Re-estimating China's lake CO2 flux considering spatiotemporal variability

The spatiotemporal variability of lake partial carbon dioxide pressure (pCO2) introduces uncertainty into CO2 flux estimates at the lake water-air interface. Knowing the variation pattern of pCO2 is important for obtaining accurate global estimation. Here we examine seasonal and trophic variations in lake pCO2 based on 13 field campaigns conducted in Chinese lakes from 2017 to 2021. We found significant seasonal fluctuations in pCO2, with decreasing values as trophic states intensify within the same region. Saline lakes exhibit lower pCO2 levels than freshwater lakes. These pCO2 dynamics result in variable areal CO2 emissions, with lakes exhibiting different trophic states (oligotrophication > mesotrophication > eutrophication) and saline lakes differing from freshwater lakes (-23.1 ± 17.4 vs. 19.3 ± 18.3 mmol m-2 d-1). These spatiotemporal pCO2 variations complicate total CO2 emission estimations. Using area proportions of lakes with varying trophic states and salinity in China, we estimate China's lake CO2 flux at 8.07 Tg C yr-1. In future studies, the importance of accounting for lake salinity, seasonal dynamics, and trophic states must be noticed to enhance the accuracy of large-scale carbon emission estimates from lake ecosystems in the context of climate change.

Integration with carbon capture technology enables a positive carbon balance for sustainable rice paddy remediation with calcium‑silicon composites

In situ stabilization technologies based on lime-derived materials are extensively used for remediating Cd-contaminated paddy soils. However, the environmental impacts and carbon budget associated with these technologies throughout the paddy soil remediation life cycle are gaining increasing attention. Herein, through paddy field trials, two representative lime-derived materials, quicklime and calcium-silicon composite (Ca/Si), are evaluated for their remediation effectiveness and environmental sustainability in the remediation of Cd-contaminated soils. The results demonstrate that both quicklime and Ca/Si can reduce Cd bioavailability and enable the safe use of rice grain. Nevertheless, the life cycle assessment score of the quicklime case is 4.4 times that of the Ca/Si case, indicating that the quicklime case has a greater negative impact on the environment. Furthermore, the net ecosystem carbon budget analysis reveals that both lime-derived materials exhibit outward carbon emissions throughout their life cycle, in which the carbon emission of the quicklime case (-20.2 t CO2-eq/ha) is 20 times that of the Ca/Si case (-1 t CO2-eq/ha). Moreover, the implementation of carbon capture technology results in the Ca/Si case achieving a positive carbon budget and contributing to a carbon neutrality plan. Conversely, the quicklime case falls short, affording only a 24.8 % reduction in carbon emissions. Overall, this study provides valuable insights into the environmental sustainability of different lime-derived materials for paddy soil remediation and carbon mitigation.

Modeling lateral carbon fluxes for agroecosystems in the Mid-Atlantic region: Control factors and importance for carbon budget

Estimating lateral carbon fluxes in agroecosystems presents challenges due to intricate anthropogenic and biophysical interactions. We used a modeling technique to enhance our comprehension of the determinants influencing lateral carbon fluxes and their significance in agroecosystem carbon budgets. The SWAT-C model was refined by incorporating a dynamic dissolved inorganic carbon (DIC) module, enhancing our ability to accurately quantify total lateral carbon fluxes. This improved model was calibrated using observed data on riverine particulate organic carbon (POC) and dissolved organic carbon (DOC) fluxes, as well as net ecosystem exchange (NEE) data monitored by a flux tower situated in a representative agricultural watershed, the Tuckahoe Watershed (TW) of the Chesapeake Bay's coastal plain. We assessed the losses of POC, DOC, and DIC across five primary rotation types: C (continuous carbon), CS (corn-soybean), CSS (corn-soybean-soybean), CWS (corn-wheat-soybean), and CWSCS (corn-wheat-soybean-corn-soybean). Our study revealed notable variations in the average annual fluxes of POC (ranging between 152 and 198 kg ha-1), DOC (74-85 kg ha-1), and DIC (93-156 kg ha-1) across the five rotation types. The primary influencing factor for annual POC fluxes was identified as sediment yield. While both annual DOC and DIC fluxes displayed a marked correlation with surface runoff across all crop rotation schemes, soil respiration also significantly influenced annual DIC fluxes. Total lateral carbon fluxes (POC + DOC+DIC) constituted roughly 11 % of both net ecosystem production (NEP) and NEE, yet they represented a striking 95 % of net biome production (NBP) in the TW's agroecosystem. Grain yield carbon accounted for 80 % of both NEP and NEE and was nearly seven times that of NBP. Our findings suggest that introducing soybeans into cornfields tends to reduce NEP, NEE, and also NBP. Conversely, integrating winter wheat into the corn-soybean rotation significantly boosted NEP, NEE, and NBP values, with NBP even surpassing the levels in continuous corn cultivation.

Particle-attached bacteria act as gatekeepers in the decomposition of complex phytoplankton polysaccharides

BACKGROUND

Marine microalgae (phytoplankton) mediate almost half of the worldwide photosynthetic carbon dioxide fixation and therefore play a pivotal role in global carbon cycling, most prominently during massive phytoplankton blooms. Phytoplankton biomass consists of considerable proportions of polysaccharides, substantial parts of which are rapidly remineralized by heterotrophic bacteria. We analyzed the diversity, activity, and functional potential of such polysaccharide-degrading bacteria in different size fractions during a diverse spring phytoplankton bloom at Helgoland Roads (southern North Sea) at high temporal resolution using microscopic, physicochemical, biodiversity, metagenome, and metaproteome analyses.

RESULTS

Prominent active 0.2-3 µm free-living clades comprised Aurantivirga, "Formosa", Cd. Prosiliicoccus, NS4, NS5, Amylibacter, Planktomarina, SAR11 Ia, SAR92, and SAR86, whereas BD1-7, Stappiaceae, Nitrincolaceae, Methylophagaceae, Sulfitobacter, NS9, Polaribacter, Lentimonas, CL500-3, Algibacter, and Glaciecola dominated 3-10 µm and > 10 µm particles. Particle-attached bacteria were more diverse and exhibited more dynamic adaptive shifts over time in terms of taxonomic composition and repertoires of encoded polysaccharide-targeting enzymes. In total, 305 species-level metagenome-assembled genomes were obtained, including 152 particle-attached bacteria, 100 of which were novel for the sampling site with 76 representing new species. Compared to free-living bacteria, they featured on average larger metagenome-assembled genomes with higher proportions of polysaccharide utilization loci. The latter were predicted to target a broader spectrum of polysaccharide substrates, ranging from readily soluble, simple structured storage polysaccharides (e.g., laminarin, α-glucans) to less soluble, complex structural, or secreted polysaccharides (e.g., xylans, cellulose, pectins). In particular, the potential to target poorly soluble or complex polysaccharides was more widespread among abundant and active particle-attached bacteria.

CONCLUSIONS

Particle-attached bacteria represented only 1% of all bloom-associated bacteria, yet our data suggest that many abundant active clades played a pivotal gatekeeping role in the solubilization and subsequent degradation of numerous important classes of algal glycans. The high diversity of polysaccharide niches among the most active particle-attached clades therefore is a determining factor for the proportion of algal polysaccharides that can be rapidly remineralized during generally short-lived phytoplankton bloom events. Video Abstract.

Modelling terrigenous DOC across the north west European Shelf: Fate of riverine input and impact on air-sea CO2 fluxes

Terrigenous carbon in aquatic systems is increasingly recognised as an important part of the global carbon cycle. Despite this, the fate and distribution of terrigenous dissolved organic carbon (tDOC) in coastal and oceanic systems is poorly understood. We have implemented a theoretical framework for the degradation of tDOC across the land to ocean continuum in a 3D hydrodynamical-biogeochemical model on the North West European Shelf. A key feature of this model is that both photochemical and bacterial tDOC degradation rates are age dependant constituting an advance in our ability to describe carbon cycling in the marine environment. Over the time period 1986-2015, 182±17 Gmol yr-1 of riverine tDOC is input to the shelf. Results indicate that bacterial degradation is by far the most important process in removing tDOC on the shelf, contributing to 73±6 % (132±11 Gmol yr-1) of the total removal flux, while 21±3 % (39±6 Gmol yr-1) of riverine tDOC was advected away from the shelf and photochemical degradation removing 5±0.5 % of the riverine flux. Explicitly including tDOC in the model decreased the air-sea carbon dioxide (CO2) flux by 112±8 Gmol yr-1 (4±0.4 %), an amount approximately equivalent to the CO2 released by the UK chemical industry in 2020. The reduction is equivalent to 62 % of the riverine tDOC input to the shelf while approximately 17 % of riverine input is incorporated into the foodweb. This work can improve the assumptions of the fate of tDOC by Earth System Models and demonstrates that the inclusion of tDOC in models can impact ecosystem dynamics and change predicted global carbon budgets for the ocean.

