Carbon Cycle Research Articles

Enhancing deadwood reporting for forest ecosystems: Bridge equations to convert deadwood measured at any diameter threshold to reference diameters

National as well as international requirements have led to an increased need to quantify deadwood stocks in forest ecosystems given their important role not only in terms of carbon storage and regulation of the carbon cycle but also as biodiversity refugia. However, differences in definitions and field monitoring as well as gaps in existing data on deadwood mean that comparisons among countries and retrospective analyses are difficult. In this research, we propose two potential approaches to solve the most common gaps in forest deadwood monitoring. First, we develop bridging functions capable of converting deadwood measurements with a specific reference diameter to 7.5 cm (minimum diameter value in Spain) and 10.0 cm (the most common minimum value for international statistics) diameters for the main forest types while also addressing the effect of raising the minimum measurable size on the quantification of deadwood. Furthermore, we aim to calculate the ratios between the amount of standing deadwood, the most common indicator monitored in National Forest Inventories, and the entire deadwood pool as a proxy for estimating complete deadwood stocks when data are not available. For this objective, we use information obtained from the Spanish National Forest Inventory, linear models and 10-fold cross-validation. We estimate the percentage of deadwood omitted when the minimum deadwood size is increased for the main eight forest types in Spain as well as for the entire country, using two different approaches. The ratio between the amount of standing deadwood and the entire deadwood pool ranged between 0.14 and 0.45 depending on the forest type. The lowest values of this ratio were found in Open woodlands and the largest in Mediterranean conifers. The validation statistics (R2 ranging from 0.82 in Evergreen broadleaves to 0.97 in Macaronesian broadleaves) indicate that the bridging functions we propose are robust and accurate. However, the ratios between the amount of standing deadwood and the entire deadwood pool performed poorer (R2 ranging from 0.26 in Macaronesian conifers to 0.65 in Macaronesian broadleaves) and led to an overestimation of the total stocks. Our results are of value not only for the purposes of comparison and harmonization but also for the implementation of new forest monitoring systems.

How will permafrost carbon respond to future climate change? A new assessment for future thaw trends of permafrost carbon on the Tibetan Plateau

Permafrost degradation on the Tibetan Plateau (TP) is anticipated to result in the thaw of permafrost carbon. Existing studies have been conducted to assess the future thaw of frozen carbon on the TP, primarily focusing on the deepening of the active layer while neglecting the impact of permafrost area shrinkage. This oversight may lead to a significant underestimation of the potential thaw of frozen carbon. Our research underscores the pivotal role of permafrost area shrinkage in estimating the future thaw of frozen carbon. Our findings reveal that when the combined effects of permafrost area shrinkage and active layer deepening are considered, the thaw rates of frozen carbon in various radiative forcing scenarios are nearly four times those based on active layer deepening alone. Notably, our results demonstrate substantial thaw of frozen organic carbon in the TP permafrost area under all four future scenarios: In the low radiative forcing scenario SSP1-2.6, it is predicted that 55.4 % of the organic carbon in the permafrost area 0–10 m soils will be in a state of thaw by 2100, and more than 90 % in the high radiative forcing scenario SSP5-8.5. This substantial thaw is poised to diminish the TP's current carbon sink function significantly. Our study emphasizes that as global warming persists, frozen carbon in permafrost areas will play a more active role in global carbon cycle processes in the future. Furthermore, we stress the importance of considering permafrost area shrinkage in understanding the thaw of frozen carbon, providing valuable insights for carbon balance studies on the TP.

Rhodobacteraceae are key players in microbiome assembly of the diatom Asterionellopsis glacialis.

Most, if not all, microeukaryotic organisms harbor an associated microbial community, termed the microbiome. The microscale interactions that occur between these partners have global-scale consequences, influencing marine primary productivity, carbon cycling, and harmful algal blooms to name but a few. Over the last decade, there has been a growing interest in the study of phytoplankton microbiomes, particularly within the context of bloom dynamics. However, long-standing questions remain regarding the process of phytoplankton microbiome assembly. The significance of our research is to tease apart the mechanism of microbiome assembly with a particular focus on identifying bacterial taxa, which may not merely be symbionts but architects of the phytoplankton microbiome. Our results strengthen the understanding of the ecological mechanisms that underpin phytoplankton-bacteria interactions in order to accurately predict marine ecosystem responses to environmental perturbations.

