Abiotic CO2 Research Articles

Emissions of CO2 and CH4 from Agricultural Soil with Kitchen Compost at Different Temperatures

Emissions of CO2 from the soil are mainly derived from soil microbial respiration, whereas CH4 emissions originate from anaerobic degradation of organic matter via microbial processes. Kitchen waste compost is used in the agricultural sector to improve soil quality. However, abiotic CO2 and CH4 emissions from soils amended with kitchen waste compost under aerobic conditions remain uncertain. Temperature plays an important role in organic matter decomposition in both biotic and abiotic pathways. This study aimed to evaluate biotic and abiotic emissions of CO2 and CH4 from soils receiving kitchen compost at different temperatures. Ten grams of soil amended with or without 0.1 g kitchen compost (1%) were sterilized or non-sterilized. The mixture and soil-only samples were incubated in 100-mL glass bottles at 20, 30, and 35 °C for 28 d under an aerobic condition. The results showed that CO2 and CH4 emissions increased at higher temperatures and compost application rates (p < 0.05). Emissions of CO2 mainly occurred via biotic pathways. Abiotic processes were potential pathways for CH4 generation, particularly at high temperatures of 35 °C. There was 20–24% of C in kitchen compost changed to CO2 and less than 0.1% to CH4. Our results suggest that global warming enhances abiotic CO2 and CH4 emissions and may contribute to further global warming.

Recent advances in CO2 reduction with renewable reductants under hydrothermal conditions: towards efficient and net carbon benefit CO2 conversion.

The ever-growing atmospheric CO2 concentration threatening the environmental sustainability of humankind makes the reduction of CO2 to chemicals or fuels an ideal solution. Two priorities are anticipated for the conversion technology, high efficiency and net carbon benefit, to ensure the mitigation of the CO2 problem both promptly and sustainably. Until now, catalytic hydrogenation or solar/electro-chemical CO2 conversion have achieved CO2 reduction promisingly while, to some extent, compromising to fulfill the two rules, and thus alternative approaches for CO2 reduction are necessary. Natural geochemical processes as abiotic CO2 reductions give hints for efficient CO2 reduction by building hydrothermal reaction systems, and this type of reaction atmosphere provides room for introducing renewable substances as reductants, which offers the possibility to achieve CO2 reduction with net carbon benefit. While the progress in CO2 reduction has been abundantly summarized, reviews on hydrothermal CO2 reduction are relatively scarce and, more importantly, few have focused on CO2 reduction with renewable reductants with the consideration of both scale of efficiency and sustainability. This review provides a fundamental and critical review of metal, biomass and polymer waste as reducing agents for hydrothermal CO2 reduction. Various products including formic acid, methanol, methane and multi-carbon chemicals can be formed, and effects of operational parameters such as temperature, batch holding time, pH value and water filing as well as detailed reaction mechanisms are illustrated. Particularly, the critical roles of high temperature and pressure water as reaction promotor and catalyst in hydrothermal CO2 conversion are discussed at the mechanistic level. More importantly, this review compares hydrothermal CO2 reduction with other methods such as catalytic hydrogenation and photo/electrocatalysis, evaluating their efficiency and potential for net carbon benefit. The aim of this review is to promote the understanding of CO2 activation under a hydrothermal environment and provide insights into the efficient and sustainable strategy of hydrothermal CO2 conversion for future fundamental research and industrial applications.

Combining methane clumped and bulk isotopes, temporal variations in molecular and isotopic composition, and hydrochemical and geological proxies to understand methane's origin in the Ronda peridotite massifs (Spain)

