions to net sink status, including earlier shifts towards net negative radiative forcing since the restoration. The study highlights the importance of adaptive, site‐specific restoration strategies and long‐term monitoring to capture switchover dynamics from sources to sinks. As global investment in wetland restoration grows, our findings underscore the need to balance climate mitigation goals with ecological realities and the self‐designing processes of vegetation succession.

Seagrass ecosystems are pivotal contributors to coastal carbon sequestration through the long-term burial of organic carbon (OC) in sediments. Yet global burial estimates remain uncertain, with early values of 138 ± 38 g OC m-2 yr-1 derived from a limited dataset biased toward highly depositional communities and indirect, production-based approaches. We call for a downward revision, supported by a data-driven assessment based on a global synthesis of 326 dated sediment cores integrating OC burial over the last century. We find that seagrass meadows bury OC at a geometric mean rate of 26 ± 2 g OC m-2 yr-1, with an area-weighted global average of 33 ± 10 g OC m-2 yr-1 accounting for bioregional differences in seagrass distribution. Globally, these rates scale to 6–16 Tg C yr-1, based on current mapped seagrass extent (247,800–366,200 km2), and reveal that ~15% of seagrass net community production is retained and buried locally. Although our estimate is roughly one-fourth of earlier values, seagrass sediments still account for 3–6% of total oceanic OC burial, despite occupying <0.1% of the seafloor. Combined with mangroves and tidal marshes, OC burial in vegetated coastal sediments represents an estimated 8–13% of total oceanic OC burial.

A high degree of uncertainty persists regarding current and future emissions of methane from both natural and constructed wetlands. Part of the problem is the existence of ‘hot spots’ of methane flux, which have not been clearly identified and studied at multiple scales. Methane has a short lifetime compared to carbon dioxide; thus, efforts to avoid methane hot spots from constructed wetlands can promptly decelerate the rate of atmospheric warming. In this study we measured methane fluxes using flux towers in a restored oligohaline wetland in the Sacramento–San Joaquin River Delta, where we previously identified a hot spot of methane flux using footprint-weighed flux maps and chambers. Our main objectives with this study were to determine why this hot spot occurs and what are the biogeochemical and microbiological conditions that lead to these high methane fluxes. We found four main mechanisms that explain the existence of the hot spot. (1) The hot spot was associated with areas where the water level was closer to the surface (2) Methane originated mostly from older unoxidized peat in deeper layers, which had a shorter migration pathway to the atmosphere at the hot spot location due to soil disturbance during wetland construction. (3) Relatively lower methane oxidation in the hot spot in the upper soil layer (10–30 cm under the surface), deduced from isotopic profiles in porewater carbon and upper-level methanotroph abundance. (4) Higher ebullition events at the hot spot that can be related to low water levels and lower bulk density throughout the soil profile. This study thus suggests that mitigating soil disturbances during wetland construction and managing water level can reduce the occurrence and magnitude of hot spots of methane flux in constructed wetlands.

Seagrass ecosystems are recognized for their capacity to sequester and store organic carbon, but there is large variability in soil organic carbon stocks associated with plant traits and environmental conditions, making the quantification and scaling of carbon storage and fluxes needed to contribute to climate change mitigation highly challenging. Here, we provide estimates of carbon stocks associated with seagrass systems (biomass and soil) through analyses of a comprehensive global database including 2700+ seagrass soil cores. The median global soil C_org stock estimate is 24.2 (12.4 – 44.9) Mg C_org ha⁻¹ in the top 30 cm of soil, 27% lower than estimates from previous global syntheses, refining the IPCC Tier 1 soil C_org stock currently used for carbon accounting in places without local data. We estimate that seagrass carbon stocks at risk of degradation could emit 1,154 Tg (665 – 1699) CO₂ with a social cost of $213 billion (2020 US dollars), if no action is taken to conserve these habitats.