High Soil CO2 Research Articles

Soil CO2 flux maps as tools to reduce the risk on soil diffuse degassing areas

Carbon dioxide released permanently from soils in diffuse degassing areas may constitute a permanent hazard for the population. Several villages in the Azores archipelago (Portugal) are placed in areas with anomalous soil CO2 degassing and lethal indoor CO2 concentration (>10 vol%) has been already recorded in some buildings. The 2021-2022 dislodgements of population at Vulcano (Italy) and La Palma (Spain) volcanic islands due to high soil CO2 degassing highlight the importance of defining criteria to produce human CO2 exposure risk maps, which are useful to mitigate the risk and should constitute valuable tools for land-use planners. Risk is assessed in the current study by combining susceptibility, exposure, and vulnerability maps. The defined criteria were applied to two villages in Furnas Volcano (São Miguel Island, Azores), showing that 58% and 98% of the buildings, respectively, at Furnas and Ribeira Quente villages are at high risk of CO2 exposure.

Warming reduces soil CO2 emissions but enhances soil N2O emissions: A long-term soil transplantation experiment

Climate warming can accelerate soil organic matter decomposition and stimulate soil CO2 and N2O emissions. However, long-term climate warming and land-use changes in relatively high-latitude regions on soil CO2 and N2O emissions remain largely unexplored, posing challenges to climate change research. Therefore, we conducted a long-term soil transplant experiment (8 years) across three relatively high-latitude northeastern regions in China to study the impacts of climate warming and land-use changes (from cropland to grassland) on soil CO2 and N2O emissions. As the temperature increased by 3 °C and 5 °C, the soil CO2 emissions from cropland were reduced by 59.07% and 56.87%, respectively, and those from grassland were reduced by 17.11% and 10.62%, respectively. The experiment duration, soil C storage, soil microbial abundance and soil moisture may be the main factors that explain why warming did not stimulate soil CO2 emissions. Soil N2O emissions increased by 76.57% in cropland and 263.81% in grassland as the temperature increased by 5 °C. Higher soil CO2 and N2O emissions were observed in grassland compared to cropland. Warming promoted aboveground plant biomass and indirectly promoted soil N2O emissions, particularly in grassland. The effects of long-term warming on soil CO2 and N2O emissions exhibited contrasting patterns, with CO2 emissions in relatively high-latitude and cold regions showing sensitivity to climate warming. When taking strategies to enhance soil C sequestration, consideration should be given to whether these strategies will be offset by stimulating soil N2O emissions, which is crucial for mitigating global warming. Overall, the impacts of long-term natural field warming and land-use changes on soil CO2 and N2O emissions and associated controls provide new insights for mitigating climate change.

Distinguishing the combined vegetation and soil component of δ13C variation in speleothem records from subsequent degassing and prior calcite precipitation effects

Abstract. The carbon isotopic signature inherited from soil and epikarst processes may be modified by degassing and prior calcite precipitation (PCP) before its imprint on speleothem calcite. Despite laboratory demonstration of PCP effects on carbon isotopes and increasingly sophisticated models of the governing processes, to date, there has been limited effort to deconvolve the dual PCP and soil–epikarst components in measured speleothem isotopic time series. In this contribution, we explore the feasibility, advantages, and disadvantages of using trace element ratios and δ44Ca to remove the overprinting effect of PCP on measured δ13C to infer the temporal variations in the initial δ13C of drip water prior to degassing and PCP. In nine examined stalagmites, the most widely utilized PCP indicators Mg/Ca and δ44Ca covary as expected. However, Sr / Ca does not show consistent relationships with δ44Ca so PCP is not the dominant control on Sr / Ca. From δ44Ca and Mg/Ca, our calculation of PCP as fCa, the fraction of initial Ca remaining in solution at the time the stalagmite layer is deposited, yields multiple viable solutions depending on the assumed δ44Ca fractionation factor and inferred variation in DMg. Uncertainty in the effective fractionation of δ13C during degassing and precipitation contributes to uncertainty in the absolute value of estimated initial δ13C. Nonetheless, the trends in initial δ13C are less sensitive to these uncertainties. In coeval stalagmites from the same cave spanning the 94 to 82 ka interval, trends in calculated initial δ13C are more similar than those in measured δ13C and reveal a common positive-anomaly initial δ13C during a stadial cooling event. During deglaciations, calculated initial δ13C implies a trend of greater respiration rates and higher soil CO2, although the higher interglacial drip water saturation favors more extensive PCP. Initial δ13C can be estimated for active and fossil speleothems from a range of settings, wherever there is confidence that Mg/Ca and/or δ44Ca provides a quantitative indication of past changes in PCP. Further study of Mg partitioning in speleothems will improve the robustness of Mg/Ca as a PCP proxy.