In serpentinised peridotite and ultramafic rock systems, methane (CH4) origin is frequently considered abiotic, but variable microbial and thermogenic components can also exist. Typically, the origin of CH4 is studied using bulk, 13C/12C and 2H/H isotopic composition, molecular gas composition, occasionally radiocarbon (14C), microbiology and geological context. Recent advances in CH4-clumped isotope methods have yielded novel insights into the formation of CH4: nonetheless, their interpretation in natural gas samples is often uncertain and requires additional research. Here, we study the origin of the gas released in hyperalkaline (pH > 10) springs in the Ronda Peridotite Massifs (southern Spain), combining bulk and clumped CH4 isotopes with molecular gas composition, hydrochemical (Total Organic Carbon and Platinum Group Elements in water), geothermal and geo-structural data. Five springs analysed in 2014 have been re-examined for changes in gas chemistry over time, and three newly discovered gas-bearing springs are analysed for the first time. Regardless of whether springs have microbial or abiotic isotopic fingerprints, we find that bulk CH4 isotopes are fairly stable over a seven-year period. This suggests that the CH4 source(s) or postgenetic processes (such as oxidation and diffusion) have not undergone significant temporal changes. Major variations in H2 and CH4 concentrations in certain springs may be the result of changes in gas pressure and migration intensity. Paired CH4 clumped isotopes (Δ12CH2D2 - Δ13CH3D) were analysed in two bubbling springs, where the presence of CH4 can be interpreted as non-microbial based on 13C enrichment, absence of 14C, and the presence of ethane and propane. However, these isotopes are in disequilibrium, which prevents the quantification of the gas formation temperature. Within the Δ12CH2D2 - Δ13CH3D diagram, the data lie within both the microbialgenic zone, suggested by previous authors, and the abiotic zone that results combining data from laboratory gas synthesis and other natural gas samples. Therefore, attributing a microbial origin to CH4 based only on clumped isotopes is less definite than previously assumed. The amount of Total Organic Carbon appears to be correlated with the origin of CH4, as it is higher in 13C-depleted CH4 samples and lower in 13C-enriched samples. Palladium (Pd) and Rhodium (Rh) dissolved in water (the more soluble Platinum Group Elements) can be a proxy for the chromitite ore deposits contained in plagioclase tectonite layers throughout the investigated area, which may act as catalysts for abiotic CO2 hydrogenation. Clumped isotope disequilibrium and the reported absence of diffuse CH4-bearing fluid inclusions in the peridotites appear to rule out high temperature gas genesis in post-magmatic inclusions. These observations, along with the moderate temperatures at the base of the peridotite massifs and the consistent occurrence of gas along tectonic contacts between serpentinised (H2-bearing) peridotite and carbon-bearing rocks, are compatible with the theory of low-temperature CO2 hydrogenation.

Do biodegradable microplastics cause soil inorganic carbon loss in calcareous soils?

The presence of biodegradable microplastics (MPs) has the potential to affect soil pH, and possibly accelerate or inhibit the loss of soil inorganic carbon (SIC) in calcareous soils. However, most researchers have focused on the release of biotic carbon dioxide (CO2) from soils following MP amendments, and few studies have investigated SIC-derived CO2. In this experiment, three typical biodegradable MPs were applied to three calcareous soils amended with 1 % 13C-labeled (99 % atom) carbonate, and the release of CO2 originating from SIC was quantified. The total CO2 emissions, soil pH, and microbial functional genes involved in soil nitrification and denitrification were also detected. Throughout the experiment, the contribution of 13C-labeled carbonate to total CO2 emissions ranged from 0.42 % to 3.31 %. The impact of biodegradable MPs on SIC-derived CO2 varied with incubation period. At the early stage (≤20 days), the amendment of three biodegradable MPs increased the abiotic CO2 in some cases, and the CO2 emissions from 13C-labeled SIC were positively correlated with the total CO2 originating from the decomposition of SOC and MPs. At the late stage (20–70 days), the presence of biodegradable MPs inhibited the release of CO2 from 13C-labeled carbonate in most treatments. Moreover, there were negative relationships of SIC-derived CO2 with soil pH and the amoA gene of ammonia-oxidizing archaea (AOA), but positive correlations of SIC-derived CO2 with amoA of ammonia oxidizing bacteria (AOB) and nirK and nirS genes encoding nitrate reductase in denitrification. Our results indicate that long-term exposure to biodegradable MPs probably regulates the release of H+ in the nitrification process by controlling AOB, and then controlling the dynamics of SIC in calcareous soils.

