Missing Carbon Sink Research Articles

Migration of dissolved carbon on bare karst slopes in soil in response to natural rainfall events

Soil carbon in karst critical zones is an essential part of the global carbon pool, and the migration of dissolved carbon is a vital component of the carbon cycle in such regions. The unique “dual hydrological structure” makes the path and process of migration of soil dissolved carbon driven by rainfall unclear in karst slopes, which ultimately hinders the accurate evaluation of the carbon sink effect in such regions. Therefore, based on field monitoring of natural rainfall, this study evaluated the process and characteristics of the spatial migration of soil dissolved carbon in bare karst slopes and its response to natural rainfall (rainfall duration and amount). The results showed that the concentration of dissolved carbon transported in surface runoff was significantly lower than that transported within the soil layer; however, carbon loss displayed the opposite trend. Rainfall factors (rainfall amount and duration) and runoff volume significantly affected soil dissolved carbon, contributing 46.91% to soil dissolved carbon. Rainfall factors significantly affect soil dissolved carbon migration by influencing runoff. In addition, the hydrological path is an important factor affecting soil dissolved carbon transport. Our study shows that the soil and bedrock interface (SBI) and shallow karst fissures (SKF) are the main routes of dissolved carbon migration in soils in bare karst slopes. Dissolved carbon migrating along SKF is a vital contributor to the “missing carbon sink” in karst regions, and dissolved carbon migrating along the SBI is potentially an important contributor to the “missing carbon sink.” This study provides primary data and is expected to serve as a reference for revealing the mechanism of karst carbon migration and transformation and accurately evaluating the carbon sink effect in karst areas.

High stability of autochthonous dissolved organic matter in karst aquatic ecosystems: Evidence from fluorescence

Biological carbon pump (BCP) in karst areas has received intensive attention for years due to their significant contribution to the global missing carbon sink. The stability of autochthonous dissolved organic matter (Auto-DOM) produced by BCP in karst aquatic ecosystems may play a critical role in the missing carbon sink. However, the source of dissolved organic matter (DOM) in inland waters and its consumption by planktonic bacteria have not been thoroughly examined. Recalcitrant dissolved organic matter (RDOM) may exist in karst aquatic ecosystem as in the ocean. Through the study of the chromophoric dissolved organic matter (CDOM) and the interaction between CDOM and the planktonic bacterial community under different land uses at the Shawan Karst Water-carbon Cycle Test Site, SW China, we found that C2, as the fluorescence component of Auto-DOM mineralised by planktonic bacteria, may have some of the characteristics of RDOM and is an important DOM source in karst aquatic ecosystems. The stability ratio (Fmax(C2/(C1+C2))) of Auto-DOM reached 89.6 ± 6.71% in winter and 64.1 ± 7.19% in spring. Moreover, correlation-based network analysis determined that the planktonic bacterial communities were controlled by different fluorescence types of CDOM, of which C1 (fresh Auto-DOM), C3 (conventional allochthonous DOM (Allo-DOM)) and C4 (the Allo-DOM mineralised by bacteria) were clustered in one module together with prevalent organic-degrading planktonic bacteria; C2 was clustered in another tightly combined module, suggesting specific microbial utilization strategies for the C2 component. In addition, some important planktonic bacterium and functional genes (including chemotrophic heterotrophs and photosynthetic bacteria) were found to be affected by high Ca2+ and dissolved inorganic carbon (DIC) concentrations in karst aquatic ecosystems. Our research showed that Auto-DOM may be as an important carbon sink as the Allo-DOM in karst ecosystems, the former generally being neglected based on a posit that it is easily and first mineralized by planktonic bacteria.