Fault-related basins as carbon reservoirs: Soil CO2 emissions in the SE Korean Peninsula

Carbon dioxide (CO2) is one of the main components of greenhouse gases, and there is a great concern about emissions mainly from human activities. However, long term CO2 emissions during the Earth’s history are largely related to natural emissions by geological factors. Natural CO2 emissions are mainly at highly permeable passages below the surface (e.g., volcanoes or faults). Unlike volcanoes, which actively release mantle-derived volatiles, faults can be affected by CO2 of more diverse origins and are relatively lacking in research. Here, we note that it is important for CO2 emissions that faults as channels with high permeability take topographical control to form sedimentary basins and produce mature soils. To this end, we performed CO2 flux measurements in the Yangsan Fault System (YFS), Republic of Korea. As this fault is an active fault, topographical variations are clear and accessible. In the YFS, high spring CO2 fluxes (119.5 g m−2 d-1 to 77,699.5 g m−2 d-1) were observed in Sector 1, and high soil CO2 fluxes (0.5 g m−2 d-1to 1,240 g m−2 d-1) were observed along the Hyeongsan River in Sector 2. The carbon isotope composition indicates that the mantle contributes to fault-related spring CO2 (δ13C = -14.03 ‰ to −7.44 ‰), and soil CO2 comes from shallow biogenic sources (δ13C = –22.82 ‰ to −13.53 ‰) deposited in fault-forming basins. In addition, the helium isotope ratios indicative of the mantle contribution (3He/4He = 1.86 to 6.02 Ra) are shown in the gas samples of the springs. Furthermore, in a post-earthquake survey, we found temporal changes in soil CO2 flux values near the epicenter, which may be related to the earthquake. A simple calculation of overall CO2 emissions suggests that significant amounts of CO2 (6.2 Mt yr−1) are being emitted along the YFS. Therefore, this study reports both large-scale structural degassing and biological CO2 emissions from fault zones, providing implications for the role of sedimentary basins and soil types formed in fault zones.

Impacts of thinning activities on boreal peatland forests

Boreal peatland forests are an important source of timber. Recently, timber harvesting has been extended to warmer months, resulting in machinery traffic over unfrozen soils, and leading to higher levels of soil disturbance, such as deeper ruts. Despite this, our knowledge of the impact of soil disturbance on peat physical properties and soil biochemistry is still limited. To address this gap, I conducted a study to examine the effects of soil disturbance caused by harvesting machinery during thinning operations on the soil physical, chemical, and biological properties and vegetation of drained boreal peatland forests. To assess the rate of recovery, I sampled six sites that formed a chronosequence covering 15 years since thinning. The results showed that soil disturbance caused an increase in the bulk density and field capacity of peat, along with a decrease in total porosity. In the vegetation, moss biomass and root production were reduced, but sedge cover increased. Furthermore, recently disturbed areas exhibited greater soil CO2 production potential, as well as higher soil CO2 and CH4 concentrations compared to control areas. However, CO2 and CH4 emissions, microbial communities, and cellulose decomposition rate were not impacted. Although the rate of recovery varied, all studied properties impacted by disturbance were fully recovered within 15 years. As the water retention characteristic (WRC) describes soil structure and its alterations, it a useful for disturbance assessment. Thus, I propose how WRC can be predicted using artificial neural networks. Overall, the study demonstrated that while drained boreal peatlands are sensitive to disturbance, they are also resilient to mechanical soil disturbance caused by thinnings.

