Carbon Sink Research Articles

Faster dieback of rainforests altering tropical carbon sinks under climate change

Carbon sinks in the tropical rainforests are restricting the global warming to attain unprecedented heights. However, deforestation and climate change is switching them to a net carbon source at some of the deforested patches. Using machine learning algorithm we predict that more than 50% of the tropical rainforests will undergo rapid “Savannisation”/transformation by the end of 21st century under high emission scenarios. Climate change projects ‘El Niño-like’ warming condition, which decreases precipitation in the rainforests and favors atmospheric dryness. In Central Amazonia vegetation degradation saturates the carbon sink and more than 25% of the rainforests will transform into a net carbon source due to increase in soil microbial respiration. This transition will accelerate if Eastern Pacific/Global temperature warms beyond 1.5◦K/2.3◦K (by 2050’s) and will undergo a steeper transit by ~2075 (2.45◦K/3.8◦K warming). This alteration will exacerbate global warming and has consequences for policies that are intended to stabilize Earth’s climate.

Estimation of carbon stocks of woody plant species in church forests of West Gojjam zone, Northwestern Ethiopia: Implications for climate change mitigation

Forests, particularly church forests, play a crucial role in mitigating climate change by absorbing and storing CO2, preserving biodiversity, and acting as carbon sinks. This study aimed to estimate the biomass and carbon stocks of various woody species in church forests in the West Gojjam zone. Twenty-six church forests were selected based on agroecology, elevation, size, and proximity to population centers. Vegetation data were collected using a systematic sampling technique, with 20 m x 20 m (400 m2) plots established along transect lines oriented at 120° intervals at 60°, 180°, and 300° within each church forest. Measurements of diameter at breast height (DBH) and height were taken for all matured woody plants with a DBH ≥ 2.5 cm. Data analysis was performed using one-way ANOVA to evaluate the effects of altitude, forest size, and human disturbance on aboveground biomass (AGB) and carbon stocks. Additionally, linear regression was applied to investigate the relationship between vegetation structure (species richness, density, and diversity) and biomass accumulation. The results revealed a total of 111 woody species, dominated by indigenous species particularly from the Fabaceae family. The study church forests had Shannon diversity index and richness ranging from 1.73 to 3.47 and 7 to 45, respectively. The results showed that the 26 church forests had an average aboveground biomass (AGB) of 31.97 ± 3.31 tons ha-1 and a CO2 equivalence of 97.15 ± 10.47 tons ha-1. The AGB and aboveground carbon (AGC) values varied among the church forests, of which Debre Mihret Mesk Kidanemihret had the highest AGB with 99.00 tons ha-1 and 49.50 tons ha-1 AGC, indicating their substantial capacity for carbon storage. Conversely, Korch Silassie church forest displayed the lowest AGB, suggesting ecological challenges that necessitate targeted conservation efforts. These findings underscore the critical role of church forests as carbon sinks, capable of sequestering atmospheric CO2 and contributing to the mitigation of climate change. The findings of the present study suggest the integration of church forests into national and international climate policies, such as REDD+ to leverage their potential in reducing emissions from deforestation and forest degradation.

Reducing CO2 emissions from rivers around bare land contributes to improving the carbon sink function of the Qinghai–Tibetan Plateau

