Ehui Tan

Seagrass meadows are pivotal in coastal carbon cycling and climate regulation, yet simultaneous measurements of CO ₂ , CH ₄ , and N ₂ O fluxes, especially in tropical systems, remain scarce. Here, we present the first synchronized seasonal assessment of these three greenhouse gases (GHGs) fluxes across water‐atmosphere in a tropical seagrass meadow (Li'an Lagoon, China). The ecosystem acted as a source of CH ₄ to the atmosphere but absorbed substantially more warming potential in the form of CO ₂ and N ₂ O, resulting in a net atmospheric carbon sink. Notably, N ₂ O uptake offset 63%–243% of the warming potential from CH ₄ emissions. At the global scale, tropical seagrass meadows exhibit higher median CH ₄ emissions yet stronger N ₂ O uptake compared to temperate systems, also constituting a net GHGs sink from the atmosphere, with N ₂ O uptake contributing 38%–164% of this sink effect. These findings highlight the necessity of multi‐gas assessments to accurately evaluate the climate function of seagrass ecosystems worldwide.

The diverse and productive blue carbon ecosystems (mangrove, seagrass, salt marsh, and macroalgae) provide many ecosystem services and play an important role in climate change mitigation and adaptation. However, less is known about how different biogeochemical processes within blue carbon ecosystems can influence seawater carbonate dynamics especially on the partial pressure of carbon dioxide ( p CO ₂ ) and pH. Through underway p CO ₂ measurements and discrete water samples, we examined the spatial and temporal distributions of the carbonate system in two tropical mangrove estuaries and one benthic‐macroalgae‐vegetated lagoon. In early summer, the mangrove estuaries behaved as weak CO ₂ sources of 2.0–4.0 mmol m ⁻² d ⁻¹ , whereas the tropical lagoon became a CO ₂ sink of −6.4 mmol m ⁻² d ⁻¹ . The chemical stoichiometry, stable carbon isotope, and other geochemical tracers reveal that carbonate dissolution and sulfate reduction followed by aerobic respiration are dominant controls on carbonate dynamics in mangrove estuaries. Without alkalinity enhancement driven by carbonate dissolution and sulfate reduction mainly from mangrove sediments, CO ₂ emission would be 23 times larger than current observation, and pH would decrease by 0.39 units on an estuary‐wide scale. However, in macroalgae‐inhabited lagoon primary production followed by carbonate formation could draw p CO ₂ down to 70 μatm and raise pH to 8.8, which are significantly distinct from the normal estuarine water. This study demonstrates that lateral export of alkalinity from mangrove sediments and the in situ metabolism of macroalgae can significantly influence the estuarine air‐sea CO ₂ flux and acidification status, and emphasizes the importance of protecting and restoring the blue carbon ecosystems.

Denitrification and anaerobic ammonium oxidation (anammox) convert reactive nitrogen to inert N₂ and play vital roles in nitrogen removal in coastal and marine ecosystems, weakening the adverse effects caused by terrestrial excessive nitrogen inputs. Given the importance of denitrification and anammox in the nitrogen cycle, several studies have measured denitrification and anammox through intact core incubations across different systems, and nitrogen loss processes are affected by a series of environmental factors such as organic carbon, nitrate, dissolved oxygen and temperature. However, a global synthesis of actual nitrogen loss rates is lacking, and how environmental factors regulate nitrogen loss remains unclear. Therefore, we have compiled a database of nitrogen loss rates, including denitrification and anammox in coastal and marine systems, from the published literature. This database includes 473, 466 and 255 measurements for total nitrogen loss, denitrification and anammox, respectively. This work deepens our understanding of the spatial and temporal distribution of denitrification, anammox and the relative contribution of anammox to total nitrogen loss, and their corresponding environmental controls. To our knowledge, the constructed database offers for the first time a comprehensive overview of actual nitrogen loss rates in coastal and marine ecosystems on a global scale. This database can be utilized to compare nitrogen loss rates of different regions, identify the key factors regulating these rates and parameterize biogeochemical models in the future. This database is available from the Figshare repository at https://doi.org/10.6084/m9.figshare.27745770.v3 (Chang et al., 2024).

Sediment nitrate respiration eliminates reactive nitrogen (Nr) and consumes organic carbon (OC) accompanying by CO ₂ and N ₂ O production to partially neutralize the climate benefit of sedimentary carbon burial. The quantitative linkage between carbon and nitrogen stoichiometry and greenhouse potential of this syndepositional process, particularly at a marginal sea scale, remains unexplored. Here we show that temperature and organic matter co‐regulate the sediment nitrate respiration and associated N ₂ O production in China's marginal seas. By establishing empirical equations, we access that 2.8 ± 0.4 Tg Nr (∼26.5% of riverine input) is annually respired via degrading 2.2 ± 0.2 Tg OC (∼12.5% of OC deposited) to produce 15.0 ± 3.5 Gg N ₂ O‐N, which may counter‐balance 15.1 ± 8.1% of the air‐sea CO ₂ influx. This link between anthropogenic Nr input and removal to carbon sequestration reveals that sedimentary nitrate respiration potentially reduces the climatic benefits of marginal seas.

Nitrification is the key process linking the oxidized and reduced forms of reactive nitrogen, playing an important role in the nitrogen biogeochemical cycle. Quantifying the nitrification rate and evaluating its environmental regulators in different aquatic environments at both regional and global scales has received increasing attention. However, the spatiotemporal variations in nitrification rates in coastal waters, particularly with different trophic states, remain unclear. By using the ¹⁵N-labeling technique, here, we quantified the nitrification rates under dark and light conditions in the surface waters of Shenzhen Bay and Dapeng Bay, representing eutrophic and oligotrophic environments, respectively. The nitrification rates were 2–3 orders of magnitude higher in Shenzhen Bay (eutrophic) than those in Dapeng Bay (oligotrophic). The concentrations of ammonium and total suspended matter play key roles in regulating the spatiotemporal distribution and difference in nitrification in these two coastal bays. The nitrification rate under the dark condition (0.13–49.37 nmol N L⁻¹ h⁻¹) was greater than that under light incubation (0–10.15 nmol N L⁻¹ h⁻¹), indicating light inhibition of 33–100% in the surface water. Such results imply that daily integrated nitrification based on the rates under dark incubation may have been overestimated. An environment with high turbidity is preferable for nitrification, as it reduces the damage caused by light to ammonia-oxidizing microbes. Collectively, the differences in nitrification rates further result in a distinct composition of dissolved inorganic nitrogen, with Shenzhen Bay dominated by nitrate and Dapeng Bay dominated by ammonium. Our results provide scientific references for the mitigation of nitrogen pollution in different trophic coastal bays.