Spatiotemporal distribution and national measurement of the global carbonate carbon sink

Characterization of Microbial Carbon Metabolism in Karst Soils from Citrus Orchards and Analysis of Its Environmental Drivers

Karst regions (KRs) have created significant karst carbon sinks globally through the carbon cycling process involving "water-carbon dioxide-carbonate rock-biota". Soil organic carbon (SOC) represents a crucial component of these carbon sinks. Microorganisms play a vital role in the soil carbon cycle, influencing the formation and preservation of SOC. Therefore, investigating the carbon metabolism of soil microorganisms in KRs is essential for clarifying the unique biogeochemical cycling mechanisms within these regions. In this paper, soils from karst regions (KRs), mixed regions (MRs) and non-karst regions (NKRs) were collected from citrus orchards in Mao Village, Karst Experimental Field, Guilin City, Guangxi Zhuang Autonomous Region, China. The ability to use different carbon sources was analyzed by Biolog-Eco microtiter plate technique; the number of microorganisms was detected by the plate colony counting method, and the microbial biomass was determined by the chloroform fumigation method. The results showed that the soil bacterial number (5.69 ± 0.39 × 106 CFU/g), microbial biomass carbon (MBC) (608.24 ± 63.80 mg/kg), microbial quotient (SMQ) (3.45 ± 0.18%), and Shannon's index (H') (3.28 ± 0.05) of the KR were significantly higher than those of the NKR. The pH showed a significant positive correlation (p < 0.05) with the bacterial number and H' (p < 0.05); SOC showed a highly significant positive correlation with bacterial number (p < 0.01), and a significant positive correlation with MBC, H', and average well change development (AWCD) (p < 0.05). Total nitrogen (TN) showed a significant positive correlation with MBC (p < 0.05); available potassium (AK) showed a significant positive correlation with bacterial number and MBC (p < 0.05). Exchangeable calcium (Ca2+) demonstrated significant positive correlations with bacterial number, MBC, and H' (p < 0.05). The above results indicate that soil bacterial number, carbon metabolic ability and diversity were highest in the KR. pH, SOC and exchangeable Ca2+ were the main influencing factors for the differentiation of soil microbial carbon metabolic diversity between the KR and NKR.

Estimation method for karst carbon sinks on the basis of a concentration prediction model

Karstification can reduce the CO2 concentration in the atmosphere/soil. Accurate estimation of karst carbon sinks is crucial for the study of global climate change. In this study, the Lijiang River Basin was taken as the research area. On the basis of the measured dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentration data from 14 consecutive months, the relationships of DIC and DOC to elevation, slope, aspect, rainfall, and temperature were established. Among six regression algorithms, the random forest (RF), boosted regression tree (BRT) and BP neural network (BP) were selected for stacking integration to construct DIC and DOC concentration prediction models, achieving accuracies of 91% and 83%, respectively. On the basis of these models, the spatial and temporal distributions of DIC and DOC concentrations in the Lijiang River Basin from 2000 to 2022 were predicted. The prediction results reveal that DIC and DOC concentrations have a stable spatial distribution, which is consistent with the lithology distribution in the basin. The solute load method was used to estimate the karst carbon sink in the Lijiang River Basin over 23 years. The carbon sink over 23 years showed an overall growth trend, although with significant fluctuations. On the basis of the estimation results of karst carbon sinks over 23 years, a time series prediction model is used to predict the Lijiang River Basin from 2023 to 2030. The prediction results continue the volatility and trend of the historical data. A comparison of the model verification results with related research findings revealed that the concentration prediction model constructed in this study has high accuracy and good applicability in the estimation of karst carbon sinks at the watershed scale.

