Soil Gas Fluxes Research Articles

Soil trace gas fluxes in living mulch and conventional agricultural systems

AbstractRow crop agriculture is a significant source of two major greenhouse gases (GHGs) (carbon dioxide [CO2] and nitrous oxide [N2O]) and the air pollutant precursor ammonia (NH3). Fluxes of these naturally occurring trace gases are often augmented by agricultural practices, such as fertilizer application and crop systems management. A living mulch system (LMS) maintains a live cover crop year‐round and is an emerging agricultural system that can reduce pesticide and fertilizer use while maintaining yields. Multiple trace gas fluxes of GHGs and NH3 had not previously been measured together in an LMS of corn (Zea mays L.) and white clover (Trifolium repens L.). This study compared soil gas fluxes in a white clover LMS with two other cover crop systems and a no‐cover‐crop system. Infrared and gas chromatography measurements were taken over 2 yr in northern Georgia. Mean soil CO2 and N2O fluxes (159.7 kg ha−1 d−1 and 0.027 kg N ha−1 d−1, respectively) observed in LMS plots exceeded those from other treatments. Soil temperature, moisture, potentially mineralizable nitrogen (N), and nitrate partially explained these differences. Mean soil NH3 emissions were greater in LMS (0.089 kg N ha−1 d−1) compared with no cover crop (0.038 kg N ha−1 d−1). Increased N2O and NH3 fluxes could be from release of N from decomposition of clover and from release of N into the soil as the corn shades the clover. Although LMS plots did not reduce trace gas emissions, labile carbon content was at least 100 mg kg−1 greater than other treatments after 2 yr, improving soil health.

Multivariate analysis of CO2, H2S and CH4 diffuse degassing and correlation with fault systems in Agua Caliente - Tzitzio, Michoacán, México

The quantitative relationships between soil gas flux anomalies and faults systems in Agua Caliente-Tzitzio geothermal system were analyzed statistically. The main degassing features are related with thermal springs. The data used in the multivariate analysis are geothermal gas fluxes and spatial structural data (i.e., local fault strike). Exploratory Data Analysis (EDA) and Multivariate Analysis of Variance (MANOVA) were implemented, for the first time, to statistically describe the relation between the location of degassing features and the presence of some local fault systems, discriminating the direction of the main discharge channels and pointing to higher permeability faults. The results show that there is a statistically significant correlation between diffuse degassing of CO2 and CH4 and the N-S and E-W fault systems. Conversely, H2S degassing does not show any correlation with the fault systems, which may be related with the higher H2S reactivity compared with that of CO2 and CH4. It is highly recommended to use EDA and MANOVA techniques in order to establish the main degassing directions in geothermal and volcanic areas.

Diel pattern driven by free convection controls leaf-cutter ant nest ventilation and greenhouse gas emissions in a Neotropical rain forest.

Leaf-cutter ant nests are biogeochemical hot spots where ants live and import vegetation to grow fungus. Metabolic activity and (in wet tropical forests) soil gas flux to the nest may result in high nest CO2 concentrations if not adequately ventilated. Wind-driven ventilation mitigates high CO2 concentrations in grasslands, but little is known about exchange for forest species faced with prolonged windless conditions. We studied Atta cephalotes nests located under dense canopy (leaf area index > 5) in a wet tropical rainforest in Costa Rica, where wind events are infrequent. We instrumented nests with thermocouples and flow-through CO2 sensing chambers. The results showed that CO2 concentrations exiting leaf-cutter ant nests follow a diel pattern with higher values at night. We developed an efflux model based on pressure differences that evaluated the observed CO2 diel pattern in terms of ventilation by (1) free convection (warm, less dense air rises out the nest more prominently at night) and (2) episodic wind-forced convection events providing occasional supplemental ventilation during daytime. Average greenhouse gas emissions were estimated through nest vents at about 78kg CO2eq nest-1year-1. At the ecosystem level, leaf-cutter ant nest vents accounted for 0.2% to1% of total rainforest soil emissions. In wet, clayey tropical soils, leaf-cutter ant nests act as free convection-driven conduits for exporting CO2 and other greenhouse gases produced within the nest (fungus and ant respiration, refuse decay), and by roots and soil microbes surrounding the nest. This allows A. cephalotes nests to be ventilated without reliable wind conditions.