Temporal and spatial variations of air-sea CO2 fluxes and their key influence factors in seagrass meadows of Hainan Island, South China Sea

Seagrass ecosystems have received a great deal of attention for contributing to uptake of atmospheric CO2, and thereby helping to mitigate global climate change ('blue carbon'). Carbon budgets for seagrass ecosystems are developed by estimating air-sea CO2 fluxes. Data for air-sea CO2 flux for tropical seagrass ecosystems are lacking, which is problematic for constraining global seagrass carbon budgets. Here, we sought to address this important data gap for tropical seagrass ecosystems (dominated by Thalassia hemprichii and Enhalus acoroides) from the Hainan Island of South China Sea, while also testing what the main factors driving the variations of air-sea CO2 fluxes are. We found that air-sea CO2 fluxes exhibited a U-shape diurnal variability from 6 a.m. to 6 a.m. of the next day, with the highest and lowest air-sea CO2 fluxes values at early morning and afternoon, respectively. Biological processes were the driving force for mediating diurnal variations of seawater pCO2. The pCO2, sea in different seasons displayed a trend of increasing from spring, reaching maximum in summer and then a decreasing trend after summer, where water temperature, wind speed and seagrass growth mainly drove the variations. This resulted in net uptake of CO2 in all seasons except during summer in our study seagrass ecosystems, with greater negative values found in autumn (-3.63 ± 0.76 mmol m-2 d-1) than those in winter (-2.84 ± 0.60 mmol m-2 d-1). While the nutrient loading induced seagrass biomass changes (especially the seagrass T. hemprichii), which mediated the air-sea CO2 fluxes changes among different seagrass meadows. Net annual CO2 uptake potential under low nutrient loading (-0.77 ± 0.16 mol m-2 yr-1) was 23-54 % greater than high nutrient loading seagrass meadows, with the average annual air-sea CO2 flux of the three seagrass meadows as -0.64 ± 0.13 mol m-2 yr-1. These results suggest that tropical seagrass meadows of Hainan Island are a significant CO2 sink of atmospheric CO2, but this capacity can be diminished by nutrient loading. Scaling up, we estimate the annual atmospheric CO2 uptake by seagrass meadows of Hainan Island (total area 55.28 km2) was 1544 t of CO2 yr-1, equivalent to the annual emissions from the wholesale, retail, accommodation and catering industries of 164,000 tourists in Hainan Island. With carbon neutrality becoming an important part of global climate governance, this study provides timely information for capitalising on the ability of seagrasses to contribute to natural climate solutions.

The carbon budget of China: 1980-2021

As one of the world's largest emitters of greenhouse gases, China has set itself the ambitious goal of achieving carbon peaking and carbon neutrality. Therefore, it is crucial to quantify the magnitude and trend of sources and sinks of atmospheric carbon dioxide (CO2), and to monitor China's progress toward these goals. Using state-of-the-art datasets and models, this study comprehensively estimated the anthropogenic CO2 emissions from energy, industrial processes and product use, and waste along with natural sources and sinks of CO2 for all of China during 1980-2021. To recognize the differences among various methods of estimating greenhouse emissions, the estimates are compared with China's National Greenhouse Gas Inventories (NGHGIs) for 1994, 2005, 2010, 2012, and 2014. Anthropogenic CO2 emissions in China have increased by 7.39 times from 1980 to 12.77 Gt CO2 a-1 in 2021. While benefiting from ecological projects (e.g., Three Norths Shelter Forest System Project), the land carbon sink in China has reached 1.65 Gt CO2 a-1 averaged through 2010-2021, which is almost 15.81 times that of the carbon sink in the 1980s. On average, China's terrestrial ecosystems offset 14.69% ± 2.49% of anthropogenic CO2 emissions through 2010-2021. Two provincial-level administrative regions of China, Xizang and Qinghai, have achieved carbon neutrality according to our estimates, but nearly half of the administrative regions of China have terrestrial carbon sink offsets of less than 10% of anthropogenic CO2 emissions. This study indicated a high level of consistency between NGHGIs and various datasets used for estimating fossil CO2 emissions, but found notable differences for land carbon sinks. Future estimates of the terrestrial carbon sinks of NGHGIs urgently need to be verified with process-based models which integrate the comprehensive carbon cycle processes.

Characteristics of carbon budget based on energy carbon emissions and vegetation carbon absorption

How to evaluate the change characteristics of energy carbon emissions (ECE) and vegetation carbon absorption (VCA) by scientific methods is particularly important for achieving carbon peak by 2030 and carbon neutrality by 2060. Based on provincial-level energy consumption data, nighttime light data, and population data, this study realized spatial simulation of energy carbon emissions and analyzed the change characteristics of energy carbon emissions (ECE) and vegetation carbon absorption (VCA) combined with the net primary productivity (NPP)of vegetation. Besides, the relationship between energy carbon emissions, vegetation carbon absorption, and economic development was also analyzed at an urban scale. The results showed that (1) the total ECE increased from 4.34 billion tons in 2000 to 14.43 billion tons in 2019, but the growth rate of ECE decreased from 15.9% during 2000-2010 to 3.1% during 2010-2019. The VCA capacity has been increasing year by year. In 2019, it could absorb 1.56 billion tons more carbon dioxide than in 2000 with an increase of 16.1%. (2) Through the identification of the increasing and decreasing regions of ECE and VCA, it was found that the continuous rise area of ECE accounts for 0.5% of the study area; the area of fluctuating rise accounted for 6.7% of the study area. The area of continuous decline of VCA accounted for 0.2% of the study area; the area of fluctuating decline accounted for 49.6% of the study area. (3) The eastern China accounted for 42% of ECE and 17% of VCA with 11.4% of land, while the western region accounted for 26% of ECE and 55% of VCA with 66.6% of land, which indicated that there were significant differences in the characteristics of carbon budget between the eastern China and the western region. (4) The carbon pressure index (CPI) of most cities was on the rise, but the carbon efficiency index (CEI) was also on the rise, and cities were developing towards the model of low energy consumption and high output value. In a word, the growth rate of ECE is slowing down, and the VCA capacity is increasing. In the process of promoting carbon neutrality, we should be aware of the different resource endowments of different regions, realize the actual role of each region in carbon neutrality and economic development, and allocate carbon neutrality tasks differently.

Contribution of water erosion to organic carbon and total nitrogen loads in agricultural discharge from boreal mineral soils

While organic carbon (OC) in agricultural mineral soils is widely studied in terms of soil carbon sequestration and gaseous emissions, discharge-induced OC loss from soil is still poorly understood and estimations of boreal soil OC loads within water erosion are lacking. Loss of organic matter from arable soils is a concern for surface water quality, climate change and soil productivity. The main aim of this study was to quantify the role of water erosion in total OC and nitrogen (N) loads exported in agricultural discharge from boreal mineral soils under various cultivation practices. Surface water and subsurface drainage were collected near-continually over 2 years in two clayey and one sandy soil in Finland. Eroded sediment was mechanically separated by centrifugation from all discharge samples to detect sediment OC% and N% by dry-combustion method. Dissolved OC and N concentrations in selected discharge samples were measured with high-temperature catalytic oxidation of unfiltered supernatant. A multiple linear regression model was used to study the significant factors affecting dissolved, sediment and total OC loads. In the clayey soils, the sediment OC (2-24 kg ha-1 y-1) and N (0.2-1.1 kg ha-1 y-1) export accounted for up to 35 % and 20 % of the annual discharge-induced total loads of OC (19-85 kg ha-1) and N (2-8 kg ha-1), respectively. In the sandy soil, erosion was negligible and dissolved loads of 17-35 kg OC ha-1 y-1and 4-7 kg N ha-1 y-1 were detected. Subsurface drainage exported most of the sediment-associated OC and N loads from clayey soils. For the total OC loads, the distribution varied between the discharge routes, while the total N loads were mostly exported in subsurface drainage in both soil types. Sediment OC and N exports were related to soil plowing and discharge intensity, while dissolved OC loss was promoted by high surface soil OC%. Our results also indicated that a single cultivation practice may affect sediment and dissolved loads in opposite ways. These findings can be used to complement carbon budget estimations for mineral agricultural soils, and to assess soil management effects on terrestrial organic matter loading to boreal surface waters.

Growing-season carbon budget of alpine meadow ecosystem in the Qinghai Lake Basin: a continued carbon sink through this century according to the Biome-BGC model

BACKGROUND

The alpine meadow is one of the most important ecosystems in the Qinghai-Tibet Plateau (QTP), and critically sensitive to climate change and human activities. Thus, it is crucial to precisely reveal the current state and predict future trends in the carbon budget of the alpine meadow ecosystem. The objective of this study was to explore the applicability of the Biome-BGC model (BBGC) in the Qinghai Lake Basin (QLB), identify the key parameters affecting the variation of net ecosystem exchange (NEE), and further predict the future trends in carbon budget in the QLB.

RESULTS

The alpine meadow mainly acted as carbon sink during the growing season. For the eco-physiological factors, the YEL (Yearday to end litterfall), YSNG (Yearday to start new growth), CLEC (Canopy light extinction coefficient), FRC:LC (New fine root C: new leaf C), SLA (Canopy average specific leaf area), C:Nleaf (C:N of leaves), and FLNR (Fraction of leaf N in Rubisco) were confirmed to be the top seven parameters affecting carbon budget of the alpine meadow. For the meteorological factors, the sensitivity of NEE to precipitation was greater than that to vapor pressure deficit (VPD), and it was greater to radiation than to air temperature. Moreover, the combined effect of two different meteorological factors on NEE was higher than the individual effect of each one. In the future, warming and wetting would enhance the carbon sink capacity of the alpine meadow during the growing season, but extreme warming (over 3.84 ℃) would reduce NEE (about 2.9%) in the SSP5-8.5 scenario.