Inorganic carbon migration and transformation in groundwater evaporation discharge area

During groundwater evaporation discharge, a series of carbon-related water-rock interactions potentially impact the terrestrial carbon cycle significantly. However, the migration and transformation of carbon in groundwater evaporation discharge area remain inadequately understood. Using the Tumochuan Plain in Inner Mongolia as a case study, this paper constructs a carbon balance equation for groundwater evaporation discharge area by employing mass balance principles and hydrogeochemical simulation methods, thereby analyzing the mechanisms of carbon diversion during groundwater evaporation. The result showed that evaporation discharge area of Tumochuan Plain was a 'carbon sink'. Carbon emission rate to atmosphere in study area was 7.35 g/(m2·a), while carbon fixation rate by calcite precipitation and dissolved inorganic carbon (DIC) into groundwater was 37.15 g/(m2·a). The precipitation of calcite and the dissolution of dolomite were the main water-rock interactions controlling the migration and transformation of DIC. The carbon absorbed by dolomite dissolution reached 21,698.02 t/a (30.56 g/(m2·a)), offsetting a significant portion of the CO2 emitted during calcite precipitation. In addition, the calcium released by the dissolution of dolomite and anorthite effectively promoted the precipitation of calcite, which was the primary factor for groundwater to become a carbon sink in this area.

Aquatic photosynthetic carbon pump driven by periodic temperature variations affects dissolved inorganic carbon and hydrochemical characteristics in a groundwater-fed reservoir

Under the influence of periodic temperature variations, biogeochemical cycling in water bodies is markedly affected by the periodic thermal stratification processes in subtropical reservoirs or lakes. In current studies, there is insufficient research on the influence and mechanism of dissolved inorganic carbon (DIC) distribution in karst carbon-rich groundwater-fed reservoirs under the coupled effects of thermal structure stratification and the biological carbon pump (BCP) effect. To address this issue, the Dalongdong (DLD) reservoir in the subtropical region of southern China was chosen as the site for long-term monitoring and research on relevant physicochemical parameters of water, DIC, and its stable carbon isotope (δ13CDIC), CO2 emission flux, as well as the reservoir's thermal stratification index. The results show that: (1) the DLD reservoir is a typical warm monomictic reservoir, which exhibits regular variations of mixing period-stratification period-mixing period on a yearly scale due to thermal structure changes; (2) DIC was consumed by aquatic photosynthetic organisms in the epilimnion during the stratification period, leading to a decrease in DIC concentration, partial pressure of CO2 (pCO2) and CO2 emission flux, and an increase in stable carbon isotope (δ13CDIC). During the mixing period, the trend was reversed; (3) During the thermal stratification, aquatic photosynthesis and water temperature were the primary factors controlling DIC variations in both the epilimnion and thermocline. Regarding the hypolimnion, calcite dissolution, organic matter decomposition, and water temperature were the dominant controlling factors. These results indicate that although carbon-rich karst groundwater provides a plentiful supply of DIC in the DLD reservoir, its availability is still influenced by variations in the reservoir's thermal structure and the metabolic processes of aquatic photosynthetic organisms. Therefore, to better estimate the regional carbon budget in a reservoir or lake, future studies should especially consider the combined effects of BCP and thermal structure variations on carbon variations.

Abiotic processes control carbon dioxide dynamics in temperate karst lakes

Inland waters are crucial in the carbon cycle, contributing significantly to the global CO2 fluxes. Carbonate lakes may act as both sources and sinks of CO2 depending on the interactions between the amount of dissolved inorganic carbon (DIC) inputs, lake metabolisms, and geochemical processes. It is often difficult to distinguish the dominant mechanisms driving CO2 dynamics and their effects on CO2 emissions. This study was undertaken in three groundwater-fed carbonate-rich lakes in central Spain (Ruidera Lakes), severely polluted with nitrates from agricultural overfertilization. Diel and seasonal (summer and winter) changes in CO2 concentration (CCO2) DIC, and CO2 emissions-(FCO2)-, as well as physical and chemical variables, including primary production and phytoplanktonic chlorophyll-a were measured. In addition, δ13C-DIC, δ13C-CO2 in lake waters, and δ13C of the sedimentary organic matter were measured seasonally to identify the primary CO2 sources and processes. While the lakes were consistently CCO2 supersaturated and FCO2 was released to the atmosphere during both seasons, the highest CCO2 and DIC were in summer (0.36-2.26 µmol L-1). Our results support a strong phosphorus limitation for primary production in these lakes, which impinges on CO2 dynamics. External DIC inputs to the lake waters primarily drive the CCO2 and, therefore, the FCO2. The δ13C-DIC signatures below -12‰  confirmed the primary geogenic influence on DIC. As also suggested by the high values on the calcite saturation index, the Miller-Tans plot revealed that the CO2 source in the lakes was close to the signature provided by the fractionation of δ13C-CO2 from calcite precipitation. Therefore, the main contribution behind the CCO2 values found in these karst lakes should be attributed to the calcite precipitation process, which is temperature-dependent according to the seasonal change observed in δ13C-DIC values. Finally, co-precipitation of phosphate with calcite could partly explain the observed low phytoplankton production in these lakes and the impact on the contribution to increasing greenhouse gas emissions. However, as eutrophication increases and the soluble reactive phosphorus (SRP) content increases, the co-precipitation of phosphate is expected to be progressively inhibited. These thresholds must be assessed to understand how the CO32- ions drive lake co-precipitation dynamics. Carbonate regions extend over 15% of the Earth's surface but seem essential in the CO2 dynamics at a global scale.