Challenges in Standardizing Global Emission Factors for Peatlands

Peatlands have a crucial role in the global carbon cycle, acting as significant carbon sinks, but become a significant source of greenhouse gas (GHG) emissions when peat extraction is taking place. This article presents a comprehensive overview of peatland ecosystems, emphasizing their classification across various climatic zones and the complex set of different characteristics that determine contribution to GHG emissions. Currently, inconsistency exists in definition of emission factors used between countries leading to varied approaches in estimating peatland emissions and posing significant challenges in the comparison and aggregation of global data on peat extraction related GHG. The aim of the study is to analyse the disparities in emission factors and calculation methodologies employed by different countries. Data from national GHG emission reports are submitted under the UNFCCC and the Kyoto Protocol. Emissions report data calculations and emission factors can be based either on nationally determined data or on data specified in the IPCC guidelines. Consequently, emission factor data for four countries - Latvia, Finland, Sweden and Germany - are collected and processed, which were compared with IPCC guideline data. The data were compared in two ways: by equating units of measurement and by modeling. The results show there is a pronounced difference between the emission factors of each country, however, all these factors are lower than the maximum values specified in the IPCC guidelines. It was also determined that comparing the total emission factors with the modeled results, no significant difference is observed between these results. The study concludes that emission factors are predetermined differently for each country, and it is not possible to specify the differences among assumptions for parameters included in the determination of the emission factors. The results suggest there is a need for development of more transparent accounting of emissions with regard to the diverse environmental and anthropogenic factors influencing peatland ecosystems. Factors like composition, depth of peat, water table levels, and local land-use practices further compound this variability in emission accounting. Addressing these challenges is crucial for enhancing the accuracy and reliability of GHG emission reporting under international frameworks like the United Nations Framework Convention on Climate Change and the Kyoto Protocol.

The novel regulator HdrR controls the transcription of the heterodisulfide reductase operon hdrBCA in Methanosarcina barkeri.

Methanogenic archaea play a key role in the global carbon cycle because these microorganisms remineralize organic compounds in various anaerobic environments. The microorganism Methanosarcina barkeri is a metabolically versatile methanogen, which can utilize acetate, methanol, and H2/CO2 to synthesize methane. However, the regulatory mechanisms underlying methanogenesis for different substrates remain unknown. In this study, RNA-seq analysis was used to investigate M. barkeri growth and gene transcription under different substrate regimes. According to the results, M. barkeri showed the best growth under methanol, followed by H2/CO2 and acetate, and these findings corresponded well with the observed variations in genes transcription abundance for different substrates. In addition, we identified a novel regulator, MSBRM_RS03855 (designated as HdrR), which specifically activates the transcription of the heterodisulfide reductase hdrBCA operon in M. barkeri. HdrR was able to bind to the hdrBCA operon promoter to regulate transcription. Furthermore, the structural model analyses revealed a helix-turn-helix domain, which is likely involved in DNA binding. Taken together, HdrR serves as a model to reveal how certain regulatory factors control the expression of key enzymes in the methanogenic pathway.IMPORTANCEThe microorganism Methanosarcina barkeri has a pivotal role in the global carbon cycle and contributes to global temperature homeostasis. The consequences of biological methanogenesis are far-reaching, including impacts on atmospheric methane and CO2 concentrations, agriculture, energy production, waste treatment, and human health. As such, reducing methane emissions is crucial to meeting set climate goals. The methanogenic activity of certain microorganisms can be drastically reduced by inhibiting the transcription of the hdrBCA operon, which encodes heterodisulfide reductases. Here, we provide novel insight into the mechanisms regulating hdrBCA operon transcription in the model methanogen M. barkeri. The results clarified that HdrR serves as a regulator of heterodisulfide reductase hdrBCA operon transcription during methanogenesis, which expands our understanding of the unique regulatory mechanisms that govern methanogenesis. The findings presented in this study can further our understanding of how genetic regulation can effectively reduce the methane emissions caused by methanogens.