CONCLUSION

Overall, the alpine meadow ecosystem in the QLB generally performs as a carbon sink at present and in the future. It is of great significance for the achievement of the goal of carbon neutrality and the management of alpine ecosystems.

Mind the gap: reconciling tropical forest carbon flux estimates from earth observation and national reporting requires transparency

BACKGROUND

The application of different approaches calculating the anthropogenic carbon net flux from land, leads to estimates that vary considerably. One reason for these variations is the extent to which approaches consider forest land to be "managed" by humans, and thus contributing to the net anthropogenic flux. Global Earth Observation (EO) datasets characterising spatio-temporal changes in land cover and carbon stocks provide an independent and consistent approach to estimate forest carbon fluxes. These can be compared against results reported in National Greenhouse Gas Inventories (NGHGIs) to support accurate and timely measuring, reporting and verification (MRV). Using Brazil as a primary case study, with additional analysis in Indonesia and Malaysia, we compare a Global EO-based dataset of forest carbon fluxes to results reported in NGHGIs.

RESULTS

Between 2001 and 2020, the EO-derived estimates of all forest-related emissions and removals indicate that Brazil was a net sink of carbon (- 0.2 GtCO2yr-1), while Brazil's NGHGI reported a net carbon source (+ 0.8 GtCO2yr-1). After adjusting the EO estimate to use the Brazilian NGHGI definition of managed forest and other assumptions used in the inventory's methodology, the EO net flux became a source of + 0.6 GtCO2yr-1, comparable to the NGHGI. Remaining discrepancies are due largely to differing carbon removal factors and forest types applied in the two datasets. In Indonesia, the EO and NGHGI net flux estimates were similar (+ 0.6 GtCO2 yr-1), but in Malaysia, they differed in both magnitude and sign (NGHGI: -0.2 GtCO2 yr-1; Global EO: + 0.2 GtCO2 yr-1). Spatially explicit datasets on forest types were not publicly available for analysis from either NGHGI, limiting the possibility of detailed adjustments.

CONCLUSIONS

By adjusting the EO dataset to improve comparability with carbon fluxes estimated for managed forests in the Brazilian NGHGI, initially diverging estimates were largely reconciled and remaining differences can be explained. Despite limited spatial data available for Indonesia and Malaysia, our comparison indicated specific aspects where differing approaches may explain divergence, including uncertainties and inaccuracies. Our study highlights the importance of enhanced transparency, as set out by the Paris Agreement, to enable alignment between different approaches for independent measuring and verification.

Multi-scenario simulation of carbon budget balance in arid and semi-arid regions

The carbon budget has emerged as a central focus in global carbon cycle research. The limited understanding of carbon budget balance dynamics has led to an increasing imbalance between ecological and socio-economic benefits. Building upon a comprehensive analysis of carbon storage and emission in Lanzhou from 2000 to 2020, this study develops a novel deep learning model (CNN-LSTM) to simulate carbon budget under various scenarios from 2030 to 2050. Additionally, scientifically grounded recommendations for carbon compensation are provided. The results demonstrate several key findings: (1) The deep learning model exhibits outstanding performance, with an average overall accuracy exceeding 0.93. The coupled model outperforms individual models, underscoring the significance and necessity of incorporating both temporal and spatial features in land use simulation. (2) Under the ecological protection redline scenario from 2030 to 2050, a noteworthy augmentation in carbon storage and a proficient constraint on carbon emissions are observed. This substantiates the effectiveness of ecological protection interventions. (3) Carbon compensation payment areas are predominantly concentrated in built-up land, with the extent of these areas expanding over time. (4) The disparities in carbon balance effects of forest were more conspicuous than that of built-up land across diverse temporal and scenarios.

Spatial difference of carbon budget and carbon balance zoning based on land use change: a case study of Henan Province, China

Land use change is one of the key reasons for the rise in global carbon emissions. Incorporating practical methods for carbon governance into the major strategic decisions of countries around the world is important for controlling carbon emissions. This study aims to carry out a regional land use carbon budget assessment and build a carbon balance zoning optimization framework. As a result, China will be better able to implement low-carbon strategies and reach carbon peaking and carbon neutrality. Using the data of land use and energy consumption for Henan Province from 2000 to 2020, a carbon budget assessment system was constructed. According to the analysis of the geographical distribution of carbon budget, an evaluation system was developed and a carbon balance partition was established from the natural, economic, ecological and resource structure. A regionally differentiated development strategy was proposed. The findings revealed that: (1) Land use carbon emissions of Henan Province reflected a significant increasing trend, while the variation in carbon absorption of land use was stable. Carbon emissions increased by 87,120.25×104 t in 2020 compared to 2000, but the carbon absorption remained at approximately 1735×104 t over the years and there was an overall state of carbon deficit. (2) The geographical distribution of carbon emissions in Henan Province was characterized by higher in the central part and lower in the surroundings, and the distribution of carbon absorption was higher in the west and lower in the east. The distribution pattern was closely related to the level of land use and the structure of energy consumption. (3) From the carbon balance analysis, the 158 counties in Henan Province were divided into four carbon balance functional areas, namely the carbon sink functional area, low-carbon development area, carbon intensity control area, and high-carbon optimization area. Different optimized development strategies were proposed for each functional area.

Spatial heterogeneity and scenario simulation of carbon budget on provincial scale in China

BACKGROUND

Conducting an extensive study on the spatial heterogeneity of the overall carbon budget and its influencing factors and the decoupling status of carbon emissions from economic development, by undertaking simulation projections under different carbon emission scenarios is crucial for China to achieve its targets to peak carbon emissions by 2030 and to achieve carbon neutrality by 2060. There are large disparities in carbon emissions from energy consumption, the extent of land used for carbon absorption, and the status of decoupling of emissions from economic development, among various regions of China.

RESULTS

Based on night light data and land use data, we investigated carbon budget through model estimation, decoupling analysis, and scenario simulation. The results show that the carbon deficit had a continuous upward trend from 2000 to 2018, and there was a significant positive spatial correlation. The overall status of decoupling first improved and then deteriorated. Altogether, energy consumption intensity, population density of built-up land, and built-up land area influenced the decoupling of carbon emissions from economic development. There are significant scenarios of carbon emissions from energy consumption for the study area during the forecast period, only in the low-carbon scenario will the study area reach the expected carbon emissions peak ahead of schedule in 2027; the peak carbon emissions will be 6479.27 million tons.

CONCLUSIONS

China's provincial-scale carbon emissions show a positive correlation with economic development within the study period. It is necessary to optimize the economic structure, transforming the economic development mode, and formulating policies to control the expansion of built-up land. Efforts must be made to improve technology and promote industrial restructuring, to effectively reduce energy consumption intensity.

Spatialization and driving factors of carbon budget at county level in the Yangtze River Delta of China

The county is the basic administrative unit of China, and the spatialization of carbon budget at the county scale plays an irreplaceable role in deepening the understanding of the carbon emission mechanism and spatial pattern. Yueqing County, an economically developed county in the Yangtze River Delta of China, was selected as the study area, the spatial pattern of the carbon budget and the optimal resolution of the spatialization at the county level were dissected on the basis of accurate accounting, and driving factors of carbon emissions were further identified using the geographically weighted regression model. The results indicated that (1) the carbon emissions were mainly generated from fossil fuel combustion related to energy, accounting for 98.8% of the total carbon budget in the study area; (2) the optimal resolution of spatialization was 200 m and carbon emissions were concentrated in the southeast of the study area; (3) energy intensity, energy structure, per capita GDP, and urbanization rate were positively correlated with carbon emissions, while population played a bidirectional role in carbon emissions. This study not only strengthens the understanding of the patterns and drivers of the carbon budget but also establishes a theoretical framework and operational tools for policymakers to formulate solutions to mitigate the carbon crisis.

A carbon-neutrality-capactiy index for evaluating carbon sink contributions

The accurate determination of the carbon-neutrality capacity (CNC) of a region is crucial for developing policies related to emissions and climate change. However, a systematic diagnostic method for determining the CNC that considers the rock chemical weathering carbon sink (RCS) is lacking. Moreover, it is challenging but indispensable to establish a fast and practical index model to determine the CNC. Here, we selected Guizhou as the study area, used the methods for different types of carbon sinks, and constructed a CNC index (CNCI) model. We found that: (1) the carbonate rock chemical weathering carbon sink flux was 30.3 t CO₂ km^-2 yr^-1. Guizhou accounted for 1.8% of the land area and contributed 5.4% of the carbonate chemical weathering carbon sink; (2) the silicate rock chemical weathering carbon sink and its flux were 1.44 × 10³ t CO₂ and 2.43 t CO₂ km^-2 yr^-1, respectively; (3) the vegetation-soil ecosystem carbon sink and its flux were 1.37 × 10⁸ t CO₂ and 831.70 t CO₂ km^-2 yr^-1, respectively; (4) the carbon emissions (CEs) were 280 Tg CO₂, about 2.8% of the total for China; and (5) the total carbon sinks in Guizhou were 160 Tg CO₂, with a CNCI of 57%, which is 4.8 times of China and 2.1 times of the world. In summary, we conducted a systematic diagnosis of the CNC considering the RCS and established a CNCI model. The results of this study have a strong implication and significance for national and global CNC determination and gap analysis.

