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Regional carbon budget assessments attribute and track changes in carbon sources and sinks and support the development and monitoring the efficacy of climate policies. We present a comprehensive assessment of the natural and anthropogenic carbon (C‐CO2) fluxes for Australasia as a whole, as well as for Australia and New Zealand individually, for the period from 2010 to 2019, using two approaches: bottom‐up methods that integrate flux estimates from land‐surface models, data‐driven models, and inventory estimates; and top‐down atmospheric inversions based on satellite and in situ measurements. Our bottom‐up decadal assessment suggests that Australasia's net carbon balance was close to carbon neutral (−0.4 ± 77.0 TgC yr−1). However, substantial uncertainties remain in this estimate, primarily driven by the large spread between our regional terrestrial biosphere simulations and predictions from global ecosystem models. Within Australasia, Australia was a net source of 38.2 ± 75.8 TgC yr−1, and New Zealand was a net CO2 sink of −38.6 ± 13.4 TgC yr−1. The top‐down approach using atmospheric CO2 inversions indicates that fluxes derived from the latest satellite retrievals are consistent within the range of uncertainties with Australia's bottom‐up budget. For New Zealand, the best agreement was found with a national scale flux inversion estimate based on in situ measurements, which provide better constrained of fluxes than satellite flux inversions. This study marks an important step toward a more comprehensive understanding of the net CO2 balance in both countries, facilitating the improvement of carbon accounting approaches and strategies to reduce emissions. Plain Language Summary: Human activities—including the extraction and use of fossil fuels (coal, oil, and natural gas), cement production, and land‐use change (e.g., land clearing), release carbon dioxide (CO2) to the atmosphere, while biospheric processes such as the CO2 uptake by forests and revegetation remove CO2 from the atmosphere. In this study, we assess the balance of natural and human‐driven sources and sinks of CO2 for Australia and New Zealand (referred to as the Australasia carbon budget) for 2010–2019. Our findings indicate that Australasia was close to carbon neutral, with large uncertainties, suggesting that the CO2 sinks from vegetation in this region largely offset the CO2 emissions from human activities. An independent assessment using the latest satellite observations and modeling shows consistent results for Australia. For New Zealand, a national system of ground observations and modeling agreed better with the bottom‐up budget than satellite‐derived flux estimates. Key Points: We synthesize Australasia's carbon C‐CO2 budget (together with Australia and New Zealand) based on bottom‐up and top‐down approachesAustralasia's bottom‐up carbon budget suggests that this region was close to neutral (−0.4 ± 77.0 TgC yr−1) from 2010 to 2019Australasia's annual CO2 balance fluctuates significantly, particularly in Australia, shifting from a strong carbon sink to a strong carbon source [ABSTRACT FROM AUTHOR]

Carbon sequestration can also occur as alkalinity, dissolved inorganic and organic carbon (DIC and DOC) as well as detritus export beyond the habitat where carbon is fixed. Aquatic carbon sequestration refers to the long-term (century-scale) storage of atmospheric carbon in sediments, biomass and/or water. GLO:4D9/02nov22:lno12268-fig-0001.jpg PHOTO (COLOR): 1 Blue carbon ecosystems (mangroves, saltmarshes, seagrass meadows, and kelp forests) are carbon sequestration hotspots. A faster turnover of contemporary autochthonous organic carbon ("fast carbon") than old allochthonous organic carbon ("slow carbon") revealed a decoupling between carbon burial and anthropogenic CO SB 2 sb emissions. [Extracted from the article]

The Mediterranean seagrass Posidonia oceanica maintains a biodiverse ecosystem and it is a world-wide important carbon sink. It grows for millennia, accumulating organic-rich soils (mats) beneath the meadows. This marine habitat is protected by the European Union; however, it is declining rapidly due to coastal development. Understanding its response to disturbances could inform habitat restoration, but many environmental impacts predate monitoring programs (<50 years)., This research explores the palaeoecological potential of Posidonia mats to reconstruct six thousand years of environmental change that could have affected Posidonia meadows and, in turn, left an imprint on the mats., Palynological, microcharcoal, magnetic susceptibility and glomalin-related soil protein ( GRSP) analyses on Posidonia mats enabled us to detect climate- and human-induced environmental processes impacting on the seagrass during the Late Holocene., The pollen and microcharcoal records reconstructed anthropogenic disturbances attributed to agriculture. The record of GRSP shows that agrarian activities affected continental soil quality. Changes in magnetic susceptibility reveal that enhanced soil erosion was caused by both climate (major flooding events in the NW Mediterranean) and humans (cultivation) which impacted on the Posidonia mat. Finally, increased human impact is linked to eutrophication of coastal waters since Roman-Medieval times., Synthesis. This study shows that climate and land-use changes in the western Mediterranean resulted in enhanced loadings of terrigenous material to the coastal zone since the Late Holocene, likely disturbing the Posidonia meadows and their mat carbon accumulation dynamics. Under the current global change scenario in which CO2 emissions are projected to increase, restoring carbon sinks is a priority. Seagrass habitat restoration should consider not only the coastal perturbations, but also the continental ones at a catchment scale to preserve the socio-economic ecosystem services provided by seagrasses. [ABSTRACT FROM AUTHOR]

Coastal vegetated ecosystems play an important role in carbon cycling and bacterial communities inhabiting coastal sediments are responsible for the remineralization and processing of organic carbon (OC). We collected 1 m-long sediment cores in Posidonia seagrass meadows from coastal and estuarine sites in Australia that differed in their sedimentary organic and inorganic carbon, nitrogen and mud contents. The metabolic diversity of sediment heterotrophic bacterial communities was characterized at different sediment depths, based on the utilization pattern of 31 individual carbon substrates using Biolog EcoPlates™. High metabolic diversity was recorded at both sites, but the carbon substrate utilization rates and the use of carbohydrates were higher at the coastal site compared to the estuarine site. The heterotrophic bacterial community in the coastal sediment appeared to metabolize a more diverse OC pool compared to the estuarine site, which might partly explain the differences in OC storage among the seagrass habitats studied. The Biolog EcoPlates™ provided a useful tool for characte rising the sediment heterotrophic bacterial communities in the meadows and sediment characteristics and biochemical composition of the organic matter played an important role in shaping heterotrophic bacterial communities and their carbon utilization rates, potentially affecting carbon accumulation and preservation within seagrass sediments. [ABSTRACT FROM AUTHOR]

The actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth-induced gradients of light availability to assess the variability in their sedimentary organic carbon (Corg) stocks and accretion rates. This study showed a fourfold decrease in Corg stocks from 2-4 m to 6-8 m depth P. sinuosa meadows (averaging 7.0 and 1.8 kg m−2, respectively; top meter of sediment) and a fourteenfold to sixteenfold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m−2 average, respectively; top 2.7 m of sediment). The average Corg accretion rates in shallow P. sinuosa meadows were higher (10.5 g m−2 yr−1) than in deeper meadows (2.1 g m−2 yr−1). The reduction of sedimentary Corg stocks and accretion rates along depth-related gradients of light reduction suggests that irradiance, controlling plant productivity, meadow density, and sediment accretion rates, is a key environmental factor affecting Corg storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal Corg stocks of all seagrasses (estimated in up to 691-770 kg m−2 in 8-13 m thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g., eutrophication or sediment regime alterations) will lead to photoacclimation responses (i.e., reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses. [ABSTRACT FROM AUTHOR]