River CO2 fluxes (FCO2) play an important role in the carbon cycle of the Qinghai‒Tibet Plateau (QTP). However, the carbon budget of river water is ignored in the assessment of the carbon sources and sinks of ecosystems on the QTP, which makes the assessment of the carbon sinks of ecosystems uncertain on the QTP. Here, we selected rivers around alpine steppe, alpine meadow, forestland and bare land areas on the QTP to determine their FCO2 values and control factors during thaw, rainy and freezing periods. The results revealed that river water around the alpine steppe absorbed CO2 during the thaw and freezing periods, making it a carbon sink. The influencing factors of FCO2 around different ecosystems significantly differed across the different periods. The pCO2 was the most direct factor affecting the FCO2 of river water. During the thaw period, the FCO2 of rivers around the alpine steppe and bare land was affected mainly by dissolved organic carbon (DOC) and precipitation, respectively. The FCO2 of rivers around the alpine meadow and forestland was affected by dissolved inorganic carbon (DIC). During the rainy period, the FCO2 of the rivers around the alpine steppe and bare land was affected mainly by total nitrogen (TN) and salinity, respectively. The FCO2 of rivers around the alpine meadow and forestland was affected by dissolved oxygen (DO) and DOC. During the freezing period, the FCO2 of the rivers around the alpine steppe and bare land was affected mainly by TN and pCO2, respectively. The FCO2 of rivers around the forestland and alpine meadow was affected by precipitation, runoff and DIC. The FCO2 in rivers around vegetated areas were affected by rock weathering, aquatic ecosystem biology and hydrological processes to varying degrees at different times. However, rivers around bare ground were less affected by these processes. The FCO2 values around the alpine steppe, alpine meadow, forestland and bare land were 1.40 g C/m2/d, 1.12 g C/m2/d, 2.29 g C/m2/d and 4.20 g C/m2/d, respectively. The river water around bare land released a large amount of CO2. Therefore, to reduce the CO2 emissions of rivers on the QTP, it is urgent that we restore bare-land vegetation and promote the transfer of bare land to steppe, which is an effective way to increase the carbon sink of the ecosystems on the QTP.

Resource availability enhances positive tree functional diversity effects on carbon and nitrogen accrual in natural forests

Forests harbor extensive biodiversity and act as a strong global carbon and nitrogen sink. Although enhancing tree diversity has been shown to mitigate climate change by sequestering more carbon and nitrogen in biomass and soils in manipulative experiments, it is still unknown how varying environmental gradients, such as gradients in resource availability, mediate the effects of tree diversity on carbon and nitrogen accrual in natural forests. Here, we use Canada’s National Forest Inventory data to explore how the relationships between tree diversity and the accumulation of carbon and nitrogen in tree biomass and soils vary with resource availability and environmental stressors in natural forests. We find that the positive relationship between tree functional diversity (rather than species richness) and the accumulation of carbon in tree biomass strengthens with increasing light and soil nutrient availability. Moreover, the positive relationship between tree functional diversity and the accumulation of carbon and nitrogen in both organic and mineral soil horizons is more pronounced at sites with greater water and nutrient availabilities. Our results highlight that conserving and promoting functionally diverse forests in resource-rich environments could play a greater role than in resource-poor environments in enhancing carbon and nitrogen sequestration in Canada’s forests.

Quantifying the Carbon Stocks in Urban Trees: The Rio de Janeiro Botanical Garden as an Important Tropical Carbon Sink

Quantifying the Carbon Stocks in Urban Trees: The Rio de Janeiro Botanical Garden as an Important Tropical Carbon Sink

Spatiotemporal distribution of pCO2 and CO2 flux and the regulatory factors: From the perspective of a subtropical canyon-shaped reservoir, southwest China

Reservoir carbon cycling is a critical component of the global carbon cycle, particularly in subtropical canyon-shaped reservoirs where unique geomorphological features and hydrological regulation under monsoon climate provide a distinct perspective for carbon cycle research. This study takes the Zipingpu Reservoir (ZPPR) in southwestern China as an example and reveals significant seasonal variations in partial pressure of carbon dioxide (pCO2) and carbon dioxide flux at the water-air interface (FCO2) through annual monitoring. The average pCO2 in ZPPR is 486 μatm, and the average FCO2 is 0.093 g C m-2d-1, with approximately 42.4 % of FCO2 measurements being negative, indicating the reservoir's role can act as a carbon sink. The study also estimated the gas transfer coefficient (K), with an average value of 1.888 md-1. Peak FCO2 during spring and summer is three times higher than in other seasons, consistent with the main influencing factors of inflow dynamics, metabolic processes, and reservoir operation strategies. Inflow and operational activities, particularly during spring and summer, drive CO2 emissions in the downstream reach and upstream tributary, while biological activity facilitates CO2 uptake in the lacustrine area and forebay tributary, with enhanced absorption occurring in the autumn. Concurrently, high-intensity FCO2 emissions from the upstream river section during summer underscore the importance of carbon emission monitoring and management during critical periods. The findings not only enhance the understanding of FCO2 assessment accuracy but also provide a framework for evaluating and optimizing carbon dynamics management strategies in canyon-shaped reservoirs, contributing valuable insights to global carbon cycle research.