Genomic and phenotypic characterization of 26 novel marine bacterial strains with relevant biogeochemical roles and widespread presence across the global ocean

Prokaryotes dominate global oceans and shape biogeochemical cycles, yet most taxa remain uncultured and uncharacterized as of today. Here we present the characterization of 26 novel marine bacterial strains from a large isolate collection obtained from Blanes Bay (NW Mediterranean) microcosm experiments made in the four seasons. Morphological, cultural, biochemical, physiological, nutritional, genomic, and phylogenomic analyses were used to characterize and phylogenetically place the novel isolates. The strains represent 23 novel bacterial species and six novel genera: three novel species pertaining to class Alphaproteobacteria (families Rhodobacteraceae and Sphingomonadaceae), six novel species and three new genera from class Gammaproteobacteria (families Algiphilaceae, Salinispheraceae, and Alteromonadaceae), 13 novel species and three novel genera from class Bacteroidia (family Flavobacteriaceae), and one new species from class Rhodothermia (family Rubricoccaceae). The bacteria described here have potentially relevant roles in the cycles of carbon (e.g., carbon fixation or energy production via proteorhodopsin), nitrogen (e.g., denitrification or use of urea), sulfur (oxidation of sulfur compounds), phosphorus (acquisition and use of different forms of phosphorus and remodeling of membrane phospholipids), and hydrogen (oxidation of hydrogen to obtain energy). We mapped the genomes of the presented strains to the Tara Oceans metagenomes to reveal that these strains were globally distributed, with those of the family Flavobacteriaceae being the most widespread and abundant, while Rhodothermia being the rarest and most localized. While molecular-only approaches are also important, our study stresses the importance of culturing as a powerful tool to further understand the functioning of marine bacterial communities.

Microbial polyphenol metabolism is part of the thawing permafrost carbon cycle.

With rising global temperatures, permafrost carbon stores are vulnerable to microbial degradation. The enzyme latch theory states that polyphenols should accumulate in saturated peatlands due to diminished phenol oxidase activity, inhibiting resident microbes and promoting carbon stabilization. Pairing microbiome and geochemical measurements along a permafrost thaw-induced saturation gradient in Stordalen Mire, a model Arctic peatland, we confirmed a negative relationship between phenol oxidase expression and saturation but failed to support other trends predicted by the enzyme latch. To inventory alternative polyphenol removal strategies, we built CAMPER, a gene annotation tool leveraging polyphenol enzyme knowledge gleaned across microbial ecosystems. Applying CAMPER to genome-resolved metatranscriptomes, we identified genes for diverse polyphenol-active enzymes expressed by various microbial lineages under a range of redox conditions. This shifts the paradigm that polyphenols stabilize carbon in saturated soils and highlights the need to consider both oxic and anoxic polyphenol metabolisms to understand carbon cycling in changing ecosystems.

Microorganisms Directly Affected Sediment Carbon–Nitrogen Coupling in Two Constructed Wetlands

Clarifying the carbon–nitrogen coupling pattern in wetlands is crucial for understanding the driving mechanism of wetland carbon sequestration. However, the impacts of plants and environmental factors on the coupling of carbon–nitrogen in wetland sediments are still unclear. Sediment samples from plant (Typha angustifolia and Phragmites australis)-covered habitats and bare land were collected in two constructed wetlands in northern China. The contents of different forms of carbon and nitrogen in sediments and plants, and the sediment microbial community were detected. It was found that the sediment carbon to nitrogen (C/N) ratios did not differ significantly in the bare sites of different wetlands, but did in the plant-covered sites, which highlighted the different role of plants in shifting the carbon–nitrogen coupling in different constructed wetlands. The effects of plants on the sediment carbon–nitrogen coupling differed in two constructed wetlands, so the structural equation model was used and found that sediment microorganisms directly affected sediment C/N ratios, while water and sediment physicochemical properties indirectly affected sediment C/N ratios by altering sediment microbial functions. Multiple linear regression models showed that water pH, sediment moisture content, water dissolved oxygen, and water depth had a greater influence on the carbon metabolism potential of the sediment microbial community, while sediment moisture content had the greatest impact on the sediment microbial nitrogen metabolism potential. The study indicates that variations in environmental conditions could alter the influence of plants on the carbon and nitrogen cycles of wetland sediments. Water environmental factors mainly affect microbial carbon metabolism functions, while soil physicochemical factors, especially water content, affect microbial carbon and nitrogen metabolism functions.