Abiotic pulses and microbial activity lags in greenhouse gas emissions due to tillage

AbstractTillage affects atmosphere–soil greenhouse gas (GHG) flux by opening soil pore spaces releasing pockets of CO2, CH4, and N2O. Tillage may also stimulate microbes responsible for GHG biogenesis and consumption at longer time scales following a discrete tillage event. I measured soil gas flux immediately after three mechanically different tillage events (moldboard plow, rip‐plow, and disc‐till) over 2 mo within the same field. Delayed effects of tillage on soil respiration were measured; CH4 and N2O fluxes via laboratory incubations on soils were collected following tillage events compared to undisturbed soils. Moldboard plowing did not result in immediate pulses of GHG. Carbon dioxide emissions and CH4 influx to soil peaked 1 h post‐plow. Later rip‐plowing caused a more sustained pulse of CO2 and CH4 uptake over 2 h in the field. Disc‐tillage produced a CO2 production pulse‐then‐decline profile, but CH4 and N2O fluxes were highly variable after this event. Greenhouse gas flux from laboratory‐incubated alfalfa soil and fallow soil were similar to soil collected after rip‐plowing. Soils incubated after the last field tillage event produced significantly lower CO2 emissions during lab incubations compared with other undisturbed and plowed soil. These results show that instantaneous tillage effects are not uniform among GHGs, tillage alteration of soil respiration persists for months following a disturbance, and repeated tillage in the field changes microbial regulation of specific GHGs. Tillage has immediate impacts on GHG flux and has the potential for persistent effects on microbial activity that need further investigation as a GHG abatement tool.

Climate change may alter mercury fluxes in northern hardwood forests

Soils are the largest terrestrial pool of mercury (Hg), a neurotoxic pollutant. Pathways of Hg accumulation and loss in forest soils include throughfall, litterfall, soil gas fluxes, and leaching in soil solution, all of which will likely be altered under changing climate. We took advantage of three ongoing climate-change manipulation experiments at the Hubbard Brook Experimental Forest, New Hampshire, USA: a combined growing-season warming and winter freeze-thaw cycle experiment, a throughfall exclusion to mimic drought, and a simulated ice storm experiment to examine the response of the forest Hg cycle to climatic disturbances. Across these three experiments, we compared Hg inputs in throughfall and leaf litterfall and Hg outputs in soil gas fluxes. Soil solution was measured only in the simulated ice storm experiment. We found that northern forest soils retained consistently less Hg by 16–60% in the three climate manipulations compared to the undisturbed controls (~ 7.4 µg Hg m−2 year−1), although soils across all three experiments still served as a net sink for Hg. Growing-season soil warming and combined soil warming and winter freeze-thaw cycles had little effect on litterfall and throughfall flux, but they increased soil Hg0 evasion by 31 and 35%, respectively, relative to the control plots. The drought plots had 5% lower litterfall Hg flux, 50% lower throughfall Hg flux, and 21% lower soil Hg0 evasion than the control plots. The simulated ice storm had 23% higher litterfall Hg flux, 1% higher throughfall Hg flux, 37% higher soil Hg0 evasion, and 151% higher soil Hg leaching than the control plots. These observations suggest that climate changes such as warmer soils in the growing season or more intense ice storms in winter are likely to exacerbate Hg pollution by releasing Hg sequestered in forest soils via evasion and leaching.

Concentric Structures and Hydrothermal Venting in the Western Desert, Egypt

Large-scale concentric structures are enigmatic geological features observed on the Earth surface and on other planetary bodies. Their formation has been attributed to several processes. Here we describe for the first time the results of mapping and characterization of ∼100 large concentric circular structures found in Early Cenomanian argillaceous strata of the Bahariya depression (Egyptian Western Desert). The geological processes that lead to the formation of these features have remained so far elusive. We investigate the concentric structures with a multidisciplinary approach combining field observations, statistical analysis, soil-flux gas measurements and laboratory analyses of rock samples. The whole depression is dissected by the ∼90 km long N60°E-striking dextral strike-slip Bahariya fault. Effusive lavas and shallow intrusions crop out in the down faulted blocks. The mapped circular structures increase in number approaching the fault zone. These features are up to 10 m high and 625 m wide, have a morphology similar in shape to impact craters with steeper external flanks and a gently dipping internal subsided zone. Halite-cemented brecciated sediments from different geological units have been sampled in the central part of the concentric circular structures implying a subsurface mechanism involved in their formation. Petrography analyses revealed also the presence of high- and low-temperature minerals (e.g., Ba-K-feldspars and ferroaluminoceladonite) suggesting former phases of hydrothermal circulation. Soil-gas flux profiles (CO₂ and CH₄) reveal a modest CO₂ increase when crossing the central part of the circular structures inferring enhanced permeability. Field and laboratory data are consistent with a scenario envisaging a diffused and vigorous hydrothermal venting. The proposed scenario includes multiple phases where several geological elements and processes interact. The Bahariya fault, which activity initiated during the Late Cretaceous, provided pathways for Miocene magma ascent toward the surface and for the development of a network of subsurface intrusions in the organic-rich sedimentary rocks of the Bahariya Formation. The interaction of the igneous intrusions with carbon-rich sedimentary deposits produced overpressured fluids, causing the formation of sparse hydrothermal vents at the surface. The elongation of the main axis of the vents and the deformed structures located within the strike-slip zone suggest that faulting controlled the emplacement and the final shape of some of the hydrothermal vents. We speculate that this system may represent a palaeo sediment-hosted hydrothermal system and could be related to the opening of the Red Sea.