Accounting of weathering enhancement by mafic mineral spreading for CO 2 storage

The reduction of atmospheric CO 2 is an urgent global issue in the mitigation of global warming. Carbonate acts as a carbon sink in Earth's surface layer, and the weathering of terrestrial silicates and the deposition of oceanic calcium carbonate and organic matter mainly determine atmospheric and oceanic carbon dynamics on geological time scales. Various CO 2 reduction approaches have been evaluated, and the weathering acceleration method is expected to be a low-cost approach. This study attempts to estimate carbonate precipitation potential and dissolution rate from alkali leaching rates using weathering tests on mafic rocks. Based on Japan's annual average rainfall, the in situ carbonate precipitation potential obtained by spreading an olivine mineral or basalt rock powder at 40 ton-powder/ha was on the order of 10 -3 ton/year/ton-rock. This weathering rate could not be greatly accelerated under natural temperature conditions and fine granulation of the minerals.

Stem Growth and Dehydration Responses of Mediterranean Tree Species to Atmospheric and Soil Drought.

Stem growth responses to soil and atmospheric drought are critical to forecasting the tree carbon sink strength. Yet, responses of drought-prone forests remain uncertain despite global aridification trends. Stem diameter variations at an hourly resolution were monitored in five Mediterranean tree species from a mesic and a xeric site for 6 and 12 years. Stem growth and dehydration responses to soil (REW) and atmospheric (VPD) drought were explored at different timescales. Annually, growth was determined by the number of growing days and hours. Seasonally, growth was bimodal (autumn growth ≈ 8%-18% of annual growth), varying among species and sites across the hydrometeorological space, while dehydration consistently responded to REW. Sub-daily, substantial growth occurred during daytime, with nighttime-to-daytime ratios ranging between 1.2 and 3.5 (Arbutus unedo ≈ Quercus faginea < Quercus ilex < Pinus halepensis in the mesic site, and Juniperus thurifera < P. halepensis in the xeric site). Overall, time windows favourable for growth were limited by soil (rather than atmospheric) drought, modulating annual and seasonal growth in Mediterranean species, and stems maintained non-negligible growth during daytime. These patterns contrast with observations from wetter or cooler biomes, demonstrating the growth plasticity of drought-prone species to more arid climate conditions.

Fire weakens land carbon sinks before 1.5 °C

To avoid the worst impacts of climate change, the Paris Agreement committed countries to pursue efforts to limit global warming to 1.5 °C by urgently reducing greenhouse gas emissions. However, the Paris temperature ambitions and remaining carbon budgets mostly use models that lack feedback among fire, vegetation and carbon, which are essential for understanding the future resilience of ecosystems. Here we use a coupled fire–vegetation model to explore regional impacts and feedbacks across global warming levels. We address whether the 1.5 °C goal is consistent with avoiding significant ecosystem changes when considering shifts in fire regimes. We find that the global warming level at which fire began to impact global carbon storage significantly was 1.07 °C (0.8–1.34 °C) above pre-industrial levels and conclude that fire is already playing a major role in decreasing the effectiveness of land carbon sinks. We estimate that considering fire reduces the remaining carbon budget by 25 Gt CO2 (~5%) for limiting temperature rise to 1.5 °C and 64 GtCO2 (~5%) for 2.0 °C compared to previous estimates. Whereas limiting warming to 1.5 °C is still essential for avoiding the worst impacts of climate change, in many cases, we are already reaching the point of significant change in ecosystems rich in carbon and biodiversity.