Response of vegetation phenology to climate factors in the source region of the Yangtze and Yellow Rivers

Abstract Exploring the impact of climate factors on vegetation phenology is crucial to understand climate-vegetation interactions as well as carbon and water cycle in ecosystems in the context of climate change. In this paper, we extracted the vegetation phenology data from 2002 to 2021 based on the dynamic threshold method in the source region of the Yangtze and Yellow Rivers. Trend and correlation analyses were used to investigate the relationship between vegetation phenology and temperature, precipitation and their spatial evolution characteristics. The results showed that (1) From 2002 to 2021, the multi-year average start of growing season, end of growing season, length of growing season for plants were concentrated in May, October and 4-6 months, with a trend of 4.9 days (earlier), 1.5 days (later), 6.3 days/10a (longer) respectively. (2) For every 100 m elevation increase in the vertical direction, the start, end, length of growing season were correspondingly delayed by 1.8 days, advanced by 0.8 days, and shortened by 2.6 days. (3) The impacts of temperature and precipitation on vegetation phenology varied at different stages of vegetation growth. Influencing factors of spring phenology experienced a shift from temperature to precipitation, while autumn phenology experienced precipitation followed by temperature. (4) The climate factors in the previous period significantly affected the vegetation phenology in study area and the spatial variability was obvious. Specifically, the temperature in April significantly affected the spring phenology and precipitation in August widely affected autumn phenology.

Forest degradation hinders soil fauna's contribution to litter decomposition and community dynamics in Andean temperate Nothofagus forests

Biodiversity losses driven by human disturbance hinder chief ecosystem functions like litter decomposition, among the most critical processes regulating forest ecosystems' carbon and nutrient cycles. Historical logging, wildfire, and ongoing livestock grazing pressure have degraded most southern temperate Nothofagus forests. Yet, the effect of forest degradation on soil fauna composition and, in turn, litter decomposition and nutrient cycling still need to be determined. This study aims to i) quantify the contribution of soil fauna to litter decomposition and mineralization of C, N, and P in Nothofagus sp. forest displaying different levels of degradation (mature, secondary, and degraded forests and a newly reforested site) and ii) assess the effect of forest degradation on the temporal composition of soil invertebrates' communities. Decomposition bags with 0.1 and 2 mm mesh were installed in long-term research plots across forest conditions. Litter decomposition and C, N, and P mineralization were estimated monthly for one year. Likewise, meso- and macrofauna's richness, abundance, and diversity were assessed bimonthly during the same period. We found that the contribution of mesofauna to decomposition was the lowest in the newly reforested site and 20 % higher in the degraded forest than in mature and secondary forests. Soil fauna substantially increased C, N, and P mineralization, particularly in the degraded forest. Differences in fauna's contribution to litter decomposition were likely related to changes in the community composition, primarily composed of detritivores in the three forested conditions and phytophagous and predators in the newly reforested site. We concluded that forest degradation modified the invertebrate community composition, and soil fauna enhances litter decomposition and nutrient mineralization in the degraded forests. In addition, our data suggest that forest degradation leads to a loss of invertebrate meso- and macrofauna orders, which alter litter decomposition processes.

Regulating the Critical Intermediates of Dual-Atom Catalysts for CO2 Electroreduction.

Electrocatalysis is a very attractive way to achieve a sustainable carbon cycle by converting CO2 into organic fuels and feedstocks. Therefore, it is crucial to design advanced electrocatalysts by understanding the reaction mechanism of electrochemical CO2 reduction reaction (eCO2RR) with multiple electron transfers. Among electrocatalysts, dual-atom catalysts (DACs) are promising candidates due to their distinct electronic structures and extremely high atomic utilization efficiency. Herein, the eCO2RR mechanism and the identification of intermediates using advanced characterization techniques, with a particular focus on regulating the critical intermediates are systematically summarized. Further, the insightful understanding of the functionality of DACs originates from the variable metrics of electronic structures including orbital structure, charge distribution, and electron spin state, which influences the active sites and critical intermediates in eCO2RR processes. Based on the intrinsic relationship between variable metrics and critical intermediates, the optimized strategies of DACs are summarized containing the participation of synergistic atoms, engineering of the atomic coordination environment, regulation of the diversity of central metal atoms, and modulation of metal-support interaction. Finally, the challenges and future opportunities of atomically dispersed catalysts for eCO2RR processes are discussed.