Effects of forest management practices on carbon dynamics of China's boreal forests under changing climates

Climate change and forest management practices influence forest productivity and carbon budgets, and understanding their interactions is necessary to develop accurate predictions of carbon dynamics as many countries in the world strive towards carbon neutrality. Here, we developed a model-coupling framework to simulate the carbon dynamics of boreal forests in China. The expected dynamics of forest recovery and change following intense timber harvesting in the recent past and projected carbon dynamics into the future under different climate change scenarios and forest management practices (e.g., restoration, afforestation, tending, and fuel management). We predict that under current management strategies, climate change would lead to increased fire frequency and intensity, eventually shifting these forests from carbon sinks towards being carbon sources. This study suggests that future boreal forest management should be altered to reduce the probability of fire occurrence and carbon losses caused by catastrophic fires through planting deciduous species, mechanical removal, and prescribed fire.

Vast ecosystem disturbance in a warming climate may jeopardize our climate goal of reducing CO2: a case study for megafires in the Australian 'black summer'

A warming climate is one of the most important driving forces of intensified wildfires globally. The unprecedented wildfires broke out in the Australian 'Black Summer' (November 2019-February 2020), which released massive heat, gases, and particles into the atmosphere. The total carbon dioxide (CO₂) emissions from wildfires were estimated at ∼963 million tons by using a top-down approach based on direct satellite measurements of CO₂ and fire radiative power. The fire emissions have led to an approximately 50-80 folds increase in total CO₂ emission in Australia compared with the similar seasons of 2014-2019. The excess CO₂ from wildfires has offset almost half of the global anthropogenic CO₂ emission reductions due to the Corona Virus Disease 2019 in 2020. When the wildfires were intense in December 2019, they caused a 1.48 watts per square meter additional positive radiative forcing above the monthly average in Australia and the vicinity. Our findings demonstrate that vast ecosystem disturbance in a warming climate can strongly influence the global carbon cycle and hamper our climate goal of reducing CO₂.

A new estimation of carbon emissions from land use and land cover change in China over the past 300 years

Scientific estimation of carbon emissions induced by historical land use and land cover change (LUCC) can improve the accuracy of terrestrial ecosystem carbon budget estimates and deepen understanding of the future carbon-sink potential of terrestrial ecosystems. The present study, using historical-document-based data for provincial cropland, forest, and grassland area in China, and experimental-data-based information for provincial vegetation and soil organic carbon density, re-estimates China's LUCC-induced carbon emissions for 1700-1980 using a bookkeeping model in which we updated tabulated functions for carbon losses and gains. The past 300 years have witnessed a dramatic LUCC in China. The cropland area has increased by 67.11 million ha, while the forest and grassland areas have decreased by 127.96 million ha and 16.72 million ha, respectively. Accordingly, the net carbon emissions for 1700-1980 are 6.17-12.35 Pg C, with 8.55 Pg C in the moderate scenario. Among the contributing factors, deforestation was the largest carbon source, accounting for over 90 % of the total carbon emissions. According to our estimates, over 70 % of carbon emissions were caused by harvesting wood, while <30 % were from converting forest and grassland to cropland. Spatially, for the whole period, carbon emissions in southwestern China (Chuan-Yu, Yunnan, and Guangxi), northeastern China (Liaoning, Jilin, and Heilongjiang), and parts of northwestern China (Gan-Ning, Qinghai, and Xinjiang) were as high as 6.03 Pg C, accounting for 70 % of the total carbon emissions. Extending previous studies, we updated the historical LUCC data, carbon density data, and tabulated functions for carbon losses and gains. The estimation results objectively reveal the historical spatiotemporal changes in LUCC-induced emissions.

Carbon footprint of perennial bioenergy crop production receiving various nitrogen fertilization rates

Perennial bioenergy crops can reduce greenhouse gas emissions compared to fossil fuels, but little is known about their C footprints. We evaluated C footprint and C balance of perennial bioenergy crops receiving various N fertilization rates and visually compared them with an annual crop from 2012 to 2014 in the semiarid region of US northern Great Plains. Perennial bioenergy crops were intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth and Dewey, IW), smooth bromegrass (Bromus inermis L., SB), and switchgrass (Panicum virgatum L., SG), and N fertilization rates were 0, 28, 56, and 84 kg N ha^-1. The annual crop was spring wheat (Triticum aestivum L., WH). The CO₂ flux increased in the summer when air temperature and precipitation were greater. Cumulative annual CO₂ flux was greater for SB and SG than IW in 2012-2013 and greater for SB than IW and SG in 2013-2014. Shoot C increased with increased N fertilization rate and was greater for SG than IW and SB at most N fertilization rates in both years. Root and rhizosphere C varied with N fertilization rates and were lower for SG than IW and SB at 0 kg N ha^-1, but greater at 84 kg N ha^-1. Carbon balance also varied with N fertilization rates, being lower for SG than IW and SB at 0 kg N ha^-1, but greater at other N rates. Cumulative CO₂ flux was higher, but shoot, root, and rhizosphere C as well as C balance were lower for WH than perennial bioenergy crops. Because of greater total C input but lower CO₂ flux, SG with N fertilization can be C positive, retaining more C in plant residue and soil than other perennial bioenergy crops. Spring wheat remained C negative compared to perennial bioenergy crops, losing more C as CO₂ flux than total C input.

Mowing mitigated the sensitivity of ecosystem carbon fluxes responses to heat waves in a Eurasian meadow steppe

The heat waves (HW) will be more frequent and intense in the future with increased human activity and uncertain implications for ecosystem carbon fluxes. The semi-arid Eurasian grassland is sensitive to climate change and under frequent HWs attacks. Mowing as one of the most common human practices in this region, combining with HW can have comprehensive effects on plant communities, biomass, and nutrient cycling. Hence, a 3-year (2019-2021) field manipulation experiment was conducted to assess how mowing influenced the carbon cycling under HWs, and the interactions between HWs and mowing on carbon fluxes at the community and ecosystem levels in a Eurasian meadow steppe. Over the three years, HW significantly reduced net ecosystem CO₂ exchange (NEE) and gross ecosystem production (GEP) by 28 % and 8 % (P < 0.05), respectively, whereas ecosystem respiration (Re) did not show significant changes. Moderate mowing (stubble height was set at 6-8 cm) for harvest effectively mitigated ecosystem sensitivity to HWs and significantly increased ecosystem carbon fluxes (NEE, Re, and GEP), biomass and the number of species. Mowing reduced the negative impact of HWs on ecosystem carbon fluxes by about 15 % compared to HWs alone, contributing to the invasion of species such as Thalictrum squarrosum and Vicia amoena, and increased the indirect effect of HW on NEE in the structural equation model. In addition, the higher soil water content (SWC) was another effective way to reduce the impact of HWs. Therefore, mowing and higher SWC would be effective ways to counteract the negative effects of HWs on carbon fluxes in future grassland management.

A study of carbon peaking and carbon neutral pathways in China's power sector under a 1.5 °C temperature control target

The clean and low-carbon transition of China's power sector is of great importance to the achievement of dual carbon targets and the control of global warming. This paper first estimates the remaining carbon budget of the power sector under a 1.5 °C temperature control target and on this basis constructs 1.5 °C and 2 °C power transition scenarios, examining key boundary conditions such as economic development and changes in the cost of power generation technologies. Second, the Genetic Algorithm-Extreme Learning Machine (GA-ELM) model is used to forecast the electricity demand for the next forty years. Finally, with the objective of minimising the total planning cost, a pathway optimisation model of the power system is constructed to explore the optimal transition path for the power system using the dual carbon target, carbon budget and electricity demand as the main constraints. The results of the study show that the carbon budget of the Chinese power sector is approximately 7.1 × 10¹⁰ t CO2 for a 1.5 °C temperature control target. The electricity demand tends to saturate after 2050 and reaches 1.58 × 10¹³ kWh in 2060. The time of the carbon peak and carbon neutralisation in the power sector is 5 years ahead of the double carbon target. By 2060, the power system will be dominated by new energy sources, with the proportion of installed non-fossil energy capacity at over 90% and the proportion of non-fossil energy generation at over 85%. Compared to that under the 2 °C temperature control target, the power sector under the 1.5 °C temperature control target needs to accelerate the pace of the low-carbon transition of electricity and deal with key issues such as the orderly withdrawal of coal power, the construction of a diversified clean energy system and the application of carbon capture devices. This study recommends that the process of building a zero-emissions power sector requires a good pace of the construction of new power systems at a suitable pace, increased efforts to tackle key technologies and improved relevant market mechanisms. China's carbon-neutral pathway in the power sector also has implications for other countries' clean, low-carbon transitions of their power systems.