Re-evaluating winter carbon sink in Southern Ocean by recovering MODIS-Aqua chlorophyll-a product at high solar zenith angles

Satellite ocean color observations are extensively utilized in global carbon sink evaluation. However, the valid coverage of chlorophyll-a concentration (Chla, mg m−3) measurements from these observations is severely limited during autumn and winter in high latitude oceans. The high solar zenith angle (SZA) stands as one of the primary contributors to the reduced quality of Chla products in the high-latitude Southern Ocean during these seasons. This study addresses this challenge by employing a random forest-based regression ensemble (RFRE) method to enhance the quality of Moderate Resolution Imaging Spectroradiometer (MODIS) Chla products affected by high SZA conditions. The RFRE model incorporates the color index (CI), band-ratio index (R), SZA, sensor zenith angle (senz), and Rayleigh-corrected reflectance at 869 nm (Rrc(869)) as predictors. The results indicate that the RFRE model significantly increased the MODIS observed Chla coverage (1.03 to 3.24 times) in high-latitude Southern Ocean regions to the quality of standard Chla products. By applying the recovered Chla to re-evaluate the carbon sink in South Ocean, results showed that the Southern Ocean’s ability to absorb carbon dioxide (CO2) in winter has been underestimated (5.9–18.6 Tg C year−1) in previous assessments. This study underscores the significance of improving the Chla products for a more accurate estimation of winter carbon sink in the Southern Ocean.

Estimation and Spatial Distribution of Individual Tree Aboveground Biomass in a Chinese Fir Plantation in the Dabieshan Mountains of Western Anhui, China

Understanding aboveground biomass (AGB) and its spatial distribution is key to evaluating the productivity and carbon sink effect of forest ecosystems. In this study, a 123-year-old Chinese fir forest in the Dabieshan Mountains of western Anhui Province was used as the research subject. Using AGB data calculated from field measurements of individual Chinese fir trees (diameter at breast height [DBH] and height) and spectral vegetation indices derived from unmanned aerial vehicle (UAV) remote sensing images, a random forest regression model was developed to predict individual tree AGB. This model was then used to estimate the AGB of individual Chinese fir trees. Combined with digital elevation model (DEM) data, the effects of topographic factors on the spatial distribution of AGB were analyzed. We found that remote sensing spectral vegetation indices obtained by UAVs can be used to predict the AGB of individual Chinese fir trees, with the normalized difference vegetation index (NDVI) and the optimized soil-adjusted vegetation index (OSAVI) being two important predictors. The estimated AGB of individual Chinese fir trees was 339.34 Mg·ha−1 with a coefficient of variation of 23.21%. At the local scale, under the influence of elevation, slope, and aspect, the AGB of individual Chinese fir trees showed a distribution pattern of decreasing from the middle to the northwest and southeast along the northeast-southwest trend. The effect of elevation on AGB was influenced by slope and aspect; AGB on steep slopes was higher than on gentle slopes, and the impact of slope on AGB was influenced by aspect. Additionally, AGB on north-facing slopes was higher than on south-facing slopes. Our results suggest that local environmental factors such as elevation, slope, and aspect should be considered in future Chinese fir plantation management and carbon sink assessments in the Dabieshan Mountains of western Anhui, China.

Timber and carbon sequestration potential of Chinese forests under different forest management scenarios

Developing forestry action plans with the goal of carbon neutrality is a critical task to identify carbon sink potential and balance the pathways of China's forests. Current research that predicts forest biomass carbon stock and sink potential has shortcomings, such as an incomplete assessment of China's forest carbon sink range, assumptions based on unchanged forest area in the base year and insufficient consideration of natural and human disturbance factors. This study utilised the national forest inventory (NFI) data to construct a model of forest growth and consumption using a machine learning algorithm (i.e. random forest), identified suitable areas for future forest expansion by integrating multi-source data, and set up three future forest management scenarios: business as usual (BAU), enhanced policy scenario (EPS) and maximum potential scenario (MPS). In addition, changes in the area, volume stock and biomass carbon stock in China's forests between 2020 and 2060 were predicted under three forestry activities (i.e. existing forest, afforested/reforested (AR) forest and forest conversion) and three climate scenarios (i.e. SSP126, SSP370 and SSP585) based on the 9th NFI (2014–2018). According to China's relevant planning goals and suitable forest space, the area of AR forests is predicted to be 55.55 × 106 hm2 by 2060, and the forest coverage rate is predicted to increase from 23% in 2018 to 28% by 2060. Biomass carbon sequestration (BCS) between 2020 and 2060 in AR forests is predicted to be 36.00 TgC per year. By 2060, the average BCS is predicted to be approximately 140.00–287.56 TgC per year in China's forests, which is mainly owing to arbor forest management. Under the BAU and EPS scenarios, BCS in China's forests is expected to decline from 2020 to 2060. However, under the MPS, BCS in China's forests is projected to increase and be maintained at 322 TgC per year or above by 2060, with wood production reaching 4.71 × 108 m3 per year. China's forests are predicted to experience an increase in biomass carbon stock in the future and play a role as a carbon sink. By taking measures to achieve the maximum growth potential of all forest types, China's forests will achieve a win‒win situation between carbon sinks and timber production.