Organic carbon source tracing and the BCP effect in the Yangtze River and the Yellow River: Insights from hydrochemistry, carbon isotope, and lipid biomarker analyses

Autochthonous organic carbon (AOC) formed by biological carbon pump (BCP) in surface waters may serve as a significant carbon sink. The locations, magnitudes, variations and mechanisms responsible for the terrestrial missing carbon sink by BCP are uncertain, especially in large river systems. In this study, hydrochemical characteristics, carbon isotope compositions of dissolved inorganic carbon (DIC) and organic carbon (OC), n-alkane homologues and C/N ratios of organic matter along the Yangtze River and the Yellow River were investigated to constrain the OC source and the significance of BCP effect. It was found that (1) DIC concentrations in the Yellow River were much higher than those in the Yangtze River, which was controlled primarily by the temperature effect; (2) AOC in the both rivers was characterized by lower C/N ratios and δ13CPOC values. Based on calculation of n-alkanes compounds, the AOC proportions ranged from 29 to 88% (49% on average, with a higher proportion (55%) in the rainy season than in the dry season (46%)) and 19–68% (41% on average; with a lower proportion in the rainy season (31%) than in the dry season (51%)) in the Yangtze River and the Yellow River, respectively, indicating intense aquatic production. Low dissolved CO2 concentration (6.17 μmol/L on average) of the Yangtze River limited the aquatic production and decreased the BCP effect in the dry season, indicated by lower AOC proportion. However, the BCP effect increased in the Yellow River in the dry season mainly due to the increased light penetration; (3) even in high turbidity riverine systems such as the Yellow River, the aquatic photosynthetic uptake of DIC could produce considerable AOC. These findings clearly show the formation of AOC by BCP in both the clear and high turbidity riverine systems, suggesting a potential direction for finding the terrestrial missing carbon sink.

Increasing Autochthonous Production in Inland Waters as a Contributor to the Missing Carbon Sink

Accounting for the residual land sink (or missing carbon sink) has become a major budget focus for global carbon cycle modelers. If we are not able to account for the past and current sources and sinks, we cannot make accurate predictions about future storage of fossil fuel combustion emissions of carbon in the terrestrial biosphere. Here, we show that the autochthonous production (AP) in inland waters appears to have been strengthening in response to changes in climate and land use, as evidenced by decreasing CO2 emissions from and increasing dissolved organic carbon storage and/or organic carbon burial in inland waters during recent decades. The increasing AP may be due chiefly to increasing aquatic photosynthesis caused by global warming and intensifying human activities. We estimate that the missing carbon sink associated with the strengthening AP in inland waters may range from 0.38 to 1.8 Gt C yr-1 with large uncertainties. Our study stresses the potential role that AP may play in the further evolution of the global carbon cycle. Quantitative estimates of future freshwater AP effects on the carbon cycle may also help to guide the action needed to reduce carbon emissions, and increase carbon sinks in terrestrial aquatic ecosystems.

Impact of differences in soil temperature on the desert carbon sink

An understanding of the global terrestrial carbon cycle is crucial to predict future climate change. Deserts are an important part of the terrestrial ecosystem, but their role in the terrestrial carbon cycle has long been neglected. Recent studies have shown that deserts may sequester enormous volumes of CO2 and play a pivotal role as a carbon sink. As the world’s second-largest shifting desert, the Taklimakan Desert (TD) contributes substantially to the global desert carbon sink. However, the contributions of the internal processes of the TD to its carbon sink and the long-term trend of the carbon sink under climate change are still unclear. Based on an observational study, we determined that both the expansion/contraction of soil air containing CO2 caused by heat fluctuation in shifting sand and the salt/alkali chemistry control the release/absorption processes of soil respiration in shifting sand. Besides, the mutual offset of these processes indicates that the shifting sand in the TD acts as a stable carbon sink at present, with an annual average CO2 uptake rate of 7.11 g m−2. However, an increase in soil temperature difference will stimulate soil air expansion and release more CO2 into the atmosphere under climate change, gradually lessening the carbon sink rate of shifting sand in TD in the future. These processes will be accelerated by a positive feedback loop, where this effect triggered by climate change will enhance regional warming. Our results highlight the status of deserts in the global carbon cycle and provide crucial information regarding the world’s missing carbon sink.