Determining whether Qinghai-Tibet Plateau waterbodies have acted like carbon sinks or sources over the past 20 years

Half of all of China's lakes are on the Qinghai-Tibet Plateau (QTP), which are mainly distributed at altitudes above 4000 m asl. Being under conditions of progressively intensifying anthropogenic activities and climate change, the debate on whether QTP lakes act as carbon (C) sinks or sources remains unresolved. This study explores QTP lake C exchange processes and characteristics over the past two decades through field monitoring and data integration. Results reveal high lake carbon dioxide (CO₂) exchange flux distribution patterns in its western and southern regions and correspondingly low values in its eastern and northern regions. Lake CO₂ exchange flux rates also show significant temporal differences where those in the 2000s and 2010s were significantly higher compared to the 2020s. Annual total CO₂ emission flux from QTP lakes has increased from 1.60 Tg C a^-1 in the 2000s to 6.87 Tg C a^-1 in the 2010s before decreasing to 1.16 Tg C a^-1 in the 2020s. However, QTP lakes have generally acted as C sinks when annual ice-cover periods are included in the estimation of annual C budgets. Consequently, QTP lakes are gradually evolving towards C sinks. Some small-sized freshwater lakes on the QTP exhibit C sequestration characteristics while low-mid altitude saltwater lakes also act as C sinks. Therefore, owing to the high uncertainties in the estimation of C exchange flux, the QTP lake C sink capacity has been largely underestimated.

Spatial and seasonal dynamics of the methane cycle in a tropical coastal lagoon and its tributary river

Coastal aquatic ecosystems such as estuaries and coastal lagoons are important atmospheric methane sources that must be better constrained. This work presents a detailed characterization of the methane cycle in a tropical coastal lagoon (La Mancha, Veracruz, Mexico) and its tributary river over three distinct seasons, along a transect from the river to the sea connection. In addition to several physicochemical parameters, the dissolved methane, carbon dioxide, and oxygen concentrations were measured with high resolution in the sediments and the water column, combined with production/uptake rates. Methane and carbon dioxide cycles were further constrained by determining atmospheric flux over the entire river and lagoon sections. The results indicate that La Mancha is a highly contrasted ecosystem. The river section is characterized by a strong pycnocline, relatively high methane concentration, and active methanogenesis and methanotrophy, discharging into a relatively homogeneous lagoon section where the methane and carbon cycles are less active. Overall, both the river and the lagoon were a net source of methane and carbon dioxide, with an annual emission of 2.9 metric tons of methane and 2757 metric tons of carbon dioxide. The spatial structure of the main components of the methane, carbon dioxide, and oxygen cycles was established, and it was observed that depthwise heterogeneities predominated in the river section. In contrast, lengthwise heterogeneities dominated in the lagoon section.

Optimization of plant harvest and management patterns to enhance the carbon sink of reclaimed wetland in the Yangtze River estuary

Estuarine wetlands are often located in economically developed and densely populated estuarine deltas, which are frequently disturbed and threatened by human activities. Reclamation, as an important way to alleviate the demand for local land resources, can lead to habitat destruction of natural coastal wetlands and weakening of ecological service functions, including carbon sink capacity. Research has shown that poor plant growth and weakened carbon fixation were the main reasons for the reduced carbon sequestration in a reclaimed wetland. This study aimed to examine the impacts of plant management on the improvement or restoration of carbon sink function in Chongming Dongtan reclaimed wetland, located in the Yangtze River Estuary, China. A management pattern that could effectively enhance the carbon sink function of the reclaimed wetland was selected based on analyses of the effects of different plant harvesting and management patterns (no harvesting, harvesting without returning to the field, direct straw return, and charred straw return) on the plant growth, carbon fixation, and soil respiration, combined with whole-life-cycle carbon footprint evaluation from straw harvest to field return. Compared with no harvesting, the aboveground biomass of direct straw return and charred straw return increased by about 12.3% and 15.5%, respectively (P < 0.05). Simultaneously, straw charring released the least amount of CO₂ (1.94 μmol m^-2 s^-1) and inhibited degradation of soil organic carbon through affecting its microbial community structure. Moreover, considering the carbon budget of different patterns, the charred straw return pattern also most effectively enhanced the carbon sink function and thus could be used for subsequent improvement of carbon sequestration in reclaimed wetlands.

Sustainability assessment and carbon budget of chemical stabilization based multi-objective remediation of Cd contaminated paddy field

The feasibility of chemical stabilization-based strategy for extensive field application is under debate due to lacking a proper framework for its sustainability assessment during its life cycle. Herein, a comprehensive framework consisting of crop production, soil quality, and carbon footprint was constructed for assessing agricultural land remediation based on a two-year paddy field trial. Results show that between the two representative agents, biochar scenario substantially benefits for environmental, social, and agricultural sustainability, because of its more positive impacts on human health and ecosystem, public acceptance, soil reproductive, and rice yield. A notably higher sustainability score of 80.7 for biochar scenario than that of 47.0 for lime is found, in spite of the economical sustainability of lime. The net ecosystem carbon budget of the biochar scenario exhibits an unprecedentedly positive value of 17.8 t CO₂-eq ha^-1, which can finely contribute to a positive carbon budget during remediation. Our finding demonstrates that biochar strategy enables a multi-objective achievement of soil quality - crop production - carbon budget during agricultural land remediation. This study provides new insights into sustainability assessment for restoring agricultural land for safe crop production and synergizing with carbon neutral plan.

The dynamics of the carbon storage and fluxes in Scots pine (Pinus sylvestris) chronosequence

To evaluate the impact of stand age on the ecosystem's C budget, as well as the post-harvest recovery of the C storages and fluxes, a chronosequence of Scots pine stands from the clear-cut stage up to the age of 110 years was studied. An age-related trend of net primary production (NPP) demonstrated effective C accumulation in the young and middle-aged stands and their levelling out thereafter. The understorey vegetation contributed 8-46% to total NPP, being lower in the pole and middle-aged stands, but without a clear age related trend. Annual cumulative soil heterotrophic respiration (Rh) demonstrated stable values along the chronosequence, varying between 3.8 and 5.4 t C ha^-1 yr^-1. The Rh flux of 2.9 t C ha^-1 yr^-1 at the clear-cut site did not exceed the corresponding value for stands. The NEP along the chronosequence followed the dynamics of the annual biomass production of the trees, peaking at the middle-aged stage and decreasing in the older stands; the NPP of the trees was the main driver directing the dynamics of NEP. There was no significant correlation between Rh and dynamics of aboveground litter or fine root production, which can partly explain why no relationship was established between annual Rh and stand age. The total ecosystem C stocks followed the same trend as cumulative tree biomass, peaking in the older stands, however, the soil C stocks varied along the chronosequence irrespective of stand age. The post-harvest C compensation point was reached at the age of 7-years and C payback occurred at a stand age of 11-12 years. Stands acted as C accumulating ecosystems and average annual C accumulation was around 2.5 t C ha^-1 yr^-1, except for the youngest stand and the clear-cut area which acted as C sources. In the oldest stand C budget was almost balanced, with a modest annual accumulation of 0.12 t C ha^-1 yr^-1.

Significance of belowground production to the long-term carbon sequestration of intertidal seagrass beds

The high biomass and sediment features of seagrass beds can make their belowground portions critical sources of blue carbon sinks. However, seagrass belowground production and decomposition have rarely been quantified in the field. To assess the significance of seagrass belowground production to carbon sequestration, belowground carbon budgets were constructed in intertidal seagrass beds of the late-successional species Thalassia hemprichii and the early-successional species Haloduleuninervis in southern Taiwan. For both species, the turnover rates of the belowground portions were much longer than that of the aboveground portion, so the belowground biomass was much higher than the aboveground biomass. The leaf productivity of both species was significantly higher than the belowground productivity, but most of the leaf production decomposed within a year. The lower turnover and slower decomposition rates of the belowground portions allowed the late-successional seagrass T. hemprichii to store more carbon in the sediments than the early-successional seagrass H. uninervis. Long-term changes for the past 20 years in the sediment depth showed that the sediments of seagrass beds were increasing in the habitats at low elevation but were decreasing or had no clear trends in the habitats at high elevation or on the windward side. The carbon storage rates according to the belowground production of T. hemprichii and H. uninervis were 0.3-4.7 and 1.5-2.3 g C m^-2 yr^-1, respectively, which can potentially contribute 53% of the long-term organic carbon storage in the low-elevation sediments.