Carbon sink efficiency of Chinese mollusk mariculture

Chinese mollusk mariculture plays a key role in boosting ocean carbon sink capacity through biological processes, supported by large-scale production. However, its full potential remains underutilized due to the absence of comprehensive efficiency evaluations. This interdisciplinary study applies a three-stage approach to assess the carbon sink efficiency of Chinese mollusk mariculture, with a focus on regional variations across nine coastal provinces from 2012 to 2022. The results show that environmental variables and statistical noise distort efficiency scores, leading to overestimation in six provinces and underestimation in three. After adjusting for these influences, regional disparities in efficiency become more apparent. High-production provinces such as Shandong, Fujian, and Guangdong exhibit higher efficiency, while low-production provinces like Hainan show diminished performance. Although there is a positive correlation between production volume and carbon sink efficiency, further improvements are constrained by the sector's low profitability. To address this, we propose two strategies: (1) reallocating subsidies from capture fisheries to mollusk mariculture, and (2) accelerating carbon trading between mollusk mariculture and other industries. These measures could enable the global mollusk mariculture to enhance both its carbon sink capacity and efficiency, contributing to food security and climate change mitigation.

A theoretical framework for value co-creation analysis in carbon sink projects

With the maturation of the carbon market, carbon sink projects have become a thriving business model, and the value co-creation of carbon sink has become a hot topic in interdisciplinary research. However, the theoretical foundation for value co-creation of carbon sink is limited, and existing theoretical frameworks do not fully cover resource allocation, power dynamics, and interaction mechanisms among different actors in project operations. Therefore, this study, based on the theory of value co-creation and supplemented by residual rights theory and circuit of capital theory, has established a comprehensive theoretical framework to consistently understand the driving factors, co-creation process, and outcomes of resource transformation in projects. Using the Forest Ecological Bank in Fujian Province, China as a case study, we applied our theoretical framework for analysis. The results show that our theoretical framework can depict the complete process of value co-creation of carbon sink, reveal the roles played by diverse actors, and capture the project's dynamic changes. The research findings can provide guidance for the sustainable development of carbon sink projects in areas such as strengthening cooperation and partnerships, creating incentive mechanisms, and enhancing the role of third-party institutions.

Wildfire Emissions Offset More Permafrost Ecosystem Carbon Sink in the 21st Century

AbstractPermafrost ecosystems in high‐latitudes stock a large amount of carbon and are vulnerable to wildfires under climate warming. However, major knowledge gap remains in the effects of direct carbon loss from increasing wildfire biomass burning on permafrost ecosystem carbon sink. In this study, we used observation‐derived data sets and Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations to investigate how carbon emissions from wildfire biomass burning offset permafrost ecosystem carbon sink under climate warming in the 21st century. We show that the fraction of permafrost ecosystem carbon sink offset by wildfire emissions was 14%–25% during the past two decades. The fraction is projected to be 28%–45% at the end of this century under different warming scenarios. The weakening carbon sink is caused by greater increase in wildfire emissions than net ecosystem production in permafrost regions under climate warming. The increased fraction of ecosystem carbon sink offset by wildfire carbon loss is especially pronounced in continuous permafrost region during the past two decades. Although uncertainties exist in simulations of wildfire emissions and ecosystem carbon budget, results from different models still show that wildfire emissions offset more permafrost ecosystem carbon sink in the 21st century. These findings highlight that carbon sink capacity of permafrost ecosystems is increasingly threatened by wildfires under the warming climate.