Plant Allelopathy in Response to Biotic and Abiotic Factors

The allelopathy definition accepted by the International Allelopathy Society is any process or phenomenon via which organisms such as plants, microorganism, viruses, and fungi release specific metabolites, known as allelochemicals, into the environment that affect the growth and development of other surrounding plants, resulting in mutual inhibition or promotion of the organisms. Allelopathy in donor plants is either self-induced or induced by specific external biotic and abiotic factors. The external factors may determine the types and quantities of allelochemicals released into the environment by a donor plants. Biotic factors inducing allelopathy include plant competition, herbivory by animals and insects, and soil-borne pathogens. For example, competing plants produce allelochemicals in response to competing weeds. Plants may emit signaling chemicals from their roots to induce the production of protective metabolites against organisms, herbivores, or competitors. Additionally, herbivory is a selection pressure that triggers defensive measures in plants, such as the synthesis of allelochemicals in response to injury. Moreover, some leaves release volatile substances that help the plant to deter or attract herbivores or pests that attack leaves. Likewise, root compounds may be produced in response to soil-borne pathogens. Furthermore, some abiotic factors, including light, temperature, drought, CO2, and nutrient deficiency, can also induce plants to release allelochemicals. Therefore, it is pertinent to understand this natural phenomenon in plants and how external factors lead plants to exert allelopathy to compete, defend themselves, and survive in a challenging environment.

Influence of carbon-coated zero-valent iron-based nanoparticle concentration on continuous photosynthetic biogas upgrading

This study assessed the influence of carbon-coated zero-valent nanoparticle concentration (70, 140 and 280 mg L−1) on the performance of photosynthetic biogas upgrading in an indoor pilot scale plant composed of an algal-bacterial photobioreactor interconnected to an external biogas absorption column. In addition, the influence of nanoparticle concentration on the abiotic CO2 gas-liquid mass transfer in the biogas absorption column was also evaluated. Microalgae productivity was enhanced by > 100 % when nanoparticles were added to the cultivation broth, which also boosted nitrogen and phosphorus assimilation from centrate. The biomethane produced complied with most international standards only when nanoparticles were supplemented, achieving CO2 concentrations < 1 % (CO2 removal efficiencies > 98 %) and CH4 concentrations > 94 % in the treated biogas. Finally, this research consistently demonstrated that the improvement of biogas upgrading performance by the addition of nanoparticles was based on a photosynthesis enhancement or stimulation (which significantly increased the pH in the algal cultivation broth) rather than on an improved nanoparticle-mediated CO2 capture in the biogas absorption column.

White-rot fungi scavenge reactive oxygen species, which drives pH-dependent exo-enzymatic mechanisms and promotes CO2 efflux.

Soil organic matter (SOM) decomposition mechanisms in rainforest ecosystems are governed by biotic and abiotic procedures which depend on available oxygen in the soil. White-rot fungi (WRF) play an important role in the primary decomposition of SOM via enzymatic mechanisms (biotic mechanism), which are linked to abiotic oxidative reactions (e.g., Fenton reaction), where both processes are dependent on reactive oxygen species (ROS) and soil pH variation, which has yet been studied. In humid temperate forest soils, we hypothesize that soil pH is a determining factor that regulates the production and consumption of ROS during biotic and abiotic SOM decomposition. Three soils from different parent materials and WRF inoculum were considered for this study: granitic (Nahuelbuta, Schizophyllum commune), metamorphic (Alerce Costero, Stereum hirsutum), and volcanic-allophanic (Puyehue, Galerina patagonica). CO2 fluxes, lignin peroxidase, manganese peroxidase, and dye-decolorizing peroxidase levels were all determined. Likewise, the production of superoxide anion (O2•-), hydrogen peroxide (H2O2), and hydroxyl radicals (•OH) were assessed in soils microcosms after 36 days of anaerobic incubation with WRF inoculum and induced Fenton reaction under pH variations ranging from 2.5 to 5.1. ROS significantly increased biotic and abiotic CO2 emissions in all tested soils, according to the findings. The highest values (217.45 mg C kg-1) were found during the anaerobic incubation of sterilized and inoculated soils with WRF at a natural pH of 4.5. At pH 4.0, the lowest levels of C mineralization (82 mg C kg-1) were found in Nahuelbuta soil. Enzyme activities showed different trends as pH changed. The Fenton reaction consumed more H2O2 between pH 3 and 4, but less between pH 4.5 and 2.5. The mechanisms that oxidized SOM are extremely sensitive to variations in soil pH and the stability of oxidant radical and non-radical compounds, according to our findings.