Carbon dynamics of organic layer in three forest stands dominated by different tree species

ABSTRACT The organic (O) layer on top of forest soil is important in forest carbon (C) cycling, but not many studies have investigated integrated C dynamics, including dissolved organic C (DOC) fluxes. To clarify the DOC contribution and C dynamics in the O layer, this study quantified the C stock of the O layer, C input (litter fall (LF) and throughfall DOC (TF-DOC)), and C output (CO2 emission and litter leachate DOC (LL-DOC)) over the course of a year for three forest stands in the same region (middle slopes of Mt. Asama, Nagano, Japan) dominated separately by Quercus serrata (Q), Larix kaempferi (L), and Pinus densiflora (P). The TF-DOC contribution to C input was high in the rainy season. Additionally, TF-DOC was greater in the L forest than in the Q and P forests because of relatively small LF production in the L forest. The contribution of LL-DOC to C output also tended to be high in the rainy season, but the contribution increased in winter owing to the small emission of CO2 at low temperatures. Annual contribution of LL-DOC was much higher in the P forest than in the Q and L forests, reflecting differences in litter quality. The DOC contribution rate to C flux was generally < 20%, suggesting that DOC flux is a minor but non-negligible process in C cycling in the O layer depending on season and forest type. Our comprehensive C flux data provide better understanding of C cycling in the O layer of temperate forests.

Current research hotspots and frontier trends on carbon budget of coastal wetlands: A bibliometric analysis

ABSTRACT Coastal wetlands are the main distribution of blue carbon in coastal zones and well known for their high carbon sequestration capacity. Investigating the variation of carbon budget is crucial for understanding the functionality of coastal wetlands and effectively addressing climate change. In this study, a bibliometric analysis of 4,509 articles was conducted to reveal research progress, hot issues, and emerging trends in the coastal wetland carbon budget field. The number of publications and citations in this field increased exponentially from 1991 to 2022. The leading subject category was Environmental Sciences with 1,844 articles (40.9%). At present, studies have been focused on blue carbon, the effects of climate change and man-made disturbances on carbon cycle, and the restoration of coastal wetlands. Based on the hotspots and trends in this field, the future researches should include (1) exploring the functional mechanisms of various factors affecting carbon cycle and establishing a methodological system for the estimation of blue carbon in coastal wetlands; (2) researching restoration techniques of coastal wetland and constructing wetland restoration evaluation index system; and (3) formulating enforceable carbon trading policy and strengthening international cooperation.

Soil Enzyme Response and Calorific Value of Zea mays Used for the Phytoremediation of Soils Contaminated with Diesel Oil

Ensuring a stable and cost-effective energy supply is a major challenge for the International Energy Agency (IEA). Additionally, the effectiveness of vermiculite and dolomite in mitigating the adverse effects of diesel oil, a petroleum-derived product, on plant growth and development, and on the biochemical activity of the soil, were assessed. Therefore, an attempt was made in the study to determine the energy properties of Zea mays, which is suitable for cultivation in contaminated areas. For these purposes, several parameters were analyzed in its biomass, including calorific value (Q), heating value (Hv), energy yield (Yep), ash content, and the presence of carbon (C), hydrogen (H), sulfur (S), nitrogen (N), and oxygen (O). Biochemical activity was measured through the evaluation of soil enzymes serving as indicators for the carbon (dehydrogenases, catalase, β-glucosidase), nitrogen (urease), sulfur (arylsulfatase), and phosphorus (acid and alkaline phosphatase) cycles. The plant greenness index was also determined. It has been demonstrated that diesel oil does not alter the calorific value of Zea mays biomass but significantly reduces the biomass quantity and destabilizes the biochemical properties of the soil. Zea mays contained an average of 6.84% ash, 49.88% C, 5.65% H, 0.17% S, 2.90% N, and 34.57% O. The calorific value of Zea mays ranged from 15.02 to 15.54 MJ kg−1 d.m. of plants, and the heating value ranged from 18.25 to 19.21 MJ kg−1 d.m. of plants. The biomass obtained from contaminated soil is recommended for energy purposes. The sorbents used—vermiculite and dolomite—proved to be less effective in the remediation of soil contaminated with diesel oil.

Effects of chironomid larvae density and mosquito biocide on methane and carbon dioxide dynamics in freshwater sediments.