Negative Asymmetric Response of Pantropical Gross Primary Productivity to Precipitation Anomalies

AbstractThe carbon sink in pantropical biomes play a crucial role in modulating the inter‐annual variations of global terrestrial carbon balance and is threatened by extreme climate events. However, it has not been carefully examined whether an increase in tropical gross primary productivity (GPP) can compensate the decrease during precipitation anomalies. Using the asymmetry index (AI) and multiple GPP products, we assessed responses of pantropical GPP to precipitation anomalies during 2001–2022. Positive AI indicates that GPP increases are greater than GPP decreases during precipitation anomalies, and vice versa. Our results showed an average negative pantropical GPP asymmetry, that is, GPP decreases exceeded the GPP increases during precipitation anomalies. In addition, a positive AI was found in tropical hyper‐arid and arid regions, which is opposite to the negative AI observed in tropical semi‐arid, sub‐humid, and humid regions. This suggest that tropical GPP asymmetry changes from positive to negative as the moisture increases. Notably, a significant decreasing trend of negative AI was observed over the entire tropical region, indicating that the negative effect of inter‐annual precipitation variations on pantropical vegetation productivity has enhanced. Considering the model predicted increasing climate variability and extremes, the negative impact of precipitation variability on tropical carbon cycle may continue to intensify. Lastly, the divergence in AI estimates among multiple GPP products highlight the need to further improve our understanding of the response of tropical carbon cycle to climate changes, especially for the tropical humid regions.

The variegated canalized-1 tomato mutant is linked to photosystem assembly

The recently described canal-1 tomato mutant, which has a variegated leaf phenotype, has been shown to affect canalization of yield. The corresponding protein is orthologous to AtSCO2 -SNOWY COTYLEDON2, which has suggested roles in thylakoid biogenesis. Here we characterize the canal-1 mutant through a multi-omics approach, by comparing mutant to wild-type tissues. While white canal-1 leaves are devoid of chlorophyll, green leaves of the mutant appear wild-type-like, despite an impaired protein function. Transcriptomic data suggest that green mutant leaves compensate for this impaired protein function by upregulation of transcription of photosystem assembly and photosystem component genes, thereby allowing adequate photosystem establishment, which is reflected in their wild-type-like proteome. White canal-1 leaves, however, likely fail to reach a certain threshold enabling this overcompensation, and plastids get trapped in an undeveloped state, while additionally suffering from high light stress, indicated by the overexpression of ELIP homolog genes. The metabolic profile of white and to a lesser degree also green tissues revealed upregulation of amino acid levels, that was at least partially mediated by transcriptional and proteomic upregulation. These combined changes are indicative of a stress response and suggest that white tissues behave as carbon sinks. In summary, our work demonstrates the relevance of the SCO2 protein in both photosystem assembly and as a consequence in the canalization of yield.

Benthic diatom community response to the sudden rewetting of a coastal peatland

The coastline of the shallow southern Baltic Sea is a highly dynamic system of spits, lagoonal systems and subtidal flats. On the land side, coastal peatlands are common. Intact peatlands provide important ecosystem services, however, many peatlands have been severely degraded due to human activities. Many coastal peatlands are also separated from the Baltic Sea by artificial dunes or dykes in order to facilitate drainage and costal protection measures. Due to their potential to act as carbon sinks, as well as buffer zones against sea level rise and flooding events, there is an increasing interest in the restoration and rewetting of coastal peatlands. Microphytobenthic communities are important primary producers in coastal systems and peatlands and diatoms are often the dominant microphytobenthic group in temperate regions. In November 2019, the removal of the dyke that separated the Polder Drammendorf (a drained, oligohaline peatland), from the adjacent Baltic Sea lagoon Kubitzer Bodden represented a unique chance to study the sudden rewetting of a degraded coastal peatland with brackish water and its effect on the microphytobenthos. The aims of this study were to investigate the poorly known diatom community composition of coastal peatlands, to determine the effects of flooding events on diatom composition and biomass in coastal peatlands as well as the effects of nutrient and substance release from the peatlands on the diatom communities of adjacent coastal waters. We investigated and documented the impact of this sudden rewetting on the benthic diatom communities of the peatland and the adjacent lagoon over the course of one year, applying the latest taxonomy and measuring key environmental factors. The results deepen the understanding of diatom community composition, taxonomy and ecology and explore the influence of land-sea exchange processes on the benthic diatoms of the Baltic Sea coast.