Geochemical features of fluid in Xiaojiang fault zone, Southeastern Tibetan plateau: Implications for fault activity

It is well established that fluid geochemistry in fault zones could provide reliable indicators for fault activity. Along these lines, in this work, Rn and CO2 fluxes were measured for soil gases along the Xiaojiang Fault Zone (XFZ), SW China. The relationship among the soil gas Rn and CO2 fluxes, hydrochemistry of hot springs, and fault activity were thoroughly examined based on the analytical results and regional geology. The water samples from the hot springs in the study area were the primary types of Ca·Mg-HCO3, Ca·Mg-HCO3·SO4, and Ca-SO4. It was believed that these types were probably affected by the interactions between the groundwater and carbonate rocks. Based on the weak correlation between the soil gas Rn and the CO2 fluxes, the existence of distinct sources was suggested. The manifestation of a relatively high temperature and ion concentration of the hot spring water samples, as well as the relatively high soil gas Rn and CO2 fluxes, led to a concentrated distribution in the northern and southern segments of the XFZ, which was consistent with the spatial distribution of the seismic activities and fault slip rate. The intense fault activity in the southern and northern segments could also enhance the permeability and accelerate both the water-rock interactions and soil gas emission within the fault zone. An assessment of the earthquake potential and radioactivity protection is considered of outmost significance for the southern and northern segments of the XFZ.

Soil greenhouse gas fluxes from tropical vegetable farms, using forest as a reference

Field-based quantification of soil greenhouse gas emissions from the Philippines’ agriculture sector is missing for vegetable production systems, despite its substantial contribution to agricultural production. We quantified soil N2O emission, CH4 uptake, and CO2 efflux in vegetable farms and compared these to the secondary forest. Measurements were conducted for 13 months in 10 smallholder farms and nine forest plots on Andosol soil in Leyte, Philippines using static chambers. Soil N2O and CO2 emissions were higher, whereas CH4 uptake was lower in the vegetable farms than in the forest. Vegetable farms had annual fluxes of 12.7 ± 2.6 kg N2O-N ha−1 yr−1, −1.1 ± 0.2 kg CH4-C ha−1 yr−1, and 11.7 ± 0.7 Mg CO2-C ha−1 yr−1, whereas the forest had 0.10 ± 0.02 kg N2O-N ha ha−1 yr−1, −2.0 ± 0.2 kg CH4-C ha−1 yr−1, and 8.2 ± 0.7 Mg CO2-C ha−1 yr−1. Long-term high N fertilization rates in vegetable farms resulted in large soil mineral N levels, dominated by NO3– in the topsoil and down to 1-m depth, leading to high soil N2O emissions. Increased soil bulk density in the vegetable farms probably increased anaerobic microsites during the wet season and reduced CH4 diffusion from the atmosphere into the soil, resulting in decreased soil CH4 uptake. High soil CO2 emissions from the vegetable farms suggested decomposition of labile organic matter, possibly facilitated by plowing and large N fertilization rates. The global warming potential of these vegetable farms was 31 ± 2.7 Mg CO2-eq ha−1 yr−1 (100-year time frame).

Trade-offs between high yields and soil CO2 emissions in semi-humid maize cropland in northern China

Optimizing yields while reducing soil CO2 emissions in cropping systems has become the focus of agricultural study. To explore effective ways to achieve the balance, short-term field experiments (2018–2020) were conducted in North China Plain. Three replications of three-factor combinations of two tillage systems (i.e., no-tillage and conventional tillage), two straw management (i.e., no straw and straw mulch), and two nitrogen fertilization levels (i.e., medium nitrogen of 180 kg N ha1 and high nitrogen of 210 kg N ha1) were arranged in a randomized complete block design. It was discovered that soil CO2 emissions were mainly dominated by soil temperature. No-tillage decreased soil CO2 emissions by 7.3% than conventional tillage, straw mulch promoted soil CO2 emissions than no straw, and high fertilization increased CO2 emissions by 21.8% compared to medium fertilization. However, maize yields were not significantly affected by above treatments. The combinations of agricultural practices that achieved the highest maize yields were medium nitrogen in combination with straw mulch under no-tillage and conventional tillage. The combination of no-tillage, straw mulch, and medium nitrogen decreased soil CO2 emissions intensity by 11.5%. It was concluded that the combination of no-tillage, straw mulch amendment, and medium nitrogen achieved a balanced yields and soil CO2 emissions, which should be recommended for local farmers in North China Plain.