Estimation of karst carbon sink and its contribution to CO2 emissions over a decade using remote sensing imagery

Research on the carbon source and sink imbalance in global carbon cycle has demonstrated that the mass of CO2 consumed by rock weathering is a portion of the “missing carbon sink.” To further understand the karst carbon sink contributions to atmospheric CO2 emissions, the present study establishes the relationship between the karst carbon sinks and the atmospheric CO2 emissions. We first estimated the mass of CO2 consumed by rock weathering using the Global Erosion Model for CO2 fluxes (GEM-CO2), based on the distribution of different types of rock in Guangxi, China. The rocks were classified using remote sensing imagery, and we estimated the relationship between the mass of CO2 consumed by rock weathering and the CO2 emitted into atmosphere from 2003 to 2012. The analyses reveal that (1) the mass of CO2 consumed by rock weathering could be rapidly estimated using remote sensing imagery, as it not only considers different types of rocks but also simultaneously considers the geological structure and meteorological data, such as rainfall, temperature, and vegetation coverage; (2) the mass of CO2 consumed by rock weathering from 2003 to 2012 displays a “wavy pattern”, which might have been primarily caused by the annual variations in rainfall, whose tendency could be fitted by a straight line at an average annual increasing rate of approximately 8.7%; (3) the mass of emitted CO2 from 2003 to 2012 in Guangxi display a “straight line” at an average annual increasing rate of approximately 12.7%; (4) the mass of CO2 consumed by rock weathering is positively correlated with the mass of emitted CO2 with a correlation coefficient of R2 = 0.62; this implies that an increasing concentration of CO2 in the atmosphere reversely fosters the CO2 consumption by rock weathering; (5) due to different increasing rates, annually 7 × 106 t of CO2 is accumulated in the atmosphere in Guangxi, and a total of 506.42 × 106 t of CO2 is accumulated over a decade; (6) the mass of the karst carbon sinks in Guangxi, China, and globally are estimated to be 1.01 × 106 tC·a−1, 1.49 × 107 tC·a−1, and 0.95 × 108 tC·a−1, respectively. Therefore, the results of this study not only help to understand the relationship between karst carbon sink and the global carbon cycle, but also provide an important basis for predicting the variations in CO2 in the atmosphere and climate change, which could further provide a scientific basis for the rational use of resources and the formulation of environmental protection policies.

Transformation of DIC into POC in a karst river system: evidence from δ13CDIC and δ13CPOC in Lijiang, Southwest China

One of the most important questions in the science of global carbon cycle is how to balance the atmospheric CO2 budget. However, there is a large terrestrial carbon sink is missing. The locations, magnitudes, variations, and mechanisms responsible for this terrestrial missing carbon sink are uncertain and the focus of much continuing debate. In order to provide a basis for the assessment of carbonate weathering-related carbon sink, this study investigated the spatial and temporal variations of dissolved inorganic carbon (DIC) and particulate organic carbon (POC), and used stable carbon isotopes and hydrochemical data to quantitatively estimate the proportion of DIC transformation to POC and analyze the potential influence of the transformation of DIC into OC on the carbon isotope composition in the Lijiang River (typical karst surface water), Southwest China. The results showed that DIC concentrations are high in winter and low in summer and increase from upstream to downstream. POC concentrations are high in summer and low in spring. δ13CPOC and δ13CDIC are negatively correlated in the Lijiang River. The δ13CDIC values are high in spring and low in summer, compared with the estimated δ13CDIC(cal.); the observed δ13CDIC values have a maximum positive shift in spring and similar in the summer; the δ13CPOC values decrease from upstream to downstream, and are high in summer and low in spring. These observations indicate that the isotopic compositions of both DIC and POC are significantly affected by aquatic photosynthesis that transforms DIC into OC. In-river primary production contribute 12–54% of POC, and the contributions of carbonate rock-sourced DIC to the riverine POC range from 7 to 30%, which suggests an important sink of atmospheric CO2 in river systems and should be taken into account in global carbon cycle.