Quantifying groundwater carbon dioxide and methane fluxes to an urban freshwater lake using radon measurements

Freshwater lakes can play a significant role in greenhouse gas budgets as they can be sources or sinks of carbon to the atmosphere. However, there is limited information on groundwater discharge being a source of carbon to freshwater lakes. Here, we measure CO₂ and CH₄ in the largest urban freshwater lake in the metropolitan area of Sydney (Australia) and quantify groundwater discharge rates into the lake using radon (²²²Rn, a natural groundwater tracer). We also assess the spatial variability of radon, CO₂ and CH₄ in the lake, in addition to surface water and groundwater nutrient and carbon concentrations. Results revealed that the lake system was a source of CO₂ and CH₄ to the atmosphere with fluxes of 113 ± 81 and 0.3 ± 0.1 mmol/m²/d, respectively. These calculated CO₂ fluxes were larger than commonly observed lake fluxes and the global average flux from lakes. However, CH₄ fluxes were lower than the average global value. Based on the radon mass balance model, groundwater discharge to the lake was 16 ± 10 cm/d, which resulted in groundwater-derived CO₂ and CH₄ fluxes contributing 25 and 13% to the overall greenhouse gas emissions from the lake, respectively. Radon, CO₂ and CH₄ maps showed similar spatial distribution trends in the lake and a strong relationship between radon, NO₃ and NH₄ suggested groundwater flow was also a driver of nitrogen into the lake from the western side of the lake, following the general regional groundwater flow. This work provides insights into groundwater and greenhouse gas dynamics in Sydney's largest urban freshwater lake with two implications for carbon budgets: to incorporate urban lakes in global carbon budgets and to account for, the often ignored, groundwater discharge as a source of carbon to lakes.

Threshold, budget and deadline: beyond the discourse of climate scarcity and control

Since its inception, the Intergovernmental Panel on Climate Change (IPCC) has always been at the centre of the global climate debate. Its authoritative reports provide cultural resources for public understanding on the challenge of climate change. While the IPCC maintains its perception as a policy-neutral adviser, the IPCC in practice acts as a powerful discursive agent that guides policy debates in a certain direction by enacting influential scientific concepts. These concepts include three prominent metaphors-temperature threshold, carbon budget and climate deadline-that have been widely circulated across science, policy and advocacy. Three metaphors differ on ways in which the risk of climate change is expressed in terms of space and time. But they all constitute the discourse of climate scarcity-the cognitive view of that we have (too) little space and time to stay below a physical limit for avoiding dangerous climate change. This discursive construction of physical scarcity on climate change has significant political and psychological implications. Politically, the scarcity discourse has the risk of increasing a post-political tendency towards managerial control of the global climate ('scarcity of politics'). Psychologically, however, scarcity has a greater risk of generating a 'scarcity mindset' that inhibits our cognitive capacity to imagine human life beyond managing physical scarcity. Under a narrow mindset of scarcity, the future is closed down to the 'point of no return' that, if crossed, is destined to be the end. To go beyond the scarcity discourse, a new discourse of emancipation has to be fostered. Climate change can be reframed not as a common single destination but as a predicament for actively reimagining human life. Such a narrative can expand our imaginative capacity and animate political action while embracing social losses.

Simulation and evaluation of sustainable climate trajectories for aviation

In 2019, aviation was responsible for 2.6% of world CO2 emissions as well as additional climate impacts such as contrails. Like all industrial sectors, the aviation sector must implement measures to reduce its climate impact. This paper focuses on the simulation and evaluation of climate scenarios for air transport. For this purpose, a specific tool (CAST for "Climate and Aviation - Sustainable Trajectories") has been developed at ISAE-SUPAERO. This tool follows a methodology for the assessment of climate impacts adapted to aviation. Firstly, models for the main levers of action, such as air traffic, aircraft energy consumption and energy decarbonization, are provided using trend projections from historical data or assumptions from the literature. Second, the evaluation of scenarios is based on aviation carbon budgets, which are also extended to non-CO2 effects using the concept of GWP*. Several scenario analyses are performed in this paper using CAST allowing different conclusions to be drawn. For instance, the modelling of the scenarios based on the more recent ATAG (Air Transport Action Group) commitments shows that aviation would consume 6.5% of the world carbon budget for +1.5 °C. Some illustrative scenarios are also proposed. By allocating 2.6% of the world carbon budget to aviation, it is shown that air transport is compatible with a +2 °C trajectory when the annual growth rate of air traffic varies between -1.8% and +2.9%, depending on the technological improvements considered. However, using the same methodology for a +1.5 °C trajectory shows that a drastic decrease in air traffic is necessary. Lastly, analyses including non-CO2 effects emphasize the importance of implementing specific strategies for mitigating contrails.

Regional carbon stock assessment and the potential effects of land cover change

Accurate assessment of carbon stocks remains a global challenge. High levels of uncertainty in Land Use, Land Use Change and Forestry reporting has hindered decision-makers and investors worldwide to support sustainable soil and vegetation management. Potential mitigation-driven activities and effects are likely to be locally/regionally unique. A spatially-targeted approach is thus required to optimise strategic carbon management. This study provides a new regional carbon assessment (tier 3) approach using biophysical-process modelling of high-resolution Land Cover (LC) data within a UK National Park (NFNP) to provide higher accuracy. Future Land Cover Change (LCC) scenarios were simulated. Vegetation-driven carbon dynamics were modelled by coupling two widely-used models, LPJ-GUESS and RothC-26.3. Transition and persistence analysis was conducted using Terrset's Land Change Modeller to predict likely future LCC for 2040 using Multi-Layer Perceptron Markov-Chain Analysis. Current total carbon in the NFNP is 7.32-8.73 Mt C, with current trajectories of LCC leading to minor losses of up to 0.39 Mt C. Alternative LCC scenarios indicated possible gains or losses of 1.27 Mt C, or 136.7 t C ha^-1. The importance of vegetation-driven carbon storage was greater than the national average, with a VegC pool 12-14% of the soil organic C pool, placing greater significance on local/regional LC and management policy. The potential storage capacity of each LC class was ranked (highest to lowest): Coniferous > Broadleaved/Mixed > Coastal > Semi-natural Grassland > Heath > Improved Grassland > Arable (Cropland). Opportunities were prioritised to inform landscape-scale management to reduce future carbon losses and/or to enhance gains through LCC. Balancing the carbon budget relies upon maintaining existing LC. The more detailed LC classification facilitated accounting of management through stock change factors and disaggregation of classes, achieving greater detail and accuracy. Forthcoming policy decisions must optimise carbon storage at a local/regional landscape-scale.

Diurnal variation in BVOC emission and CO2 gas exchange from above- and belowground parts of two coniferous species and their responses to elevated O3

Increased tropospheric ozone (O₃) concentrations in boreal forests affect the emission of biogenic volatile organic compounds (BVOCs), which play crucial roles in biosphere-atmosphere feedbacks. Although it has been well documented that BVOC emissions are altered in response to elevated O₃, consequent effects on the carbon budget have been largely unexplored. Here, we studied the effects of elevated O₃ (80 nmol mol^-1) on diurnal variation of BVOC emissions and gas exchange of CO₂ from above- and belowground parts of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) and further investigated effects on the carbon budget. In spring, elevated O₃ decreased BVOC emissions and net photosynthesis rate (Pn) from above-ground parts of both species. As BVOC emissions have a causal relationship with dormancy recovery, O₃-induced decreases in BVOC emissions indicated the inhibition of dormancy recovery. Contrary to the spring results, in summer BVOC emissions from aboveground parts were increased in response to elevated O₃ in both species. Decreases in Pn indicated O₃ stress. O₃-induced monoterpene emissions from aboveground were the main volatile defense response. Elevated O₃ had little effect on BVOC emissions from belowground parts of either species in spring or summer. In spring, elevated O₃ decreased the proportion of carbon emitted as BVOCs relative to that assimilated by photosynthesis (the proportion of BVOC-C loss) at the soil-plant system levels in both species. In summer, elevated O₃ resulted in a net CO₂-C loss at the soil-plant system level of Scots pine. During this process, O₃-induced BVOC-C loss can represent a significant fraction of carbon exchange between the atmosphere and Scots pine. In Norway spruce, the effects of O₃ were less pronounced. The current results highlight the need for prediction of BVOC emissions and their contributions to the carbon budget in boreal forests under O₃ stress.

Utility of radium quartet for evaluating porewater-derived carbon to a saltmarsh nearshore water: Implications for blue carbon export

Saltmarshes are global hotspots of carbon sequestration and storage and are known as effective blue carbon ecosystems. However, the role of porewater exchange in saltmarshes as a source of carbon to the nearshore waters is still poorly constrained. Herein, we examined the radium quartet, dissolved inorganic (DIC) and organic (DOC) carbon in the porewater and nearshore surface water of Chongming Dongtan saltmarsh, China. Multiple methods based on the radium quartet were applied to estimate the porewater exchange, including the three-endmember model, mass balance model and time series observation. All methods revealed that the porewater exchange rate in Chongming Dongtan saltmarsh equaled 3.37 ± 1.23 cm d^-1. The porewater-derived DIC and DOC fluxes were then estimated to be (1.51 ± 0.64) × 10⁷ and (9.97 ± 6.96) × 10⁵ mol d^-1, respectively, which correspondingly made up 64.6% and 35.6%, of the total inputs into the Chongming Dongtan saltmarsh nearshore water. Considering the intertidal area covered by saltmarsh vegetation, carbon export through the porewater exchange was 3.87 ± 1.55 g C m^-2 d^-1, and was 1.2-fold greater than the carbon burial rate, accounting for approximately 29% of carbon outwelling in Chongming Dongtan saltmarsh. This study highlights the significance of porewater exchange for evaluating carbon sequestration capacity, and suggests that porewater exchange should not be overlooked in blue carbon assessments of saltmarshes.