Spatial and temporal variation of CO2 and CH4 emissions from a septic tank soakaway

CO2 and CH4 flux measurements over a septic tank soakaway located in a northern maritime climate (Ireland) were conducted for a period of 81 days using a multi-chamber automated soil gas flux chamber system with high spatial and temporal resolution. Overall median CO2 fluxes were 7.28; 6.40 μmol CO2 m−2 s−1 from the soakaway and control soil, respectively. Overall median CH4 fluxes were − 0.28; −0.67 nmol CH4 m−2 s−1 from the soakaway and control soil, respectively. While CO2 fluxes expressed strong diurnal variability driven by soil temperature, CH4 fluxes were less affected by environmental factors and effectively limited to the first few meters from the septic tank. However, localised CH4 degassing events were observed during drying conditions with up to 60-times higher fluxes compared to the overall median. The soakaway was found to be a net emitter of both CO2 and CH4, releasing a total of 7.327 kg CO2 yr−1 and 0.033 kg CO2Eq. yr−1, respectively. The apparent spatio-temporal heterogeneity of observed soil gas fluxes identified in this study emphasises the importance of integrating measurements with both high spatial and temporal resolution from on-site installations as engineered nature-based solutions.

Denitrifiers, nitrogen-fixing bacteria and N2O soil gas flux in high Arctic ice-wedge polygon cryosols.

Climate warming and subsequent permafrost thaw may result in organic carbon and nutrient stores being metabolized by microbial communities, resulting in a positive feedback loop of greenhouse gas (GHG) soil emissions. As the third most important GHG, understanding nitrous oxide (N2O) flux in Arctic mineral ice-wedge polygon cryosols and its relationship to the active microbial community is potentially a key parameter for understanding future GHG emissions and climatic warming potential. In the present study, metatranscriptomic analyses of active layer Arctic cryosols, at a representative ice-wedge polygon site, identified active nitrogen-fixing and denitrifying bacteria that included members of Rhizobiaceae, Nostocaceae, Cyanothecaceae, Rhodobacteraceae, Burkholderiaceae, Chloroflexaceae, Azotobacteraceae and Ectothiorhodospiraceae. Unique microbial assemblages with higher proportion of Rhodobacteriales and Rhocyclales were identified by targeted functional gene sequencing at locations with higher (P = 0.053) N2O emissions in the wetter trough soils compared with the dryer polygon interior soils. This coincided with a higher relative abundance of the denitrification nirS gene and higher nitrate/nitrite concentrations in trough soils. The elevated N2O flux observed from wetter trough soils compared with drier polygon interior soils is concerning from a climate warming perspective, since the Arctic is predicted to become warmer and wetter.

Gas Diffusivity in the Forest Humus Layer

While several diffusivity models have been developed for mineral soils, there are few data for the forest humus layers that overlie, and are distinct from, the mineral soil. We measured soil gas diffusivity of the organic layer of spruce and beech stands. Total pore space and relative soil gas diffusivity were much higher in the humus layer than in mineral soil. An exponential diffusivity function was used to describe relative soil gas diffusivity in the humus layer of all spruce sites using one parameter set. The best-fit parameters exceeded the range recommended for mineral soil, indicating the distinctness in pore structure of mineral soil and the organic layer. Different model parameter sets were required to describe the different types of humus at the beech sites. As a consequence, traditional predictive models developed for mineral soil should not be used to derive diffusivity values to interpret the aeration status of the forest humus layer. Specific parameter sets should be used for calculating soil gas fluxes through these layers using the gradient flux approach.