Using copper-based biocathodes to improve carbon dioxide conversion efficiency into methane in microbial methanogenesis cells

Copper can be used as a metal catalyst for abiotic CO2 conversion into methane and organic chemicals, but it has not been examined as a catalyst for enhancing biotic methane generation in microbial methanogenesis cells (MMCs). In this study, copper-based electrodes prepared using several different techniques were compared to the performance of MMCs containing graphite block cathodes. Gas production was examined under both abiotic and biotic conditions at a fixed cathode potential of –0.9 V vs. Ag/AgCl in two-chamber electrochemical cells. All copper-based cathodes showed better methane production than plain graphite blocks except for the cathode made from copper foil which lacked a biocompatible surface. The cathode prepared by an electroless Cu deposition (electroless-Cu) method had the highest current density production of 0.6 A/m2 and methane production rate of 201 nmol/cm3/d, and its performance was stable over time. Both the electroless-Cu and electro-deposited Cu electrodes produced more current than that obtained with copper powders with a Nafion binder (Nafion-Cu), likely due to different surface characteristics such as hydrophobicity and uniformity of the copper layer. The results of this study showed that copper-based biocathodes improved methane production relative to plain graphite materials and techniques for preparing copper electrodes impact bioelectrochemical performance with the highest performance in the electroless-Cu reactors.

Dryland irrigation increases accumulation rates of pedogenic carbonate and releases soil abiotic CO2

Agricultural fields in drylands are challenged globally by limited freshwater resources for irrigation and also by elevated soil salinity and sodicity. It is well known that pedogenic carbonate is less soluble than evaporate salts and commonly forms in natural drylands. However, few studies have evaluated how irrigation loads dissolved calcium and bicarbonate to agricultural fields, accelerating formation rates of secondary calcite and simultaneously releasing abiotic CO2 to the atmosphere. This study reports one of the first geochemical and isotopic studies of such “anthropogenic” pedogenic carbonates and CO2 from irrigated drylands of southwestern United States. A pecan orchard and an alfalfa field, where flood-irrigation using the Rio Grande river is a common practice, were compared to a nearby natural dryland site. Strontium and carbon isotope ratios show that bulk pedogenic carbonates in irrigated soils at the pecan orchard primarily formed due to flood-irrigation, and that approximately 20–50% of soil CO2 in these irrigated soils is calcite-derived abiotic CO2 instead of soil-respired or atmospheric origins. Multiple variables that control the salt buildup in this region are identified and impact the crop production and soil sustainability regionally and globally. Irrigation intensity and water chemistry (irrigation water quantity and quality) dictate salt loading, and soil texture governs water infiltration and salt leaching. In the study area, agricultural soils have accumulated up to 10 wt% of calcite after just about 100 years of cultivation. These rates will likely increase in the future due to the combined effects of climate variability (reduced rainfall and more intense evaporation), use of more brackish groundwater for irrigation, and reduced porosity in soils. The enhanced accumulation rates of pedogenic carbonate are accompanied by release of large amounts of abiotic CO2 from irrigated drylands to atmosphere. Extensive field studies and modelling approaches are needed to further quantify these effluxes at local, regional and global scales.

Hydrothermal synthesis of long-chain hydrocarbons up to C24 with NaHCO3-assisted stabilizing cobalt

Abiotic CO2 reduction on transition metal minerals has been proposed to account for the synthesis of organic compounds in alkaline hydrothermal systems, but this reaction lacks experimental support, as only short-chain hydrocarbons (<C5) have been synthesized in artificial simulation. This presents a question: What particular hydrothermal conditions favor long-chain hydrocarbon synthesis? Here, we demonstrate the hydrothermal bicarbonate reduction at ∼300 °C and 30 MPa into long-chain hydrocarbons using iron (Fe) and cobalt (Co) metals as catalysts. We found the Co0 promoter responsible for synthesizing long-chain hydrocarbons to be extraordinarily stable when coupled with Fe-OH formation. Under these hydrothermal conditions, the traditional water-induced deactivation of Co is inhibited by bicarbonate-assisted CoOx reduction, leading to honeycomb-native Co nanosheets generated in situ as a new motif. The Fe-OH formation, confirmed by operando infrared spectroscopy, enhances CO adsorption on Co, thereby favoring further reduction to long-chain hydrocarbons (up to C24). These results not only advance theories for an abiogenic origin for some petroleum accumulations and the hydrothermal hypothesis of the emergence of life but also introduce an approach for synthesizing long-chain hydrocarbons by nonnoble metal catalysts for artificial CO2 utilization.