The sensitivity of the carbon sink by coupled carbonate weathering to climate and land-use changes: Sediment records of the biological carbon pump effect in Fuxian Lake, Yunnan, China, during the past century

Recent studies show that the carbon sink attributable to the weathering of carbonate rocks may have been greatly underestimated if the biological carbon pump (BCP) effect in transferring dissolved inorganic carbon (DIC) to organic carbon (autochthonous OC) by aquatic photoautotrophs is neglected. The uptake of DIC by aquatic photoautotrophs may reach 0.2 to 0.7 Pg C/a globally, indicating that the carbon sink by the coupled carbonate weathering with aquatic photosynthesis mechanism (CCW) may be an important control in climate change. In order to understand the sensitivity of the CCW carbon sink to changes of climate and land-use, a systematic study of modern trap and 100-year-long core sediments was conducted in Fuxian Lake, (Yunnan, SW China), the second-deepest plateau oligotrophic freshwater lake in China. It was found that (1) the autochthonous OC in the lake sediments was characterized by lower C/N ratios and higher δ13Corg. By means of an n-alkanes compound calculation, the proportions of autochthonous OC were determined to be in the range, 60–68% of all OC; (2) increase in the autochthonous OC accumulation rate (OCARauto) was accompanied by an increase in the inorganic carbon accumulation rate (ICAR) in both the trap and core sediments. In particular, the post-1950 OCARauto was estimated to be about 6.9 times that for the period, 1910–1950; (3) OCARauto in core sediments increased significantly with global warming and land-use change, from 1.06 g C m−2 yr−1 in 1910 to 21.74 g C m−2 yr−1 in 2017. The increasing carbon sink may act as a negative feedback on global warming if the trend holds for all lakes globally. This study is the first to quantify the burial flux of organic carbon generated by the BCP effect in lakes and may contribute to solving the problem of the missing carbon sink in the global carbon cycle.

A probabilistic case for a large missing carbon sink on Mars after 3.5 billion years ago

Mars has a thin (6 mbar) CO2 atmosphere currently. There is strong evidence for paleolakes and rivers formed by warm climates on Mars, including after 3.5 billion years (Ga) ago, which indicates that a CO2 atmosphere thick enough to permit a warm climate was present at these times. Since Mars no longer has a thick CO2 atmosphere, it must have been lost. One possibility is that Martian CO2 was lost to space. Oxygen escape rates from Mars are high enough to account for loss of a thick CO2 atmosphere, if CO2 was the main source of escaping O. But here, using H isotope ratios, O escape calculations, and quantification of the surface O sinks on Mars, we show for the first time that O escape from Mars after 3.5 Ga must have been predominantly associated with the loss of H2O, not CO2, and therefore it is unlikely that ≥250 mbar Martian CO2 has been lost to space in the last 3.5 Ga, because such results require highly unfavored O loss scenarios. It is possible that the presence of young rivers and lakes on Mars could be reconciled with limited CO2 loss to space if crater chronologies on Mars are sufficiently incorrect that all apparently young rivers and lakes are actually older than 3.5 Ga, or if climate solutions exist for sustained runoff on Mars with atmospheric CO2 pressure <250 mbar. However, our preferred solution to reconcile the presence of <3.5 Gya rivers and lakes on Mars with the limited potential for CO2 loss to space is a large, as yet undiscovered, geological C sink on Mars.

Characteristics of soil profile CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; concentrations in karst areas and their significance for global carbon cycles and climate change

Abstract. CO2 concentrations of 21 soil profiles were measured in Zhaotong City, Yunnan Province. The varying characteristics of soil profile CO2 concentrations are distinguishable between carbonate and noncarbonate areas. In noncarbonate areas, soil profile CO2 concentrations increase and show significant positive correlations with soil depth. In carbonate areas, however, deep-soil CO2 concentrations decrease and have no significant correlations with soil depth. Soil organic carbon is negatively correlated with soil CO2 concentrations in noncarbonate areas. In carbonate areas, such relationships are not clear. This means that the special geological process in carbonate areas – carbonate corrosion – absorbs part of the deep-soil-profile CO2. Isotope and soil pH data also support such a process. A mathematical model simulating soil profile CO2 concentration was proposed. In noncarbonate areas, the measured and the simulated values are almost equal, while the measured CO2 concentrations of deep soils are less than the simulated in carbonate areas. Such results also indicate the occurrence of carbonate corrosion and the consuming of deep-soil CO2 in carbonate areas. The decreased CO2 concentration was roughly evaluated based on stratigraphic unit and farming activities. Soil pH and the purity of CaCO3 in carbonate bedrock deeply affect the corrosion. The corrosion in carbonate areas decreases deep-soil CO2 greatly (accounting for 5.2 %–66.3 % with average of 36 %) and naturally affects the soil CO2 released into the atmosphere. Knowledge of this process is important for karst carbon cycles and global climate changes and it may be a part of the “missing carbon sink”.