Reconstructing Hydrothermal Fluid Pathways and Storage at the Solfatara Crater (Campi Flegrei, Italy) Using Seismic Scattering and Absorption

Imaging of fluid pathways is crucial to characterize processes taking place in hydrothermal systems, a primary cause of volcanic unrest and associated hazards. The joint imaging of seismic absorption and scattering is an efficient instrument to map fluid flow at crustal scale, and specifically in volcanoes; however, this technique has so far been applied to image volcanoes and hydrothermal systems at the kilometre scale. Here, we use data from a meter-scale, active seismic survey inside the shallow structure of the Solfatara crater to obtain the first frequency-dependent near-surface scattering and absorption model of a hydrothermal system. The Solfatara crater is the place used to monitor historic unrest at Campi Flegrei caldera (Italy), a high-risk volcano under continuous surveillance due to its closeness to a densely populated area. Improving the imaging of the shallow part of this system is crucial to broaden the understanding of unrest processes that are progressively characterizing other portions of the eastern caldera. The scattering contrasts highlight the primary structural feature, a fault separating the hydrothermal plume from zones of CO2saturation nearing fumaroles. While high-absorption anomalies mark zones of high soil temperatures and CO2fluxes, low-absorption anomalies indicate zones of very shallow upflow and are caused by contrasts between liquid-rich and vapour-rich fluids coming from mud pools and fumaroles, respectively. All maps show a SW-NE trend in anomalies consistent with fluid-migration pathways towards the eastern fumaroles. The results provide structural constraints that clarify mechanisms of fluid migration inside the crater. The techniques offer complementary geophysical images to the interpretation of hydrothermal processes and prove that seismic attenuation measurements are suitable to map fluid pathways in heterogeneous media at a detailed scale.

Controls on Physical and Chemical Denudation in a Mixed Carbonate‐Siliciclastic Orogen

AbstractMixed siliciclastic‐carbonate active orogens are common on Earth's surface, yet most studies have focused on erosion and weathering in silicate‐rich landscapes. Relative to purely siliciclastic landscapes, the response of erosion and weathering to uplift may differ in mixed‐lithology regions. However, our knowledge of weathering and erosion in mixed carbonate‐silicate lithologies is limited and, thus, so is our understanding of the mechanistic coupling between uplift, weathering, and the carbon cycle. Here, we partition denudation fluxes into erosion and weathering fluxes of carbonates and silicates in the Northern Apennines—a mixed carbonate‐siliciclastic active orogen—using dissolved solutes, the carbonate sand fraction, and existing 10Be denudation rates. Erosion generally dominates total denudation fluxes relative to weathering by an order of magnitude. Carbonate and silicate contributions to erosion vary between lithologic units, but weathering fluxes are systematically dominated by carbonates. Silicate weathering may be kinetically limited, whereas carbonate weathering may be limited by acid supply. Carbonate re‐precipitation estimated by comparing ion ratios (Sr, Ca, Na) from rivers and bedrock suggests that up to 90% of dissolved Ca2+ is lost from carbonate‐rich catchments. Corresponding [Ca2+] estimates for the weathering zone are high, likely driven by high soil CO2 partial pressures (pCO2); however, re‐equilibration with atmospheric pCO2 in rivers converts solutes back into grains that become part of the physical denudation flux. Weathering limits in this landscape therefore differ between the subsurface weathering zone and riverine exports, and our findings suggest that carbon cycle models may overestimate the sensitivity to erosion of solute exports (Ca2+ and HCO3−) derived from carbonate weathering.