Faunal mediated carbon export from mangroves in an arid area

The outwelling paradigm argues that mangrove and saltmarsh wetlands export much excess production to downstream marine systems. However, outwelling is difficult to quantify and currently 40-50% of fixed carbon is unaccounted for. Some carbon is thought outwelled through mobile fauna, including fish, which visit and feed on mangrove produce during tidal inundation or early life stages before moving offshore, yet this pathway for carbon outwelling has never been quantified. We studied faunal carbon outwelling in three arid mangroves, where sharp isotopic gradients across the boundary between mangroves and down-stream systems permitted spatial differentiation of source of carbon in animal tissue. Stable isotope analysis (C, N, S) revealed 22-56% of the tissue of tidally migrating fauna was mangrove derived. Estimated consumption rates showed that 1.4% (38 kg C ha^-1 yr^-1) of annual mangrove litter production was directly consumed by migratory fauna, with <1% potentially exported. We predict that the amount of faunally-outwelled carbon is likely to be highly correlated with biomass of migratory fauna. While this may vary globally, the measured migratory fauna biomass in these arid mangroves was within the range of observations for mangroves across diverse biogeographic ranges and environmental settings. Hence, this study provides a generalized prediction of the relatively weak contribution of faunal migration to carbon outwelling from mangroves and the current proposition, that the unaccounted-for 40-50% of mangrove C is exported as dissolved inorganic carbon, remains plausible.

Stabilization of carbon sequestration in a Chinese desert steppe benefits from increased temperatures and from precipitation outside the growing season

The carbon (C) dynamics of desert steppes play an important role in the C budget of temperate steppes. Using the Terrestrial Ecosystem Regional model (TECO-R) model for desert steppes, we examined the dynamics and potential driving mechanisms for C stocks at different temporal and spatial scales from 2000 to 2017 in northern China. The ecosystem C density averaged 2.73 kg C m^-2 and soil organic C accounted for 91.6%. The grassland biome stored 2.85 kg C m^-2, which is higher than the shrub biome (2.19 kg C m^-2). The ecosystem storage increased by an average of 27.75 g C m^-2 yr^-1, with the fastest increase in the southeastern part of the study area. The grassland biome storage increased by an average of 33.54 g C m^-2 yr^-1, versus 25.74 g C m^-2 yr^-1 for the shrub biome. The desert steppe C stock totaled 288.29 Tg C, and increased at 3.09 Tg C yr^-1. An average of >45% of the aboveground biomass was browsed by livestock. The growing season precipitation was significantly positively correlated with changes in the C stock. Increasing temperature was negatively correlated with the C stock, especially for soil carbon. Precipitation was an important driving factor, but warming interacted with precipitation to affect C sequestration during the growing season. Outside the growing season, the increased precipitation and temperature stabilized C sequestration in the desert steppe. This improved understanding of feedbacks between the desert steppe's C cycle and climate will improve predictions of C dynamics in terrestrial ecosystems and of the impacts of climate change.

Spatiotemporal distribution and national measurement of the global carbonate carbon sink

The magnitudes, spatial distributions and contributions to global carbon budget of the global carbonate carbon sink (CCS) still remain uncertain, allowing the problem of national measurement of CCS remain unresolved which will directly influence the fairness of global carbon markets and emission trading. Here, based on high spatiotemporal resolution ecological, meteorological raster data and chemical field monitoring data, combining highly reliable machine learning algorithm with the thermodynamic dissolution equilibrium model, we estimated the new CCS of 0.89 ± 0.23 petagrams of carbon per year (Pg C yr^-1), amounting to 74.50% of global net forest sink and accounting for 28.75% of terrestrial sinks or 46.81% of the missing sink. Our measurement for 142 nations of CCS showed that Russia, Canada, China and the USA contribute over half of the global CCS. We also presented the first global fluxes maps of the CCS with spatial resolution of 0.05°, exhibiting two peaks in equatorial regions (10°S to 10°N) and low latitudes (10°N to 35°N) in Northern Hemisphere. By contrast, there are no peaks in Southern Hemisphere. The greatest average carbon sink flux (CCSF), i.e., 2.12 tC ha^-1 yr^-1, for 2000 to 2014 was contributed by tropical rainforest climate near the equator, and the smallest average CCSF was presented in tropical arid zones, showing a magnitude of 0.26 tC ha^-1 yr^-1. This research estimated the magnitudes, spatial distributions, variations and contributions to the global carbon budget of the CCS in a higher spatiotemporal representativeness and expandability way, which, via multiple mechanisms, introduced an important sink in the terrestrial carbon sink system and the global missing sink and that can help us further reveal and support our understanding of global rock weathering carbon sequestration, terrestrial carbon sink system and global carbon cycle dynamics which make our understanding of global change more comprehensive.

Carbon economy of Mediterranean seagrasses in response to thermal stress

Increased plant mortality in temperate seagrass populations has been recently observed after summer heatwaves, although the underlying causes of plant death are yet unknown. The potential energetic constrains resulting from anomalous thermal events could be the reason that triggered seagrass mortality, as demonstrated for benthic invertebrates. To test this hypothesis, the carbon balance of Posidonia oceanica and Cymodocea nodosa plants from contrasting thermal environments was investigated during a simulated heatwave, by analyzing their photosynthetic performance, carbon balance (ratio photosynthesis:respiration), carbohydrates content, growth and mortality. Both species were able to overcome and recover from the thermal stress produced by the six-week exposure to temperatures 4 °C above mean summer levels, albeit plants from cold waters were more sensitive to warming than plants from warm waters as reflected by their inability to maintain their P:R ratio unaltered. The strategies through which plants tend to preserve their energetic status varied depending on the biology of the species and the thermal origin of plants. These included respiratory homeostasis (P. oceanica warm-plants), carbon diversion from growth to respiration (C. nodosa cold-plants) or storage (P. oceanica warm-plants) and changes in biomass allocation (C. nodosa warm-plants). Findings suggest an important geographic heterogeneity in the overall response of Mediterranean seagrasses to warming with potential negative impacts on the functions and services offered by seagrass meadows including among others their capacity for carbon sequestration and carbon export to adjacent ecosystems.

Spatial and temporal variability of carbon budgets of shallow South African subtropical estuaries

Estuarine carbon fluxes constitute a significant component of coastal CO₂ emissions and nutrients recycling, but high uncertainty is still present due to the heterogeneity of these areas. Although South Africa has nearly 300 estuaries, very little is known about their contribution to carbon emissions or sequestration. This study aims to provide a first estimation of the carbon emissions and nutrient fluxes of South African sub-tropical estuaries through a direct quantification of respiration, primary production and nutrient regeneration of benthic and planktonic communities. In order to account for the extreme variability in subtropical estuarine areas, due to seasonality in rainfall, two estuaries with opposite characteristics were studied; the temporarily open/closed Mdloti Estuary subjected to strong anthropic pressure, and the permanently open Mlalazi Estuary located in a natural reserve. Field deployment of benthic chambers and clear/dark bottles assessed oxygen, ammonia and phosphate fluxes of both benthic and planktonic communities. An inverse pattern between benthic and pelagic primary production was found in both estuaries. Different drivers related to mouth status and sediment characteristics were identified in the two estuaries. The annual average carbon emission indicates that the two systems are heterotrophic over the year releasing substantial CO₂ emissions into the atmosphere. Results show that carbon fluxes in subtropical estuaries are extremely variable in space and time. Future up-scaling carbon estimations need to account for those small scale and regional dynamics.

Geomorphic controls on fluvial carbon exports and emissions from upland swamps in eastern Australia

Temperate Highland Peat Swamps on Sandstone (THPSS) are upland wetlands, similar to fens in the Northern Hemisphere and are found at the headwaters of low-order streams on the plateaus of Eastern Australia. They are classified as endangered ecological communities under State and National legislation. Previous works have identified particular geomorphic characteristics that are important to carbon storage in these low energy sediment accumulation zones. Changes in the geomorphic structure of THPSS, such as channelisation, may have profound implications for carbon storage. To assess the effect of channelisation on carbon budgets in these ecosystems it is essential to identify and quantify differences in carbon export, emissions and stocks of carbon of intact swamps and those that have become channelised. We undertook seasonal sampling of the perched swamp aquifers and surface waters of two intact swamps and two channelised fills in the Blue Mountains of New South Wales, Australia, to investigate differences in carbon exports and emissions between the two swamp types. We found that channelised fills' mean CO₂ emissions were almost four times higher than intact swamps with mean CH₄ emissions up to five times higher. Annual fluvial carbon exports for channelised fills were up to 18 times that of intact swamps. Channelised fill exports and emissions can represent up to 2% of the total swamp carbon stocks per annum which is 40 times higher than the intact swamps. This work clearly demonstrates that changes in geomorphic structure brought about by incision and channelisation results in profound changes to the carbon storage function of THPSS.