Influences of temperature and moisture on abiotic and biotic soil CO2 emission from a subtropical forest

BackgroundSoil CO2 efflux is considered to mainly derive from biotic activities, while potential contribution of abiotic processes has been mostly neglected especially in productive ecosystems with highly active soil biota. We collected a subtropical forest soil to sterilize for incubation under different temperature (20 and 30 °C) and moisture regimes (30%, 60 and 90% of water holding capacity), aiming to quantify contribution of abiotic and biotic soil CO2 emission under changing environment scenarios.Main findings:Results showed that abiotic processes accounted for a considerable proportion (15.6−60.0%) of CO2 emission in such a biologically active soil under different temperature and moisture conditions, and the abiotic soil CO2 emission was very likely to derive from degradation of soil organic carbon via thermal degradation and oxidation of reactive oxygen species. Furthermore, compared with biotically driving decomposition processes, abiotic soil CO2 emission was less sensitive to changes in temperature and moisture, causing reductions in proportion of the abiotic to total soil CO2 emission as temperature and moisture increased.ConclusionsThese observations highlight that abiotic soil CO2 emission is unneglectable even in productive ecosystems with high biological activities, and different responses of the abiotic and biotic processes to environmental changes could increase the uncertainty in predicting carbon cycling.

Response of abiotic soil CO2 flux to the difference in air-soil temperature in a desert

Abiotic soil CO2 flux (Fa) has been observed in drylands worldwide, while its generation mechanism is not determined. The remarkable features for the occurrence time of Fa, that the positive Fa (CO2 emission from soil) and negative Fa (CO2 absorption by soil) generally occurred in the daytime and nighttime, respectively, suggested that the generation of Fa is related to temperature. Previous studies have focused on the sole effect of soil temperature (Ts) or air temperature (Ta) on Fa, but very little is regarding to the overall response of Fa to both Ta and Ts in the air-soil system. In the Mu Us Desert, China, we continuously measured the hourly Fa, Ts, Ta and the difference between Ta and Ts (Ta-Ts) at 4-day intervals from June to October 2016. During this period, soil abiotically absorbed atmospheric CO2 at a rate of 0.32 ± 0.19 g m−2 d−1. Fa was not correlated with Ts and was weakly correlated with Ta, however, it was strongly and positively correlated with Ta-Ts. A quadratic model adequately fitted the relationship between Fa and Ta-Ts, and similar relationships were also observed between positive Fa and Ta-Ts and between negative Fa and Ta-Ts. The results of Fa measurement highlight that desert soils can absorb CO2 from the atmosphere through abiotic processes. The relationships of Fa with Ts, Ta and Ta-Ts indicate that, abiotic CO2 exchange between soil and atmosphere depends on air-soil temperature gradient in desert. Furthermore, the thermal convection driven by Ta-Ts may play a major role in the generation of nocturnal negative Fa. Our findings provide a new possible perspective for revealing the reasons of Fa generation in drylands.

