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Coastal wetlands play an important role in regulating atmospheric carbon dioxide (CO2) concentrations and contribute significantly to climate change mitigation. However, climate change, reclamation, and restoration have been causing substantial changes in coastal wetland areas and carbon exchange in China during recent decades. Here we compiled a carbon flux database consisting of 15 coastal wetland sites to assess the magnitude, patterns, and drivers of carbon fluxes and to compare fluxes among contrasting natural, disturbed, and restored wetlands. The natural coastal wetlands have the average net ecosystem exchange of CO2 (NEE) of −577 g C m−2 year−1, with −821 g C m−2 year−1 for mangrove forests and −430 g C m−2 year−1 for salt marshes. There are pronounced latitudinal patterns for carbon dioxide exchange of natural coastal wetlands: NEE increased whereas gross primary production (GPP) and respiration of ecosystem decreased with increasing latitude. Distinct environmental factors drive annual variations of GPP between mangroves and salt marshes; temperature was the dominant controlling factor in salt marshes, while temperature, precipitation, and solar radiation were co‐dominant in mangroves. Meanwhile, both anthropogenic reclamation and restoration had substantial effects on coastal wetland carbon fluxes, and the effect of the anthropogenic perturbation in mangroves was more extensive than that in salt marshes. Furthermore, from 1980 to 2020, anthropogenic reclamation of China's coastal wetlands caused a carbon loss of ~3720 Gg C, while the mangrove restoration project during the period of 2021–2025 may switch restored coastal wetlands from a carbon source to carbon sink with a net carbon gain of 73 Gg C. The comparison of carbon fluxes among these coastal wetlands can improve our understanding of how anthropogenic perturbation can affect the potentials of coastal blue carbon in China, which has implications for informing conservation and restoration strategies and efforts of coastal wetlands. [ABSTRACT FROM AUTHOR]

Biomass-based negative emission technologies (NETs) such as bioenergy with carbon capture and storage (BECCS) and afforestation/reforestation (AR) are regarded as important options to achieve the 2 °C and 1.5 °C targets stipulated in the Paris agreement, but the feasibility of their large-scale deployments remains very uncertain. This study focused on the speed of expansions of land-use area related to the biomass-based NETs and assessed the feasibility of climate change mitigation scenarios to achieve the temperature targets. Our model analysis shows that expansions at unprecedented speeds are required for total cropland area (including energy cropland) in Sub-Saharan Africa and for planted forest area for carbon sink in many regions in the next decades, under the assumption of global least-cost measures for CO2 emission reduction. On the other hand, when the speed of the land-use expansions is limited as observed in the real world, the CO2 emission reduction costs become unrealistically high around the middle of this century, particularly in scenarios for the 1.5 °C target; relatively low-cost measures such as BECCS in Sub-Saharan Africa and AR in many regions are limited in deployment due to the limited speed of the land-use expansion, and yet energy systems must be transformed to nearly net-zero/negative CO2 emissions for the 2 °C/1.5 °C target, which necessitates using other mitigation technologies of much higher costs. These results may cause concern over the feasibility of achieving the temperature targets, especially for the 1.5 °C target, and point to technical and scenario design aspects that will need further research for biomass-based NETs and their allowable expansion speed. [ABSTRACT FROM AUTHOR]

• C org burial was evaluated in seagrass sediments at the northern Mexican Caribbean. • Thalassia testudinum seagrass meadows showed C org storage within ~ 40 to ~ 100 years. • C org stock increments were related with increasing sediment accumulation rates. • Seagrasses might be gradually impacted by population growth and land use changes. Carbon sequestration in seagrass meadows mitigates the currently rising CO 2 atmospheric levels, as these ecosystems are highly productive and preserve sediment organic carbon over long periods. Five 210Pb dated sediment cores, collected from seagrass meadows dominated by Thalassia testudinum in the northern Mexican Caribbean touristic corridor Cancun-Riviera Maya, were used to establish temporal trends of organic carbon (C org) content, burial rates and stock variation over the past 100 years. C org contents (0.17–1.94%) and burial rates (2.0–252 g m−2 yr−1) were generally within the lower end range reported for seagrass meadow sediments worldwide. C org stock gradually increased over the past decades, associated with increasing sediment accumulation, possibly caused by inland erosion promoted by agricultural and forestry activities and, in the recent decades, by a rapid increase of urban development due to a fast growth of the tourist industry and population, likely causing fertilization of coastal waters. Seagrass meadow sediments in the northern Mexican Caribbean coastline showed long-term C org storage and preservation, since ~ 40 to ~ 100 years depending on the core, which emphasizes the need to protect these ecosystems against impacts of global change, and their incorporation into climate change mitigation strategies should be considered. [ABSTRACT FROM AUTHOR]