Soil CO2 emissions from a rubber plantation on tropical peat during a strong El Niño year

El Niño event potentially enhances the soil CO2 emissions due to the prolonged of the dry season which lowers the groundwater level more. However, field information on the effect of the El Niño years on the soil respiration is limited in Indonesia’s peatlands. The aim of this study is to compare the soil respiration between normal and El Niño years. We measured soil CO2 efflux using a closed chamber system monthly, in near and far positions from rubber trees, on the dry season from July to October, in normal year of 2014 and strong El Niño year of 2015, on a rubber plantation established on peat soil. No significant relationship was found between soil respiration with groundwater level or soil temperature. We found that soil respiration in the strong El Niño year of 2015 of 9.56 μ mol m−2 s−1 was significantly larger than that in the normal year of 2014 at 5.11 μ mol m−2 s−1. We expected this finding can be used as the first baseline information regarding soil CO2 emissions under different climatic conditions and challenging how to manage the peatland for mitigating high soil CO2 emissions during El Niño events.

Impact of an extreme monsoon on CO2 and CH4 fluxes from mangrove soils of the Ayeyarwady Delta, Myanmar

Mangrove ecosystems can be both significant sources and sinks of greenhouse gases (GHGs). Understanding variability in flux and the key factors controlling emissions in these ecosystems are therefore important in the context of accounting for GHG emissions. The current study is the first to quantify GHG emissions using static chamber measurements from soils in disused aquaculture ponds, planted mangroves, and mature mangroves from the Ayeyarwady Delta, Myanmar. Soil properties, biomass and estimated net primary productivity were also assessed. Field assessments were conducted at the same sites during the middle of the dry season in February and end of the wet season in October 2019. Rates of soil CO2 efflux were among the highest yet recorded from mangrove ecosystems, with CO2 efflux from the 8 year old site reaching 86.8 ± 17 Mg CO2 ha−1 yr−1 during February, an average of 862% more than all other sites assessed during this period. In October, all sites had significant rates of soil CO2 efflux, with rates ranging from 31.9 ± 4.4 Mg CO2 ha−1 yr−1 in a disused pond to 118.9 ± 24.3 Mg CO2 ha−1 yr−1 in the 8 year old site. High soil CO2 efflux from the 8 year old site in February is most likely attributable to high rates of primary production and belowground carbon allocation. Elevated CO2 efflux from all sites during October was likely associated with the extreme 2019 South Asian monsoon season which lowered soil pore salinity and deposited new alluvium, stimulating both autotrophic and heterotrophic activity. Methane efflux increased significantly (50–400%) during the wet season from all sites with mangrove cover, although was a small overall component of soil GHG effluxes during both measurement periods. Our results highlight the critical importance of assessing GHG flux in-situ in order to quantify variability in carbon dynamics over time.

Quantitative assessment of deep-seated CO2 leakage around CO2-rich springs with low soil CO2 efflux using end-member mixing analysis and carbon isotopes