Soil CO2 influx in drylands: A conceptual framework and empirical examination

Soil CO2 flux is dominated in most ecosystems by respiration of soil organisms and roots, and is therefore expected to be positive (i.e., efflux) during dark hours. Growing evidence from various ecosystems contradicts this expectation, showing nocturnal soil CO2 influx. Little is known about this puzzling phenomenon and the underlying mechanisms governing it. We attempted to bridge this knowledge gap by proposing the “dryland carbon influx” (DCI) framework. Based on Henry's law, decreases in soil temperature stimulate CO2 dissolution in soil solutions, creating a CO2 concentration gradient between soil air and the atmosphere. This may lead to CO2 diffusion from the atmosphere into the soil. This influx is expected to be stronger at lower temperatures, high soil to atmosphere temperature gradients, and in CaCO3 rich alkaline soils that characterize drylands. The soil abiotic influx is expected to govern the overall CO2 flux when dry conditions and low soil organic matter limit organismal respiration. We examined the underlying conditions predicted by the DCI framework using a literature review and soil CO2 flux measurements from the Negev Desert. We found 24 studies that reported dark soil CO2 influx, almost exclusively from drylands. CO2 influx was recorded mainly in alkaline soils, was positively correlated with pH levels, and was associated predominantly with low soil temperature, decreasing soil temperature or a positive soil to air thermal gradient. CO2 influx was found not only in dry soils but also in poor organic content wet soils. In the Negev Desert, CO2 influx occurred only when (a) soil temperature was higher than air temperature and was decreasing, and (b) soil moisture was 7.76% or lower. Only above this threshold, a positive association between CO2 efflux and microbial biomass was found. Our results largely support the DCI framework, suggesting that soil CO2 influx is prevalent across drylands worldwide, facilitated by the edaphic and climatic conditions that induce abiotic CO2 exchange and restrain biotic respiration. Considering the abiotic contribution to soil CO2 flux, and determining the conditions that regulate it may substantially improve our carbon budget calculations, and estimations of soil biotic respiration.

Modeling carbon dioxide exchange in a single-layer urban canopy model

Carbon dioxide (CO2) is the primary greenhouse gas that drives the global climate change in past centuries. Much effort of carbon mitigation as a countermeasure to global changes has focused on the urban areas – the hotspots of fossil fuel and concentrated emission and pollutants. Despite their critical role in CO2 exchange in urban ecosystem, emission from vegetation and soils are largely overlooked in existing urban land surface models. In this study, we parameterized the biogenic CO2 exchange in cities using an advanced single-layer urban canopy model, by incorporating plant physiological functions in the built environment. In addition, the proposed model also includes the anthropogenic CO2 fluxes especially that from traffic emissions based on gridded dataset. We evaluate the proposed model using field measurements from an eddy covariance flux tower located at west Phoenix, Arizona, USA. The model results are in good agreement with the observed carbon flux over the built terrain, with a root mean squared error of 0.21 mg m−2s−1. Furthermore, our simulations show that the abiotic traffic-emitted CO2 amounts the largest source in cities, as expected. Nevertheless, the biogenic carbon exchange can be significantly enhanced in the built environment, making an equally important contributor to the total carbon emission especially in sub-urban areas.

Experimental Evidence That Permafrost Thaw History and Mineral Composition Shape Abiotic Carbon Cycling in Thermokarst-Affected Stream Networks

Mounting evidence suggests that biogeochemical processing of permafrost substrate will amplify dissolved inorganic carbon (DIC=E[CO2,HCO3–,CO32–]) production within Arctic freshwaters. The effects of permafrost thaw on DIC may be particularly strong where terrain subsidence following thaw (thermokarst) releases large amounts of sediment into fluvial networks. The mineral composition and chemical weathering of these sediments has critical yet untested implications for the degree to which streams represent a source of CO2 to the atmosphere versus a source of bicarbonate to downstream environments. Here, we experimentally determine the effects of mineral weathering on fluvial CO2 by incubating sediments collected from three retrogressive thaw slump features on the Peel Plateau (NWT, Canada). Prehistoric warming and contemporary thermokarst have exposed sediments on the Peel Plateau to varying degrees of thaw and chemical weathering, allowing us to test the role of permafrost and substrate mineral composition on CO2:HCO3– balance. We found that recently-thawed sediments (within years to decades) and previously un-thawed diamicton from deeper permafrost generated significant solutes and DIC. These solutes and the mineralogy of sediments suggested that carbonate weathering coupled with sulfide oxidation was a net source of abiotic CO2. Yet, on average, 30% of this CO2 was converted to bicarbonate via carbonate buffering reactions. In contrast, the mineralogy and geochemical trends associated with sediments from the modern and paleo-active layer, which were exposed to thaw over longer timescales, more strongly reflected silicate weathering. In treatments with sediment from the modern and paleo-active layer, minor carbonate and sulfide weathering resulted in some DIC and net CO2 production. This CO2 was not measurably diminished by carbonate buffering. Together, these trends suggest that prior exposure to thaw and weathering on the Peel Plateau reduced carbonate and sulfide in upper soil layers. We conclude that thermokarst unearthing deeper tills on the Peel Plateau will amplify regional inorganic carbon cycling for decades to centuries. However, previously-undocumented CO2 consumption via carbonate buffering may partly counterbalance CO2 production and release to the atmosphere. Regional variability in the mineral composition of permafrost, thaw history, and thermokarst intensity are among the primary controls on mineral weathering within permafrost carbon-climate feedbacks.