Assessing the long-term effects of zero-tillage on the macroporosity of Brazilian soils using X-ray Computed Tomography

Zero-tillage (ZT) is being increasingly adopted globally as a conservationist management system due to the environmental and agronomic benefits it provides. However, there remains little information on the tillage effect on soil pore characteristics such as shape, size and distribution, which in turn affect soil physical, chemical and biological processes. X-ray micro Computed Tomography (μCT) facilitates a non-destructive method to assess soil structural properties in three-dimensions. We used X-ray μCT at a resolution of 70 μm to assess and calculate the shape, size and connectivity of the pore network in undisturbed soil samples collected from a long-term experiment (~30 years) under zero tillage (ZT) and conventional tillage (CT) systems in Botucatu, Southeastern Brazil. In both systems, a single, large pore (>1000 mm3) typically contributed to a large proportion of macroporosity, 91% in CT and 97% in ZT. Macroporosity was higher in ZT (19.7%) compared to CT (14.3%). However the average number of pores was almost twice in CT than ZT. The largest contribution in both treatments was from very complex shaped pores, followed by triaxial and acircular shaped. Pore connectivity analysis indicated that the soil under ZT was more connected that the soil under CT. Soil under CT had larger values of tortuosity than ZT in line with the connectivity results. The results from this study indicate that long-term adoption of ZT leads to higher macroporosity and connectivity of pores which is likely to have positive implications for nutrient cycling, root growth, soil gas fluxes and water dynamics.

Boreal forest soil CO2 and CH4 fluxes following fire and their responses to experimental warming and drying

Boreal forests store large amounts of organic carbon and are susceptible to climate changes, particularly rising temperature, changed soil water and increased fire frequency. The young post-fire ecosystems might occupy larger proportions of the boreal forests region with the expected increases in fire frequency in the future and change the carbon (C) balance of this region. However, it is unclear how soil C fluxes in the post-fire boreal forest response to the climate changes. Therefore, a two-year field experiment was conducted in a boreal forest to investigate the effects of fire on the soil C (CO2 and CH4) fluxes and the responses of these fluxes to simulated warmer and drier climate conditions. The results showed that the boreal forest recovered form wildfire 7–8 years had higher soil CO2 flux than the mature forest. Furthermore, the treatments of warming, drying and the combination of warming and drying increased growing season cumulative soil CO2 flux in the post-fire forest by 15.8%, 20.4% and 34.2%, respectively. However, the boreal forest soil changed from a weak CH4 source to a weak CH4 sink after fire disturbance. Although CH4 absorption increased by warming and drying treatments, the interaction of warming and drying led to a decrease in soil CH4 uptake. The results indicated that the post-fire soil showed CO2 and CH4 fluxes with a greater global warming potential than before burning and that the global warming potential of the soil gas fluxes further increased by warming and drying. The predictive power of models of C cycle-climate feedbacks could be increased by incorporating the distinct ecosystem following fire with permafrost degradation and climate change across the boreal zone.

Response of Black Ash Wetland Gaseous Soil Carbon Fluxes to a Simulated Emerald Ash Borer Infestation

The rapid and extensive expansion of emerald ash borer (EAB) in North America since 2002 may eliminate most existing ash stands, likely affecting critical ecosystem services associated with water and carbon cycling. To our knowledge, no studies have evaluated the coupled response of black ash (Fraxinus nigra Marsh.) wetland water tables, soil temperatures, and soil gas fluxes to an EAB infestation. Water table position, soil temperature, and soil CO2 and CH4 fluxes were monitored in nine depressional headwater black ash wetlands in northern Michigan. An EAB disturbance was simulated by girdling (girdle) or felling (ash-cut) all black ash trees with diameters greater than 2.5 cm within treated wetlands (n = 3 per treatment). Soil gas fluxes were sensitive to water table position, temperature, and disturbance. Soil CO2 fluxes were significantly higher, and high soil CH4 fluxes occurred more frequently in disturbed sites. Soil CH4 fluxes in ash-cut were marginally significantly higher than girdle during post-treatment, yet both were similar to control sites. The strong connection between depressional black ash wetland study sites and groundwater likely buffered the magnitude of disturbance-related impact on water tables and carbon cycling.