This study examined a mountainous area with two hydrochemically distinct CO2-rich springs to understand the origin, flow, and leakage of CO2, which may provide implications for precise monitoring of CO2 leakage in geological carbon storage (GCS) sites. The carbon isotopic compositions of dissolved inorganic carbon (DIC) in CO2-rich water (δ13CDIC) and those of soil CO2 (δ13CCO2) indicated a deep-seated CO2 supply to the near-surface environment in the study area. The hydrochemical difference (e.g. pH, total dissolved solids) for the two CO2-rich springs separated by 7 m, despite similar δ13CDIC and partial pressure of CO2, was considered as the result of different evolution of shallow groundwater affected by deep-seated CO2 preferentially rising along fracture zones. Electrical resistivity tomography also suggested flow through fracture zones beneath the CO2-rich springs, showing low resistivity compared to other surveyed zones. However, soil CO2 efflux was low compared to that in other natural CO2 emission sites, and in particular it was noticeably low near the CO2-rich springs, whereas δ13CCO2 was high close the CO2-rich springs. The dissolution of CO2 in the near-surface water body seemed to decrease the deep-seated CO2 leakage through the soil layer, while δ13CCO2 imprinted the source. End-member mixing analysis was performed to assess the contribution of deep-seated CO2 to the low soil CO2 efflux by assuming that atmospheric CO2 and soil CO2 (by respiration) as well as deep-seated CO2 contribute to the soil CO2 efflux. For each end-member, characteristic δ13CCO2 and CO2 concentrations were defined, and then their apportionment to soil CO2 efflux was estimated. The resultant proportion of deep-seated CO2 was up to 8.8%. Unlike the spatial distribution of high soil CO2 efflux, high proportions exceeding 3% were found around the CO2-rich springs along the east-west valley. The study results indicate that soil CO2 efflux measurement should be combined with carbon isotopic analysis in GCS sites for CO2 leakage detection because CO2 dissolution in the underground water body may blur leakage detection on the surface. The implication of this study is the need to quantitatively assess the contribution of deep-seated CO2 using the soil CO2 concentration, soil CO2 efflux, and δ13CCO2 at each measurement site.

Effects of Land Use Types on CH4 and CO2 Production Potentials in Subtropical Wetland Soils

Changes in land use types can alter the soil and environmental characteristics of wetlands, which in turn influence the magnitude of greenhouse gas production by soil microbes. However, the effects of land use change on the production potential of methane (CH4) and carbon dioxide (CO2) in subtropical wetland soils and the underlying controls are still largely unknown. In this study, we examined the soil CH4 and CO2 production potentials under five different land use types (natural mangrove, Gei Wai water channel, Gei Wai forest, reedbed, and freshwater pond) and their relationships with soil physico-chemical properties in a subtropical wetland in Hong Kong using aerobic and anaerobic laboratory incubation experiments. Our results showed an overall decreasing trend of CH4 and CO2 production potentials down the soil profile at all sites, which could be attributed to a reduction in the concentrations of soil organic matter (SOM), total Kjeldahl nitrogen (TKN) and ammonium nitrogen (NH4+-N). Moreover, the soil CH4 and CO2 production potentials varied significantly in the surface soils among land use types, but were more similar across the sites in the deeper soils. The conversion of natural mangrove to other land use types significantly reduced both the aerobic and anaerobic CO2 production potentials in the top 10 cm soils, except for Gei Wai forest, which demonstrated significantly higher CO2 production rates (61.15–97.91 μg g−1 day−1). Meanwhile, the mean CH4 production potential in the surface soils of natural mangrove (0.05 μg g−1 d−1) was significantly lower than that in the Gei Wai forest and Gei Wai channel (0.26–0.27 μg g−1 day−1) but slightly higher than that in the freshwater pond and reedbed (0.00–0.02 μg g−1 day−1). The high soil CH4 and CO2 production potentials observed in the Gei Wai forest could be explained by the high soil concentrations of SOM, TKN and NH4+-N. On the other hand, the lower anaerobic CH4 and aerobic CO2 productions observed in the reedbed could be attributed to the lower concentrations of NH4+-N and available phosphorus. Our findings highlighted the significant impacts of land use types on the CH4 and CO2 production potentials of subtropical wetland soils, which had practical implications for wetland management for climate change mitigation.