Review of lime carbon sink.

Under the background of "missing carbon sink" mystery and carbon capture and storage (CCS) technology development, this paper summarized the lime material flow process carbon sink from the lime carbonation principles, impact factors, and lime utilization categories in chemical industry, metallurgy industry, construction industry, and lime kiln ash treatment. The results showed that the lime carbonation rate coefficients were mainly impacted by materials and ambient conditions; the lime carbon sink was mainly in chemical, metallurgy, and construction industries; and current researches focused on the mechanisms and impact factors for carbonation, but their carbon sequestration calculation methods had not been proposed. Therefore, future research should focus on following aspects: to establish a complete system of lime carbon sequestration accounting method in view of material flow; to calculate lime carbon sequestration in both China and the world and explain their offset proportion of CO2emission from lime industrial process; to analyze the contribution of lime carbon sequestration to missing carbon sink for clarifying part of missing carbon sinks; to promote the development of carbon capture and storage technology and provide some scientific bases for China's international negotiations on climate change.

Mangroves in the global climate and environmental mix

AbstractMangroves provide numerous ecosystem services to humanity and yet remain some of the most undervalued and poorly treated ecosystems in the world. Mangroves are distributed in the land‐sea interphase of tropical and subtropical coasts in 123 countries. Up until two decades ago, scholars have focused on the provisioning, supporting, and cultural services associated with mangroves, but recent efforts have pivoted towards mangroves role in regulating the global carbon cycle. Today, it is established that mangroves sequester 50 times more carbon than tropical forests, with strong indications that mangroves are the missing carbon sink. Despite their importance, mangroves remain under threat from human activities, and action is required to protect them. This paper provides an overview of the importance of mangroves to humanity, their role in carbon sequestration and nutrient cycling, and highlights the threats they continue to face across their range.

Perturbations and 3R in carbon management

Perturbations in various carbon pools like biological, geological, oceanic, and missing carbon sink affect its global data, which are generally neglected or ignored in routine calculations. These natural and anthropogenic events need to be considered before projecting a sustainable carbon management plan. These plans have both general and experimental aspects. General plans should focus on (a) minimizing emission; (b) maximizing environmentally sound reuse, reduce, and recycling; (c) effective treatment; and (d) converting carbon into valuable products with atom economy. Experimental carbon management plans involving various biological and chemical techniques with limitation in terms of research level and economic feasibility. Chemical options have benefits of higher productivity and wider product range, but it suffers from its higher-energy requirements and environmental unfriendliness. In contrast to this, biological options are more selective and less energy intensive, but their productivity is very low. Hence, there is a requirement of hybrid process where the benefits of both the options, i.e., biological and chemical, can be reaped. In view of above, the proposed review targets to highlight the various perturbations in the global carbon cycle and their effects; study the currently practiced options of carbon management, specifically in light of 3R principle; and propose various new hybrid methods by compatible combinations of chemical and biological processes to develop better and safer carbon management. These methods are hypothetical so they may require further research and validations but may provide a comprehensive base for developing such management methods.

Organic carbon source tracing and DIC fertilization effect in the Pearl River: Insights from lipid biomarker and geochemical analysis