Carbon budget of a rainfed spring maize cropland with straw returning on the Loess Plateau, China

Assessing the carbon budget of rainfed agricultural ecosystems is a vital component in the process of estimating the global carbon balance. We used eddy covariance techniques combined with soil respiration measurements to estimate the carbon budget of a rainfed spring maize field where straw returning was practiced, on the Loess Plateau, China, during 2012-2014. Carbon fluxes and their components (except heterotrophic respiration, R_h) exhibited single-peak seasonal patterns, and linear relationships were found between daily gross primary productivity (GPP) and net ecosystem exchange (NEE), and between daily GPP and ecosystem respiration (R_e), with goodness of fit value of 0.96 and 0.85, respectively. The green leaf area index was the most important factor controlling seasonal variations in daily NEE, R_e, and GPP during the growing season, followed by photosynthetically active radiation and air temperature (T_a). Daily R_e was mainly controlled by air temperature during the non-growing season, when R_e accounted for only ~17% of the annual R_e due to winter temperatures. Growing season plant respiration (R_p) was the most important source of carbon emissions from the maize field, with aboveground plant respiration being the major part of R_p. R_h accounted for ~60% of total soil respiration. Only ~60% of the annual GPP was lost as R_e, resulting in an average annual net CO₂ uptake of 509gCm^-2. Taking into account carbon exported (483gCm^-2) and carbon imported (10gCm^-2), the average annual net biome productivity was 37gCm^-2, indicating that the spring maize field with straw returning on the Loess Plateau was a weak carbon sink.

Taking climate, land use, and social economy into estimation of carbon budget in the Guanzhong-Tianshui Economic Region of China

Carbon sequestration is an indispensable ecosystem service provided by soil and vegetation, so mapping and valuing the carbon budget by considering both ecological and social factors is an important trend in evaluating ecosystem services. In this work, we established multiple scenarios to evaluate the impacts of land use change, population growth, carbon emission per capita, and carbon markets on carbon budget. We quantified carbon sinks (aboveground and belowground) under different scenarios, using the Carnegie-Ames-Stanford Approach (CASA) model and an improved carbon cycle process model, and studied carbon sources caused by human activities by analyzing the spatial distribution of human population and carbon emission per capita. We also assessed the net present value (NPV) for carbon budgets under different carbon price and discount rate scenarios using NPV model. Our results indicate that the carbon budget of Guanzhong-Tianshui Economic Region is surplus: Carbon sinks range from 1.50 × 10¹⁰ to 1.54 × 10¹⁰ t, while carbon sources caused by human activities range from 2.76 × 10⁵ to 7.60 × 10⁵ t. And the NPV for carbon deficits range from 3.20 × 10¹¹ RMB to 1.52 × 10¹² RMB. From the perspective of ecological management, deforestation, urban sprawl, population growth, and excessive carbon consumption are considered as the main challenges in balancing carbon sources and sinks. Levying carbon tax would be a considerable option when decision maker develops carbon emission reduction policies. Our results provide a scientific and credible reference for harmonious and sustainable development in the Guanzhong-Tianshui Economic Region of China.

Seasonal exports and drivers of dissolved inorganic and organic carbon, carbon dioxide, methane and δ13C signatures in a subtropical river network

Riverine systems act as important aquatic conduits for carbon transportation between atmospheric, terrestrial and oceanic pools, yet the magnitude of these exports remain poorly constrained. Interconnected creek and river sites (n=28) were sampled on a quarterly basis in three subcatchments of the subtropical Richmond River Catchment (Australia) to investigate spatial and temporal dynamics of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO₂), methane (CH₄), and carbon stable isotope ratios (δ¹³C). The study site is an area of high interest due to potential unconventional gas (coal seam gas or coal bed methane) development. DIC exports were driven by groundwater discharge with a small contribution by in situ DOC remineralization. The DIC exports showed seasonal differences ranging from 0.10 to 0.27mmolm^-2catchmentd^-1 (annual average 0.17mmolm^-2catchmentd^-1) and peaked during winter when surface water discharge was highest. DOC exports (sourced from terrestrial organic matter) had an annual average 0.07mmolm^-2catchmentd^-1 and were 1 to 2 orders of magnitude higher during winter compared to spring and summer. CO₂ evasion rates (annual average of 347mmolm^-2water aread^-1) were ~2.5 fold higher during winter compared to spring. Methane was always supersaturated (0.19 to 62.13μM), resulting from groundwater discharge and stream-bed methanogenesis. Methane evasion was highly variable across the seasons with an annual average of 3.05mmolm^-2water aread^-1. During drier conditions, stable isotopes implied enhanced CH₄ oxidation. Overall, carbon losses from the catchment were dominated by CO₂ evasion (60%) followed by DIC exports (30%), DOC exports (9%) and CH₄ evasion (<1%). Our results demonstrated broad catchment scale spatial and temporal variability in carbon dynamics, and that groundwater discharge and rain events controlled carbon exports.

Constructed wetlands may lower inorganic nutrient inputs but enhance DOC loadings into a drinking water reservoir in North Wales

The objective of this study was to monitor a newly constructed wetland (CW) in north Wales, UK, to assess whether it contributes to an improvement in water quality (nutrient removal) of a nearby drinking water reservoir. Inflow and outflow of the Free Water Surface (FWS) CW were monitored on a weekly basis and over a period of 6 months. Physicochemical parameters including pH, conductivity and dissolved oxygen (DO) were measured, as well as nutrients and dissolved organic and inorganic carbon (DOC, DIC) concentration. The CW was seen to contribute to water quality improvement; results show that nutrient removal took place within weeks after construction. It was found that 72 % of initial nitrate (N03 (-)), 53 % of initial phosphate (PO4 (3-)) and 35 % of initial biological oxygen demand (BOD) were removed, calculated as a total over the whole sampling period. From our study, it can be concluded that while inorganic nutrients do decline in CWs, the DOC outputs increases. This may suggest that CWs represent a source for DOC. To assess the carbon in- and output a C budget was calculated.

Estimating litter carbon stocks on forest land in the United States

Forest ecosystems are the largest terrestrial carbon sink on earth, with more than half of their net primary production moving to the soil via the decomposition of litter biomass. Therefore, changes in the litter carbon (C) pool have important implications for global carbon budgets and carbon emissions reduction targets and negotiations. Litter accounts for an estimated 5% of all forest ecosystem carbon stocks worldwide. Given the cost and time required to measure litter attributes, many of the signatory nations to the United Nations Framework Convention on Climate Change report estimates of litter carbon stocks and stock changes using default values from the Intergovernmental Panel on Climate Change or country-specific models. In the United States, the country-specific model used to predict litter C stocks is sensitive to attributes on each plot in the national forest inventory, but these predictions are not associated with the litter samples collected over the last decade in the national forest inventory. Here we present, for the first time, estimates of litter carbon obtained using more than 5000 field measurements from the national forest inventory of the United States. The field-based estimates mark a 44% reduction (2081±77Tg) in litter carbon stocks nationally when compared to country-specific model predictions reported in previous United Framework Convention on Climate Change submissions. Our work suggests that Intergovernmental Panel on Climate Change defaults and country-specific models used to estimate litter carbon in temperate forest ecosystems may grossly overestimate the contribution of this pool in national carbon budgets.

The carbon budget of a large catchment in the Argentine Pampa plain through hydrochemical modeling

Mar Chiquita is a coastal lagoon located in the Argentine Buenos Aires province in South America. The aim of this study is to estimate the annual contribution of inland waters to the carbon cycle in this lagoon's catchment by estimating the corresponding local carbon budget. Fifteen pairs of water samples were chosen to carry out hydrogeochemical modeling using PHREEQC software. Groundwater samples were considered as recharge water (initial solutions), while streamwater samples were taken as groundwater discharge (final solutions for inverse modeling/reference solutions for direct modeling). Fifteen direct models were performed, where each groundwater sample was constrained to calcite equilibrium under two different carbon dioxide partial pressure (PCO2) conditions: atmospheric conditions (log PCO2 (atm) = -3.5) and a PCO2 value of log PCO2 (atm) = -3. Groundwater samples are close to calcite equilibrium conditions. The calcite precipitation process is kinetically slower than gas diffusion, causing oversaturation of this reactant phase in streamwater samples. This was accompanied by a pH increase of approximately two units due to a PCO2 decrease. From the fifteen inverse models it was estimated that, of the total carbon that enters per year in the hydrological cycle of the study area, about 11.9% is delivered to the atmosphere as CO2 and around 6.7% is buried in sediments. This would indicate that 81.4% of the remaining carbon is retained in equilibrium within the system or discharged into the Mar Chiquita lagoon and/or directly to the ocean through regional flows.

Inferring time-variable effects of nutrient enrichment on marine ecosystems using inverse modelling and ecological network analysis

We combined data from an outdoor mesocosm experiment with carbon budget modelling and an ecological network analysis to assess the effects of continuous nutrient additions on the structural and functional dynamics of a marine planktonic ecosystem. The food web receiving no nutrient additions was fuelled by detritus, as zooplankton consumed 7.2 times more detritus than they consumed algae. Nutrient supply instantly promoted herbivory so that it was comparable to detritivory at the highest nutrient addition rate. Nutrient-induced food web restructuring reduced carbon cycling and decreased the average number of compartments a unit flow of carbon crosses before dissipation. Also, the efficiency of copepod production, the link to higher trophic levels harvestable by man, was lowered up to 35 times by nutrient addition, but showed signs of recovery after 9 to 11 days. The dependency of the food web on exogenous input was not changed by the nutrient additions.