The fusion of multiple scale data indicates that the carbon sink function of the Qinghai-Tibet Plateau is substantial

BACKGROUND

The Qinghai-Tibet Plateau is the "sensitive area" of climate change, and also the "driver" and "amplifier" of global change. The response and feedback of its carbon dynamics to climate change will significantly affect the content of greenhouse gases in the atmosphere. However, due to the unique geographical environment characteristics of the Qinghai-Tibet Plateau, there is still much controversy about its carbon source and sink estimation results. This study designed a new algorithm based on machine learning to improve the accuracy of carbon source and sink estimation by integrating multiple scale carbon input (net primary productivity, NPP) and output (soil heterotrophic respiration, Rh) information from remote sensing and ground observations. Then, we compared spatial patterns of NPP and Rh derived from the fusion of multiple scale data with other widely used products and tried to quantify the differences and uncertainties of carbon sink simulation at a regional scale.

RESULTS

Our results indicate that although global warming has potentially increased the Rh of the Qinghai-Tibet Plateau, it will also increase its NPP, and its current performance is a net carbon sink area (carbon sink amount is 22.3 Tg C/year). Comparative analysis with other data products shows that CASA, GLOPEM, and MODIS products based on remote sensing underestimate the carbon input of the Qinghai-Tibet Plateau (30-70%), which is the main reason for the severe underestimation of the carbon sink level of the Qinghai-Tibet Plateau (even considered as a carbon source).

CONCLUSIONS

The estimation of the carbon sink in the Qinghai-Tibet Plateau is of great significance for ensuring its ecological barrier function. It can deepen the community's understanding of the response to climate change in sensitive areas of the plateau. This study can provide an essential basis for assessing the uncertainty of carbon sources and sinks in the Qinghai-Tibet Plateau, and also provide a scientific reference for helping China achieve "carbon neutrality" by 2060.

Ecological differentiation and assembly processes of abundant and rare bacterial subcommunities in karst groundwater

The ecological health of karst groundwater has been of global concern due to increasing anthropogenic activities. Bacteria comprising a few abundant taxa (AT) and plentiful rare taxa (RT) play essential roles in maintaining ecosystem stability, yet limited information is known about their ecological differentiation and assembly processes in karst groundwater. Based on a metabarcoding analysis of 64 groundwater samples from typical karst regions in southwest China, we revealed the environmental drivers, ecological roles, and assembly mechanisms of abundant and rare bacterial communities. We found a relatively high abundance of potential functional groups associated with parasites and pathogens in karst groundwater, which might be linked to the frequent regional anthropogenic activities. Our study confirmed that AT was dominated by Proteobacteria and Campilobacterota, while Patescibacteria and Chloroflexi flourished more in the RT subcommunity. The node-level topological features of the co-occurrence network indicated that AT might share similar niches and play more important roles in maintaining bacterial community stability. RT in karst groundwater was less environmentally constrained and showed a wider environmental threshold response to various environmental factors than AT. Deterministic processes, especially homogeneous selection, tended to be more important in the community assembly of AT, whereas the community assembly of RT was mainly controlled by stochastic processes. This study expanded our knowledge of the karst groundwater microbiome and was of great significance to the assessment of ecological stability and drinking water safety in karst regions.

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.