Small lentic water bodies are important emitters of methane (CH4) and carbon dioxide (CO2), but the processes regulating their dynamics and susceptibility to human-induced stressors are not fully understood. Bioturbation by chironomid larvae has been proposed as a potentially important factor controlling the dynamics of both gases in aquatic sediments. Chironomid abundance can be affected by the application of biocides for mosquito control, such as Bti (Bacillus thuringiensis var. israelensis). Previous research has attributed increases in CH4 and CO2 emissions after Bti application to reduced bioturbation by chironomids. In this study, we separately tested the effect of chironomid bioturbation and Bti addition on CH4 production and emission from natural sediments. In a set of 15 microcosms, we compared CH4 and CO2 emission and production rates with high and low densities of chironomid larvae at the bioturbating stage, and standard and five times (5x) standard Bti dose, with control sediments that contained neither chironomid larvae nor Bti. Regardless of larvae density, chironomid larvae did not affect CH4 nor CO2 emission and production of the sediment, although both rates were more variable in the treatments with organisms. 5xBti dosage, however, led to a more than three-fold increase in CH4 and CO2 production rates, likely stimulated by bioavailable dissolved carbon in the Bti excipient and priming effects. Our results suggest weak effects of bioturbating chironomid larvae on the CH4 and CO2 dynamics in aquatic ecosystems. Furthermore, our results point out towards potential functional implications of Bti for carbon cycling beyond those mediated by changes in the macroinvertebrate community.

Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control

Abstract. Phytoplankton form the base of the marine food web by transforming CO2 into organic carbon via photosynthesis. Despite the importance of phytoplankton for marine ecosystems and global carbon cycling, projections of phytoplankton biomass in response to climate change differ strongly across Earth system models, illustrating uncertainty in our understanding of the underlying processes. Differences are especially large in the Southern Ocean, a region that is notoriously difficult to represent in models. Here, we argue that total (depth-integrated) phytoplankton biomass in the Southern Ocean is projected to largely remain unchanged under climate change by the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble because of a shifting balance of bottom-up and top-down processes driven by a shoaling mixed-layer depth. A shallower mixed layer is projected on average to improve growth conditions, consequently weaken bottom-up control, and confine phytoplankton closer to the surface. An increase in the phytoplankton concentration promotes zooplankton grazing efficiency, thus intensifying top-down control. However, large differences across the model ensemble exist, with some models simulating a decrease in surface phytoplankton concentrations. To reduce uncertainties in projections of surface phytoplankton concentrations, we employ an emergent constraint approach using the observed sensitivity of surface chlorophyll concentration, taken as an observable proxy for phytoplankton, to seasonal changes in the mixed-layer depth as an indicator for future changes in surface phytoplankton concentrations. The emergent constraint reduces uncertainties in surface phytoplankton concentration projections by around one-third and increases confidence that surface phytoplankton concentrations will indeed rise due to shoaling mixed layers under global warming, thus favouring intensified top-down control. Overall, our results suggest that while changes in bottom-up conditions stimulate enhanced growth, intensified top-down control opposes an increase in phytoplankton and becomes increasingly important for the phytoplankton response to climate change in the Southern Ocean.

Role of sea spray aerosol at the air–sea interface in transporting aromatic acids to the atmosphere

Abstract. Aromatic acids are ubiquitous in seawater (SW) and can be transported to the atmosphere via sea spray aerosol (SSA). Despite their importance in affecting the global radiative balance, the contribution of marine aromatic acids and their transport mechanisms through SSA remain unclear. Herein, the distribution of particle size and number concentration of SSA produced in SW containing nine different aromatic acids (i.e., benzoic acids, benzenedicarboxylic acids, hydroxybenzoic acids, vanillic acid, and syringic acid) was studied using a custom-made SSA simulation chamber; moreover, the enrichment of aromatic acids in SSA and their emission flux to the atmosphere were analyzed. Transmission electron microscopy (TEM) images clearly revealed that aromatic acids can be transferred to the nascent SSA. Interestingly, the morphology associated with benzenedicarboxylic-acid-coated particles showed that aromatic acids can promote the growth of other surfaces of sea salt, thus making the sea salt core spherical. Aromatic acids showed a significant enrichment behavior at the air–sea interface, which clearly indicated that SSA represents a source of aromatic acids in the atmosphere. Vanillic acid had the largest global emission flux through SSA (962 t yr−1), even though its concentration in SW was lower. The calculated results indicated that the global annual flux of aromatic acids was affected not only by the concentration in SW, but also by their enrichment factor (EF). These data are critical for further quantifying the contribution of organic acids to the atmosphere via SSA, which may provide an estimate of the potential influence of the atmospheric feedbacks to the ocean carbon cycle.