Short-term effects of biochar on soil CO2 efflux in boreal Scots pine forests

Key messageDuring the first summer, wood biochar amendments increased soil temperature, pH, and soil CO2effluxes in a xeric boreal Scots pine forest. The increase of soil CO2efflux could be largely explained by increases in by soil temperature. Higher biochar application rates (1.0 vs 0.5 kg m−2) led to higher soil CO2efflux while the pyrolysis temperature of biochar (500 or 650 °C) had no effect on soil CO2efflux.ContextUsing biochar as a soil amendment has been proposed to increase the carbon sequestration in soils. However, a more rapid soil organic matter turnover after biochar application might reduce the effectiveness of biochar applications for carbon sequestration. By raising the pyrolysis temperature, biochar with lower contents of labile carbohydrates can be produced.AimsTo better understand the effects of biochar on boreal forest soil, we applied two spruce biochar with different pyrolysis temperatures (500 °C and 650 °C) at amounts of 1.0 and 0.5 kg m−2 in a young xeric Scots pine forest in southern Finland.MethodsSoil CO2, microbial biomass, and physiochemical properties were measured to track changes after biochar application during the first summer.ResultsSoil CO2 increased 14.3% in 1.0 kg m−2 treatments and 4.6% in 0.5 kg m−2. Soil temperature and pH were obviously higher in the 1.0 kg m−2 treatments. Differences in soil CO2 among treatments disappear after correcting by soil temperature and soil moisture.ConclusionBiochar increased soil CO2 mainly by raising soil temperature in the short term. Higher biochar application rates led to higher soil CO2 effluxes. The increase in soil CO2 efflux may be transient. More studies are needed to get the optimum biochar amount for carbon sequestration in boreal forest.

Highly efficient colorimetric CO2 sensors for monitoring CO2 leakage from carbon capture and storage sites

Carbon capture and storage (CCS) technology used for reducing anthropogenic CO2 emissions involves the capture of CO2 from industrial sources and its injection into geological sinks, such as oil reservoirs and abandoned gas fields. To ensure environmental and public safety in implementing CCS technology, efficient CO2-monitoring technology must be developed to detect potential CO2 leakage from CCS sites. Conventional CO2 sensors used for monitoring CCS sites are typically high in cost and require professional staff for maintenance. In this study, we developed a portable and low-cost colorimetric CO2 sensor with high soil CO2 detection efficiency for CCS sites. The sensor consists of a detection solution that contains the pH indicator cresol red encapsulated with a gas-permeable membrane. When CO2 enters the sensor through the membrane, the color of the pH indicator changes and this was quantified using an RGB (red, green, blue) application (app), an app that measures the RGB values of a given color. The change in G and B values of the detection solution showed a significant linear relationship with soil CO2 concentration determined via non-dispersive infra-red (NDIR) CO2 sensor (r2 = 0.98, p = 0.001), and thus these values were used for quantification of CO2 concentration. Tests using CO2-injection chamber showed that the optical CO2 sensors can detect soil CO2 concentration of 0.1 to 30% within a few minutes. Field studies conducted at a natural CO2 vent and an artificial CO2 leakage site showed that the optical CO2 sensors can be applied in analyzing surficial CO2 leakage patterns. The advantage of this optical CO2 sensor when applied to field monitoring is that it is inexpensive and has few installation restrictions. Therefore, this optical CO2 sensor has a strong potential for use in monitoring CO2 leakages from CCS sites.

Herbivory modulates soil CO2 fluxes after windthrow: a case study in temperate mountain forests

Ungulate herbivory can alter functional plant communities of early-successional forest ecosystems. The consequences of such vegetation changes on soil carbon cycling are still not fully understood. Here, we used an ungulate exclusion experiment to investigate how different levels of herbivory and associated changes in vegetation succession modulate soil CO2 efflux and its heterotrophic and autotrophic sources following windthrow in temperate mountain forests. Our results indicate that only high levels of ungulate herbivory and associated vegetation shifts from tree to rather grass dominated plant communities affect soil CO2 fluxes. We did not find evidence that a moderate herbivory level and accompanied smaller shifts in the functional plant community affect soil CO2 fluxes. A greater soil CO2 efflux under the influence of high herbivory pressure was primarily attributed to accelerated heterotrophic respiration, likely due to warmer soil conditions. Moreover, autotrophic respiration from grass roots and associated microbial communities is suggested to contribute to higher soil CO2 fluxes. We conclude that intense herbivory and accompanied successional changes in the functional plant community enhance soil carbon losses following forest windthrow. This might have negative consequences for the soil carbon stocks and for the climate system.

Anomalous Discharge of Endogenous Gas at Lavinio (Rome, Italy) and the Lethal Accident of 5 September 2011.