High-resolution mapping of the global silicate weathering carbon sink and its long-term changes

Climatic and non-climatic factors affect the chemical weathering of silicate rocks, which in turn affects the CO₂ concentration in the atmosphere on a long-term scale. However, the coupling effects of these factors prevent us from clearly understanding of the global weathering carbon sink of silicate rocks. Here, using the improved first-order model with correlated factors and non-parametric methods, we produced spatiotemporal data sets (0.25° × 0.25°) of the global silicate weathering carbon-sink flux (SCSF_α ) under different scenarios (SSPs) in present (1950-2014) and future (2015-2100) periods based on the Global River Chemistry Database and CMIP6 data sets. Then, we analyzed and identified the key regions in space where climatic and non-climatic factors affect the SCSF_α . We found that the total SCSF_α was 155.80 ± 90 Tg C yr^-1 in present period, which was expected to increase by 18.90 ± 11 Tg C yr^-1 (12.13%) by the end of this century. Although the SCSF_α in more than half of the world was showing an upward trend, about 43% of the regions were still showing a clear downward trend, especially under the SSP2-4.5 scenario. Among the main factors related to this, the relative contribution rate of runoff to the global SCSF_α was close to 1/3 (32.11%), and the main control regions of runoff and precipitation factors in space accounted for about 49% of the area. There was a significant negative partial correlation between leaf area index and silicate weathering carbon sink flux due to the difference between the vegetation types. We have emphasized quantitative analysis the sensitivity of SCSF_α to critical factors on a spatial grid scale, which is valuable for understanding the role of silicate chemical weathering in the global carbon cycle.

Spatiotemporal Variations of Forest Vegetation Phenology and Its Response to Climate Change in Northeast China

Vegetation phenology is an important indicator of vegetation dynamics. The boreal forest ecosystem is the main part of terrestrial ecosystem in the Northern Hemisphere and plays an important role in global carbon balance. In this study, the dynamic threshold method combined with the ground-based phenology observation data was applied to extract the forest phenological parameters from MODIS NDVI time-series. Then, the spatiotemporal variation of forest phenology is discussed and the relationship between phenological change and climatic factors was concluded in the northeast China from 2011 to 2020. The results indicated that the distribution of the optimal extraction threshold has spatial heterogeneity, and the changing rate was 3% and 2% with 1° increase in latitude for SOS (the start of the growing season) and EOS (the end of the growing season). This research also notes that the SOS had an advanced trend at a rate of 0.29 d/a while the EOS was delayed by 0.47 d/a. This variation of phenology varied from different forest types. We also found that the preseason temperature played a major role in effecting the forest phenology. The temperature in winter of the previous year had a significant effect on SOS in current year. Temperature in autumn of the current year had a significant effect on EOS.

Where Anthropogenic Activity Occurs, Anthropogenic Activity Dominates Vegetation Net Primary Productivity Change

Anthropogenic activities and climate change affect the type, structure and function of ecosystems, resulting in important changes in vegetation net primary productivity (NPP). Therefore, in this study we used the vegetation photosynthesis model (VPM) to reveal the spatiotemporal variations in NPP in Xinjiang from 2000 to 2019. The impacts of climate change and anthropogenic activities on NPP changes were quantified and separated by the residual analysis-control variables (RES-CON) method. The results showed that the average NPP in Xinjiang increased by 17.77% from 2000 to 2019. Anthropogenic activities and climate change generally had a positive impact on NPP from 2000 to 2019. The most important anthropogenic activity was land use and land cover (LULC) transformation from grass to arable land, which significantly increased vegetation productivity. Regarding climate change, precipitation has played a significant role in promoting the productivity of vegetation. Overall, the average contribution of climate change (temperature and precipitation) to NPP variation (21.44%) is much greater than the contribution of anthropogenic activities (3.46%), but in areas where anthropogenic activities occur, the average contribution of anthropogenic activities to NPP variation (75.01%) is much greater than the average contribution of climate change (15.53%). Where there are no anthropogenic activities, the average contribution of climate change to NPP variation is 21.72%. In summary, anthropogenic activities are the main driver of NPP variation in areas where anthropogenic activities occur, while the total area in Xinjiang where climate change is the most important driver is larger than the total area where anthropogenic activities are the dominant driver.