One third of the abiotically-absorbed atmospheric CO2 by the loess soil is conserved in the solid phase

It has frequently been noticed that alkaline soils in arid and semiarid areas can absorb atmospheric CO2 abiotically. However, the fate of the absorbed CO2 is largely unknown, limiting our ability to accurately assess the contribution of abiotic CO2 absorption to carbon sequestration. The hypothesis that some abiotically-absorbed carbon could be fixed in the soil solid phase is yet to be explicitly tested. From April to November 2016, we added 13CO2 to the natural alkaline loess soil at a depth of 0–17 cm and quantified the proportion of 13CO2 conserved in the soil solid phase on the central Loess Plateau of China. The results showed that natural surface soil absorbed 13CO2 at a rate of 475.9 mg m−2 d−1. Of the absorbed 13CO2, 33.2% was conserved in the soil solid phase. The rate of added 13CO2 conserved in the solid phase was positively correlated with mean monthly soil water content but negatively correlated with mean monthly soil temperature. Our results indicated that, loess soil can absorb CO2 from the atmosphere through abiotic processes, and part of the abiotically-absorbed CO2 is stably fixed in the soil solid phase. Soil with a higher water content and lower temperature may promote the abiotic CO2 fixation. The rate of CO2 fixation through abiotic processes at the surface soil accounted for approximately 10%-20% of the reported local annual net ecosystem exchange, implying that the abiotic CO2 uptake by loess soil may play an important role in assessing the capacity of CO2 sequestration in these areas.

The effect of microorganisms on soil carbonate recrystallization and abiotic CO2 uptake of soil

Desert ecosystems with a high soil pH are considered as a sink for atmospheric CO2. Although high CO2-uptakevia chemical processes is the major CO2-uptake process in saline and alkaline soil, microorganisms may affect the exchange of inorganic carbon between the soil solution and the solid phase. Some studies related to microbial processes on carbonate formation have been published, but seldom focus on saline and alkaline soils. To determine the microbial effects on the inorganic C flux and CaCO3 recrystallization, sterile and non-sterile Solonchak samples were incubated in a 14CO2-labelled atmosphere at two CO2 concentrations (0.2% and 2%). The 14C activity was measured in the soil solution, air, and in CaCO3 after 2, 7, 21, 92 and 197 days. The 14C in dissolved organic carbon in the sterile and non-sterile soils was below 5% of the 14C input. 14C trapping in CaCO3 increased with the decrease of the 14C remaining in the gaseous CO2. Carbonate recrystallization increased logarithmically with time (R2 > 0.96). A two percent initial CO2 concentration lead to higher CaCO3 recrystallization (5–50 × 10−3% of initial CaCO3) compared to a 0.2% initial CO2 concentration (0.8–7 × 10−3% of initial CaCO3) from 2 to 197 days. CaCO3 recrystallization (% of initial CaCO3) in the sterile Solonchak was higher by 10% to 190% than the non-sterile Solonchak, indicating that the microorganisms decreased the 14C trapping of CaCO3 by 4–23% from 2 to 197 days. We assume that microorganisms grew on the carbonate surface like a coating. This must have retarded the carbonate recrystallization. We concluded that the soil CO2 concentration is the most important factor for CaCO3 recrystallization. The presence of microorganisms strongly decreased the CaCO3 recrystallization. CaCO3 recrystallization process including CO2 in soil water could be the abiotic CO2 uptake from the atmosphere by saline and alkaline soil.