Impacts of shellfish and macroalgae mariculture on the seawater carbonate system and air-sea [formula omitted] flux in Haizhou Bay, China

China is the largest mariculture country, and shellfish and algae output ranks first, showing high carbon sink capacity. In recent years, the single cultivation of macroalgae (Pyropia yezoensis) has been changed to macroalgae-shellfish mariculture in Haizhou Bay to increase the yield of P. yezoensis and improve the water environment quality. In this study, four surveys were carried out in July 2022 during the monoculture period of oyster (Magallana gigas), as well as at different stages of P. yezoensis culture (head-crop period, November 2022, peak growing season, January 2023, and end of harvesting, March 2023) in the mariculture and the surrounding waters of Haizhou Bay. The effects of different stages of culture on the seawater environment and seasonal and spatial variations in the carbonate system were analyzed, and the carbon sink capacity was preliminarily estimated. The results showed that in summer, the calcification of M. gigas and the primary production process of phytoplankton effectively reduced the dissolved inorganic carbon (DIC) level in the culture area. The culture area acts as a CO2 sink, with an average air-sea CO2 flux of −4.5 mmol m−2 d−1. During the polyculture period, the P. yezoensis culture activities maintained the stability of the seawater carbonate system, and the culture area shows strong CO2 sinks, with the average air-sea CO2 flux of −24.10 mmol m−2 d−1, -37.68 mmol m−2 d−1, and -38.99 mmol m−2 d−1, respectively. The absorption of CO2 by large-scale cultured P. yezoensis through the "biological pump" effect is the main factor affecting the CO2 exchange process at the air-sea interface, and the absorption rate of CO2 by P. yezoensis at the mature stage is higher than that at the growth stage before harvesting. The study revealed that macroalgae-shellfish mariculture could promote mutual growth, alleviate environmental pressure, and enhance the carbon sink of the culture area. The relationship between mariculture and the carbon cycle of a mariculture ecosystem is very complicated, and its biochemical process should be given great attention for further study.

Assessing and explaining rising global carbon sink capacity in karst ecosystems

Global karst vegetation is crucial for carbon sequestration and biodiversity conservation. However, research on the carbon sink capacity of vegetation and its potential driving mechanisms across different geographic types of global karst remains scant. This research delineates the trend characteristics and differences of net primary productivity (NPP), heterotrophic respiration (Rh), and net ecosystem productivity (NEP) across various bedrock and climatic zones within global karst landscapes. Extreme Gradient Boosting (XGBoost), Shapley Additive Explanations (SHAP), Neural Network fitting, and Partial Least Squares Structural Equation Modeling were utilized to investigate the nonlinear impacts and underlying mechanisms exerted by the explanatory variables on NPP, Rh, and NEP. The findings indicate that from 1981 to 2019, the rates of increase in NPP (2.02 gC·m⁻2 yr⁻1), Rh (1.47 gC·m⁻2 yr⁻1), and NEP (0.54 gC·m⁻2 yr⁻1) were marginally higher in global karst regions compared to non-karst areas. Additionally, when contrasted with various bedrock types, the differences in mean values and trends of these three indicators were more pronounced across different climatic zones of karst areas. XGBoost effectively captures the nonlinear relationship between NEP and its explanatory variables, while SHAP provides a detailed explanation of the contribution and interaction of these variables. The leaf area index (LAI) was the major contributor to NEP. Additionally, the nonlinear effects of individual critical drivers on NEP varied across different aridity gradients. The cascade effect of explanatory variables on NEP follows a pathway where geography affects climate, climate change subsequently alters vegetation characteristics, and these changes in vegetation directly drive NPP and Rh, ultimately leading to an indirect impact on NEP. This impact pathway exhibits minimal difference across different bedrock and climate types. This study enhances our comprehension of the role of vegetation carbon sinks and their underlying mechanisms across various bedrock and climate types in global karst regions.