The responses of weathering carbon sink to eco-hydrological processes in global rocks

Eco-hydrological processes affect the chemical weathering carbon sink (CS) of rocks. However, due to data quality limitations, the magnitude of the CS of rocks and their responses to eco-hydrological processes are not accurately understood. Therefore, based on Global Erosion Model for CO₂ fluxes (GEM-CO₂ model), hydrological site data, and multi-source remote sensing data, we produced a 0.05° × 0.05° resolution dataset of CS for 11 types of rocks from 2001 to 2018. The results show that the total amount of CS of global rocks is 0.32 ± 0.02 Pg C, with an average flux of 2.7 t C km^-2 yr^-1, accounting for 53% and 3% of the "missing" carbon sink and fossil fuel emissions, respectively. This is 23% higher than previous research results, which may be due to the increased resolution. Although about 60% of the CS of global rocks are in a stable state, there are obvious differences among rocks. For example, the CS of carbonate rocks exhibited a significant increase (0.30 Tg C/yr), while the CS of siliceous clastic sedimentary rocks exhibited a significant decrease (-0.06 Tg C/yr). Although temperature is an important factor affecting the CS, the proportion of soil moisture in arid and temperate climate zones is higher (accounting for 24%), which is 3.6 times that of temperature. Simulations based on representative concentration pathways scenarios indicate that the global CS of rocks may increase by about 28% from 2050 to 2100. In short, we produced a set of high-resolution datasets for the CS of global rocks, which makes up for the lack of datasets in previous studies and improves our understanding of the magnitude and spatial pattern of the CS and its responses to eco-hydrological processes.

Characterizing Spatiotemporal Variations of Soil Salinization and Its Relationship with Eco-Hydrological Parameters at the Regional Scale in the Kashi Area of Xinjiang, China from 2000 to 2017

Soil salinization is one of the most serious issues of land degradation, especially in inland drylands, such as the Kashgar region in the Xinjiang province, western China. The investigation of the spatiotemporal variations of soil salinization and its causes is critical for regional ecological restoration and social development. In this study, salinization severity was firstly interpreted in Kashgar region for the years 2000, 2010, and 2017 using multitemporal Landsat images, and the spatiotemporal variations of salinized soil area, salinization severity index, and important index of salinization change were then analyzed using transition matrix method. Finally, the relationship between salinization and eco-hydrological parameters at the regional scale was investigated using correlation analysis and multivariate linear regression. The results show that salinized soil is mainly concentrated in irrigated oasis areas. Although the decrease rate of total salinized soil area is decreasing, the decrease rate of average salinization severity is increasing gradually. There is an increasing trend for the improved area of salinized soil, whereas an opposite trend was observed for the deteriorated area of salinized soil. The conversion from extremely severe salinized soil to the severe ones was the dominant transforming type from 2000 to 2017; meanwhile, the transformation from non-salinized soil to salinized soil for the newly reclaimed farmland was observed, indicating that some necessary irrigation control measures must be taken to avoid further soil salinizing. A significant negative correlation between salinization severity and evapotranspiration, normalized difference vegetation index (NDVI) was observed, implying that soil structure change induced by vegetation, associated with high evapotranspiration (ET) and low land surface temperature (LST), played a positive role in alleviating soil salinization in this region. It is concluded that the soil salinization had been alleviated from 2000 to 2017, mainly due to the combined effects of the farmland expansion and the reasonable irrigation system.

Change Detection of Soil Formation Rate in Space and Time Based on Multi Source Data and Geospatial Analysis Techniques

Spatialization of soil formation rate (SFR) is always a difficult problem in soil genesis. In this study, the dissolution rate in karst areas of China during the period 1983–2015 was estimated on the basis of geospatial analysis techniques and detection of variation via the law of chemical thermodynamics in conjunction with long-term serial ecohydrology data. SFR at different lithological backgrounds was calculated on the basis of the content of acid-insoluble substances. Results showed that the spatial dissolution rate of carbonate rock ranges between 0 and 106 mm/ka, averaged at 22.51 mm/ka, and the SFR ranges between 10 and 134.93 t km−2 yr−1, averaged at 18.59 t km−2 yr−1. The dissolution rate and SFR exhibit a slight increasing trend with 0.04 mm/ka and 0.003 t km−2 yr−1, respectively. The risk for soil erosion was reevaluated on the basis of the SFR results, and the area with erosion risk and the ecologically safe area were corrected. Results indicated that the area with erosion risk is four times higher than the ecologically safe area. This study will hopefully instigate and facilitate the application and popularization of geospatial analysis technology to the research field of rock weathering and soil formation.