The photosynthetic conversion of dissolved inorganic carbon (DIC) into organic carbon (OC) by using aquatic phototrophs in rivers may serve as a potential carbon sink, especially in the carbonate rock areas, thereby offering a clue for finding the missing carbon sink. However, primary-produced autochthonous OC is erroneously considered as terrestrial-derived allochthonous OC. Thus, carbonate weathering-related carbon sink is underestimated if only DIC concentrations sampled at river mouths are considered, and the transformation of DIC to autochthonous OC is neglected. Therefore, distinguishing sources of autochthonous and allochthonous OC is vital in the assessment of carbon sink. In this study, source-specific biomarkers, in association with chemical compositions and phytoplankton proxies in water samples collected from the Pearl River, were analyzed to determine OC sources. Results showed that biomarkers in the Pearl River were quite abundant, and the calculated average autochthonous OC was approximately 65% of the total OC, indicating intense in-river primary productivity. Moreover, phytoplankton biomass and DIC concentration were positively related, indicating the DIC fertilization effect on aquatic photosynthesis. High total suspended solid (TSS) on the water surface blocked the sunlight and then reduced phytoplankton production. However, in situ photosynthesis of phytoplankton could also produce autochthonous OC, even larger than the allochthonous source at sites with high DIC, and even with higher TSS concentrations. These findings comprehensively elucidated the formation of autochthonous OC based on the coupling action of rock weathering and photosynthetic activity in the riverine system, suggesting a potential direction for finding the missing carbon sink.

Approaching the Truth of the Missing Carbon Sink

Approaching the Truth of the Missing Carbon Sink

Carbon Sequestration Within Silica Bodies Extracted from Kelp Cultured in the East China Sea

To re-evaluate ecological effects of kelp culture in marginal seas, the silica body content in kelp cultured in the East China Sea and the carbon content in the silica body extracted from kelp were measured. The silica body content in kelp cultured in the sea ranged from 0.20 to 0.60 % and the carbon content in the silica body extracted from kelp was 0.27 ± 0.06 %. The carbon sequestration rate in silica bodies extracted from kelp cultured in the East China Sea was 2.47 ± 0.40 t.a−1 from 2000 to 2011. In the East China Sea, the production of the silica body of kelp in the kelp culture zone was 6.56 ± 1.06 g.m−2.a−1. The carbon sequestration rate in the silica body of cultured kelp in the East China Sea was 0.02 ± 0.00 g.m−2.a−1, which accounted for 1.43 % of CO2 flux from air to sea. The carbon fixed in the silica body extracted from kelp might be one part of the missing carbon sink. Considering carbon content in silica bodies extracted from kelp cultured in the East China Sea, carbon sequestration of kelp culture in the marginal sea should be re-evaluated and some missing carbon sinks caused by anthropogenic factors might be reduced.

气象要素及土壤理化性质对不同土地利用方式下冬夏岩溶作用的影响

Many studies showed that the carbon sink resulted from karst process is fast and stable,which compose the very important part of "Missing carbon sink". Different land-use patterns can affect the karst process by influence the soilproperties. Commonly,with the land-use change from forest land to cultivated field the soil organic matter will reduce,the stability of soil structure get worse,the soil moisture retaining quality get down and the quantity of soil microbiology will diminish,all the above would affect the karst process. This study took 6 different land-use patterns( Tilled land( code: Ha),Paddy field( St),Reforested land( Tu), Abandoned Land( Hu),Mixed Woodland with Chinese fir-bamboo( Sz) and Bamboo land( Zh)) with the same geological backgrounds but different human influence intensity in Qingmuguan karst valley as a case study to determine the influence factors on karst processes. Standard limestone tablets were buried in the field to test the dissolution process rate. HOBO automatic weather station was located in the roof of local people's house to get the precipitation and temperature data. CO2 content in different soil layers were tested by Gas-Tec CO2tester. Soil samples in different layers were taken back to laboratory to detect the pH value,the organic matter,soil moisture content,and soil bulk density. The dissolution data from tablets showed that,firstly,the mean dissolution mass per unit area of different land-use patterns in different layers in Summer were 3.87 times than in Winter. The max mean dissolution mass changed from-80cm layer in Summer to-20—-50cm layers in Winter. Secondly,the mean dissolution mass in these two land-use patterns Ha and St with strong human influence intensity( Abbr. HII) was higher than that inTu and Hu in which the HII get weaker, while that in Sz and Zh which had the weakest HII with the lowest mean dissolution mass. Also,the range between them was higher in summer with strong HII than that in winter. Afteranalyzing the temperature( T),precipitation( P) and the soil physicochemical properties data,we can find that, there was a positive correlation between the change of dissolution mass with seasons and T,P which were the most important factors affecting the former. The soil CO2content can only play an important role on dissolution in summer which has more precipitation,and these two factors can react each other enough to form more carbonic acid so more carbonate tablets can be dissoluted. There was a negative correlation between soil organic matter and soil pH,and these two factors affected the dissolution jointly. They can influence dissolution greatest only in surface layer with high organic matter and low pH,while less in deeper layers. The influence of soil texture determined by soil bulk density and soil porosity on dissolution was bidirectional. The clay with good moisture retaining quality but bad permeability was benefit for the dissolution in winter for there was less precipitation that time,but bad in summer for there was no so much water pass through the soil,Hu was a good example of this. However,there was an oppose process in sandy soil with good permeability but bad moisture retaining quality,corresponded more dissolution in summer and less in winter,there was a good example of tilled land also. Additionally,the use of farmyard manure and chemical fertilizer which include NO-3and SO2-4can promote the dissolution process too. Above all,the results of this study can provide consultation for evaluate karst carbon sink more scientifically and accurately.