The Rome region contains several sites where endogenous gas is brought to the surface through deep reaching faults, creating locally hazardous conditions for people and animals. Lavinio is a touristic borough of Anzio (Rome Capital Metropolitan City) that hosts a country club with a swimming pool and an adjacent basement balance tank. In early September 2011, the pool and the tank had been emptied for cleaning. On 5 September, four men descended into the tank and immediately lost consciousness. On 12 August 2012, after a long coma the first person died, the second one reported permanent damage to his central nervous system, and the other two men recovered completely. Detailed geochemical investigations show that the site is affected by a huge release of endogenous gas (CO2 ≈ 96 vol.% and H2S ≈ 4 vol.%). High soil CO2 and H2S flux values were measured near the pool (up to 898 and 7.155 g·m−2·day−1, respectively), and a high CO2 concentration (23–25 vol.%) was found at 50–70 cm depth in the soil. We were able to demonstrate that gas had been transported into the balance tank from the swimming pool through two hubs connected to the lateral overflow channels of the pool. We show also that the time before the accident (60 hr), during which the balance tank had remained closed to external air, had been largely sufficient to reach indoor nearly lethal conditions (oxygen deficiency and high concentration of both CO2 and H2S).

CO2 leakage detection in the near-surface above natural CO2-rich water aquifer using soil gas monitoring

Conventional soil gas monitoring methods were applied above a natural low-level CO2 reservoir to locate natural CO2 leakage and assess effectiveness in monitoring geological carbon storage sites. The concentrations of the soil gas (N2, O2, and CO2) and carbon-13 isotopes of soil CO2 (δ13CCO2) were determined for ninety-four soil gas samples that were collected at a depth of ∼60 cm. The CO2 flux was also measured at the same sampling locations. The 93 soil gas samples were divided into two groups: Group A with low soil CO2 concentrations and high δ13CCO2, which was influenced by the atmospheric air, and Group B with high soil CO2 concentrations and low δ13CCO2, which originated from microbial processes. Sample M17, which was close to a CO2-rich water well, had an exceptionally high soil CO2 (36.0% v/v), δ13CCO2 (-5.7‰), and flux (546.2 g/m2/d), indicating geogenic CO2 inflow to the soil layer and discharge through the surface. This study shows that conventional soil gas monitoring methods are useful for locating CO2 leakage. A dense grid soil CO2 sampling near wells and periodic investigations are crucial for further understanding of the CO2 flow paths in the soil layer.

Changes in soil N2O and CH4 emissions and related microbial functional groups in an artificial CO2 gassing experiment

Potential CO2 leakage is a major concern for carbon capture and storage (CCS). The effects of high soil CO2 concentration on microbes is a major element of impact assessments of CO2 leakage on terrestrial ecosystems. We conducted a field experiment to investigate the responses of microbial functional groups of ammonia-oxidizers, methanogens, and methanotrophs in high soil CO2 conditions. A single-point injection gassing plot (2.5 m × 2.5 m in size), which had 52.2% CO2 in the center (radius = 0.5 m) and 5.5% in the edge (radius = 1.7 m) at 10 cm depth, was employed. N2O and CH4 emissions increased after 1 day of injection because injected CO2 was instantly utilized by nitrifiers and methanogens. This suggests that the activities of the selected microbes could be stimulated by high soil CO2 concentrations. Prolonged CO2 injection has toxic effects on aerobic nitrifiers, but may favor anaerobic methanogens. However, the early stimulatory effects of high soil CO2 on N2O and CH4 production did not last to the end of injection. These results imply that increased N2O and CH4 emissions could be the minor side effects of high soil CO2. Microbes responded faster than plants to high soil CO2, with responses observed as late as 7 days after injection. The inhibition of plant absorption of soil water and nutrients by high soil CO2 concentrations may also influence microbial responses. Moreover, high soil water content could retard underground CO2 diffusion, which would magnify CO2 impacts on plants and microbes. Our results suggest that microbial response could be used as an early indicator of the impact assessments of CO2 leakage on soil ecosystems. An understanding of the interaction among soils, plants, and microbes would be helpful in assessing the biological risks of potential CO2 leakage.