Biotic and Abiotic Contribution to Diurnal Soil CO2 Fluxes from Saline/Alkaline Soils

As the second largest carbon flux in terrestrial ecosystems, the soil CO2 flux is closely related to the atmospheric CO2 concentration. The soil CO2 flux is the sum of biotic respiration and abiotic geochemical CO2 exchange; however, little is known about abiotic CO2 fluxes in arid areas. To investigate the relative contribution of abiotic and biotic soil CO2 fluxes over a diurnal course, the abiotic CO2 flux was distinguished by autoclaving sterilization in both saline and alkaline soils at an arid site in northwestern China. The results demonstrated that: (1) Over the diurnal course, the abiotic CO2 was a significant component of the soil CO2 flux in both saline and alkaline soil, which accounted for more than 56% of the diurnal soil CO2 flux. (2) There was a dramatic difference in the temperature response between biotic and abiotic CO2 fluxes: the response curves of biotic respiration were exponential in the saline soil and quadratic in the alkaline soil, while the abiotic CO2 flux was linearly correlated with soil temperature. They were of similar magnitude but with opposite signs: resulting in almost neutral carbon emissions on daily average. (3) Due to this covering up effect of the abiotic CO2 flux, biotic respiration was severely underestimated (directly measured soil CO2 flux was only one-seventh of the biotic CO2 flux in saline soil, and even an order of magnitude lower in alkaline soil). In addition, the soil CO2 flux masked the temperature-inhibition of biotic respiration in the alkaline soil, and veiled the differences in soil biological respiration between the saline and alkaline soils. Hence, the soil CO2 flux may not be an ideal representative of soil respiration in arid soil. Our study calls for a reappraisal of the definition of the soil CO2 flux and its temperature dependence in arid or saline/alkaline land. Further investigations of abiotic CO2 fluxes are needed to improve our understanding of arid land responses to global warming and to assist in identifying the underlying abiotic mechanisms.

Effect of Plant Growth-Promoting Rhizobacteria on Salicornia ramosissima Seed Germination under Salinity, CO2 and Temperature Stress

In a scenario of climate change and growing population, halophyte root microbiota interactions may be a sustainable solution to improve alternative crop production while combating abiotic stress. In this work, seeds of the cash crop halophyte Salicornia ramosissima were inoculated with five different plant growth-promoting rhizobacteria consortia, isolated from the rhizosphere of five halophytes in southwestern Spain salt marshes. For the first time, we recorded seed germination response to three interactive abiotic stressors, CO2 (400 and 700 ppm), temperature (25 and 29 ℃) and salinity (171, 510 and 1030 mM NaCl), all of them related to climate change. Salinity played a decisive role, as no significant differences were registered between treatments at 171 mM NaCl and no germination took place at 1030 mM NaCl. At 510 mM NaCl, one rhizobacterial consortium improved seed parameters notably, increasing up to 114% germination percentage and 65% seedlings biomass. These first findings encourage us to think that cash crop halophytes like S. ramosissima and halophyte root microbiota may be valuable resources for human or animal feeding in a future climate reality.

Oxidative stress and biomarker responses in the Atlantic halibut after long term exposure to elevated CO2 and a range of temperatures.

CO2 emissions from human activities are increasing, resulting in greater rates of change in the oceans, exceeding any other event in geological and historical records over the past 300 million years. Oceans are warming and pH levels are decreasing. Marine organisms will need to respond to multiple stressors and the potential consequences of global change-related effects in fish needs to be investigated. Fish are affected by many biotic and abiotic environmental variables, including temperature and CO2 fluctuations, and it is therefore critical to investigate how these variables may affect physiological and biochemical processes. We investigated the effects of elevated CO2 levels (pH of 8.0, which served as a control, or 7.6, which is predicted for the year 2100) combined with exposure to different temperatures (5, 10, 12, 14, 16, and 18 °C) in the Atlantic halibut (Hippoglossus hippoglossus) during a three month experiment. Since regulation of reactive oxygen species (ROS) is crucial for physiological processes the focus was on the antioxidant defense system and we assessed the effects on catalytic activities of antioxidant enzymes (SOD, CAT, GR, GST, GPx). In addition we also analyzed effects on cholinesterase enzymes (AChE and BChE), and CYP1A enzyme activities (EROD). The treatments resulted in oxidative stress, and damage was evident in the form of protein carbonyls which were consistently higher in the elevated CO2-treated fish at all temperatures. Analyses of antioxidant enzymes did not show the same results, suggesting that the exposure to elevated CO2 increased ROS formation but not defences. The antioxidant defense system was insufficient, and the resulting oxidative damage could impact physiological function of the halibut on a cellular level.