Early Mississippian global δ13C excursion is not a diagenetic artifact

Shallow-water platform carbonate δ13C may provide a record of changes in ocean chemistry through time, but early marine diagenesis and local processes can decouple these records from the global carbon cycle. Recent studies of calcium isotopes (δ44/40Ca) in shallow-water carbonates indicate that δ44/40Ca can be altered during early marine diagenesis, implying that δ13C may also potentially be altered. Here, we tested the hypothesis that the platform carbonate δ13C record of the Kinderhookian-Osagean boundary excursion (KOBE), ∼353 m.y. ago, reflects a period of global diagenesis using paired isotopic (δ44/40Ca and clumped isotopes) and trace-element geochemistry from three sections in the United States. There is little evidence for covariation between δ44/40Ca and δ13C during the KOBE. Clumped isotopes from our shallowest section support primarily sediment-buffered diagenesis at relatively low temperatures. We conclude that the δ13C record of the KOBE as recorded in shallow-water carbonate is consistent with a shift in the dissolved inorganic carbon reservoir and that, more generally, ancient shallow-water carbonates can retain records of primary seawater chemistry.

Different impacts of artificial thicketization of Caragana korshinskii and Hippophae rhamnoides on soil organic carbon in alpine grasslands of the Qinghai–Tibetan Plateau

Grassland shrub encroachment is known to change soil organic carbon (SOC) storage, but the effect of artificial thicketization on SOC in alpine grasslands of the Qinghai–Tibetan Plateau remains unclear. In this study, we investigated the effects of two common shrubs, leguminous Caragana korshinskii and non‐leguminous Hippophae rhamnoides, on SOC at a reforestation site on the Qinghai–Tibetan Plateau. While C. korshinskii had no significant effect on SOC content, H. rhamnoides significantly increased SOC content. In both shrub communities, there was a significant positive correlation between SOC content and the coverage, plant aboveground biomass, dominant plant species biomass, total and available nitrogen, total phosphorus, and proportion of water‐stable aggregate of greater than 250 μm. Structural equation modeling showed that the common path by which the two shrub communities affected SOC was that the dominant species affected soil structure and the available nitrogen content by influencing community biomass, which had indirect effects on SOC. Additionally, C. korshinskii participated in the regulation of SOC. This result provides an important empirical reference and key insights into the carbon cycling mechanisms of reforested grasslands on the Qinghai–Tibetan Plateau.

An Improved Gross Primary Production Model Considering Atmospheric CO2 Fertilization: The Qinghai–Tibet Plateau as a Case Study

Involving the effect of atmospheric CO2 fertilization is effective for improving the accuracy of estimating gross primary production (GPP) using light use efficiency (LUE) models. However, the widely used LUE model, the remote sensing-driven Carnegie–Ames–Stanford Approach (CASA) model, scarcely considers the effects of atmospheric CO2 fertilization, which causes GPP estimation uncertainties. Therefore, this study proposed an improved method for estimating GPP by integrating the atmospheric CO2 concentration into the CASA model and generated a long time series GPP dataset with high precision for the Qinghai–Tibet Plateau. The CASA model was improved by considering the impact of atmospheric CO2 on vegetation productivity and discerning variations in CO2 gradients within the canopy and leaves. A 500 m monthly GPP dataset for the Qinghai–Tibet Plateau from 2003 to 2020 was generated. The results showed that the improved GPP estimation model achieved better performances on estimating GPP (R2 = 0.68, RMSE = 406 g C/m2/year) than the original model (R2 = 0.67, RMSE = 499.32 g C/m2/year) and MODIS GPP products (R2 = 0.49, RMSE = 522.56 g C/m2/year). The GPP on the Qinghai–Tibet Plateau increased significantly with the increase in atmospheric CO2 concentration and the gradual accumulation of dry matter. The improved method can also be used for other regions and the generated GPP dataset is valuable for further understanding the ecosystem carbon cycles on the Qinghai–Tibet Plateau.