Remote Sensing of CO2Absorption by Saline-Alkali Soils: Potentials and Constraints

CO2absorption by saline-alkali soils was recently demonstrated in the measurements of soil respiration fluxes in arid and semiarid ecosystems and hypothetically contributed to the long-thought “missing carbon sink.” This paper is aimed to develop the preliminary theory and methodology for the quantitative analysis of CO2absorption by saline-alkali soils on regional and global scales. Both the technological progress of multispectral remote sensing over the past decades and the conjectures of mechanisms and controls of CO2absorption by saline-alkali soils are advantageous for remote sensing of such absorption. At the end of this paper, the scheme for remote sensing is presented and some unresolved issues related to the scheme are also proposed for further investigations.

Can soil respiration estimate neglect the contribution of abiotic exchange?

This study examines the hypothesis that soil respiration can always be interpreted purely in terms of biotic processes, neglecting the contribution of abiotic exchange to CO2 fluxes in alkaline soils of arid areas that characterize 5% of the Earth’s total land surface. Analyses on flux data collected from previous studies suggested reconciling soil respiration as organic (root/microbial respiration) and inorganic (abiotic CO2 exchange) respiration, whose contributions in the total CO2 flux were determined by soil alkaline content. On the basis of utilizing meteorological and soil data collected from the Xinjiang and Central Asia Scientific Data Sharing Platform, an incorporated model indicated that inorganic respiration represents almost half of the total CO2 flux. Neglecting the abiotic module may result in overestimates of soil respiration in arid alkaline lands, which partly explains the long-sought “missing carbon sink”.

Estimating the net ecosystem exchange for the major forests in the northern United States by integrating MODIS and AmeriFlux data

The eddy covariance technique provides long-term continuous monitoring of site-specific net ecosystem exchange of CO2 (NEE) across a large range of forest types. However, these NEE estimates only represent fluxes at the scale of the tower footprint and need to be scaled up to quantify NEE over regions or continents. In the present study, we expanded a method developed previously and generated a new NEE model exclusively based on the Moderate Resolution Imaging Spectroradiometer (MODIS) products, including enhanced vegetation index (EVI), land surface water index (LSWI), land surface temperature (LST) and Terra nighttime LST′. This method, in our previous research, provided substantially good predictions of NEE and well reflected the seasonal dynamics of the deciduous broadleaf forest at the Harvard forest site. Studying NEE of forests in the middle-latitude regions of the Northern Hemisphere is significant because it may help to understand the ‘missing carbon sink’ from terrestrial ecosystems. In this study we selected eight eddy flux sites to represent the major forest ecosystems in the northern United States. Compared with the model based on a single site, we also established the general models that apply to evergreen needleleaf forest (ENF) and deciduous broadleaf forest (DBF), respectively. The results showed that our simpler model based entirely on MODIS products promised well to estimate NEE by the eddy covariance technique. The modeled annual mean NEE from DBF deviated from the measured NEE by 44.4%, whereas the modeled NEE from ENF was extremely close to the measured NEE within 5.5%. In the end, we also validated both general models for ENF and DBF using independent flux sites. It demonstrated this method performed well for estimating NEE.