Soil δ13C-CO2 and CO2 flux in the H2S-rich Rotorua Hydrothermal System utilizing Cavity Ring Down Spectroscopy

Diffuse soil CO2 flux and stable carbon isotope surveys facilitate interpretation of the nature and structure of hydrothermal systems. Although soil CO2 flux measurements are completed in the field, isotopic carbon in CO2 (i.e., δ13C-CO2) analyses are more commonly performed in a stable isotope mass spectrometry laboratory, thereby, increasing the effort, time and resources required to complete the survey. Cavity Ring Down Spectroscopy (CRDS) overcomes these challenges by enabling both CO2 isotopic and flux analyses in field settings. However, H2S interference has limited CRDS applications in H2S-rich hydrothermal settings. Here, we assess the application of a mobile CRDS system equipped with a simple Cu wire trap to complete δ13C-CO2 and CO2 flux surveying in the relatively high H2S Rotorua Hydrothermal System (RHS), North Island, New Zealand. Laboratory experiments demonstrate that the Cu trap was able to remove H2S (~250 ppmv) enabling accurate and precise δ13C-CO2 measurements. In the RHS, the Cu trap successfully removed H2S concentrations during in situ soil gas analyses. Carbon dioxide fluxes determined using the CRDS were found be 102% of those values produced by a traditional soil gas flux meter (i.e., Licor). Carbon dioxide flux determined by CRDS was similar to previous studies in the RHS; however, zones of high CO2 flux in central Kuirau Park have moved ~100–200 m in the past twelve years. Isoscapes of δ13C-CO2 for the RHS were constructed from our soil gas survey results and were used to assess the apportionment of magmatic-hydrothermal versus biogenic CO2 release. Our findings demonstrate that a CRDS enabled system allows for faster in field survey measurement, lower cost, and reactive sampling for both δ13C-CO2 and CO2 flux measurement in H2S-enriched hydrothermal systems.

Portable Automation of Static Chamber Sample Collection for Quantifying Soil Gas Flux.

Quantification of soil gas flux using the static chamber method is labor intensive. The number of chambers that can be sampled is limited by the spacing between chambers and the availability of trained research technicians. An automated system for collecting gas samples from chambers in the field would eliminate the need for personnel to return to the chamber during a flux measurement period and would allow a single technician to sample multiple chambers simultaneously. This study describes hamber utomated ampling quipment (Flux) to collect and store chamber headspace gas samples at assigned time points for the measurement of soil gas flux. The FluxCASE design and operation is described, and the accuracy and precision of the FluxCASE system is evaluated. In laboratory measurements of nitrous oxide (NO), carbon dioxide (CO), and methane (CH) concentrations of a standardized gas mixture, coefficients of variation associated with automated and manual sample collection were comparable, indicating no loss of precision. In the field, soil gas fluxes measured from FluxCASEs were in agreement with manual sampling for both NO and CO. Slopes of regression equations were 1.01 for CO and 0.97 for NO. The 95% confidence limits of the slopes of the regression lines included the value of one, indicating no bias. Additionally, an expense analysis found a cost recovery ranging from 0.6 to 2.2 yr. Implementing the FluxCASE system is an alternative to improve the efficiency of the static chamber method for measuring soil gas flux while maintaining the accuracy and precision of manual sampling.

Long-term grazing effects on soil-atmosphere exchanges of CO2, CH4 and N2O at different grasslands in Inner Mongolia: A soil core study

Regional greenhouse gas (GHG) budgets in vast grasslands may be changing due to overgrazing and grassland types. However, the comprehensive effects of grazing patterns, environmental factors and grassland types on soil carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) exchanges have been poorly studied. This study investigates the effects of long-term grazing on the soil-atmosphere exchanges of CO 2 , CH 4 and N 2 O in important processes within different grasslands in Inner Mongolia, China. Using manual static chamber and gas chromatography, we measured the fluxes of CO 2 , CH 4 and N 2 O from intact soil cores of paired grazed/ungrazed sites collected from two typical steppes (S tipa grandis and Leymus chinensis ): one wetland in a flood plain and one desert steppe in the region of the Xilin River catchment, Inner Mongolia. Soil gas flux and concentration measurements were conducted in four simulated conditions ( i.e. , drought, dry-wet, intense rainfall and freeze-thaw), which represent important processes in annual GHG exchanges. Extreme drought significantly inhibited CO 2 and N 2 O emissions in all plots but did not change the CH 4 uptake by typical steppes. Dry-wet transition and intense rainfall could remarkably promote soil CO 2 emission pulses at different types, significantly decrease CH 4 uptake by typical steppes, and arouse N 2 O emission pulses at all plots with different times of occurrence. During the freeze-thaw simulation, temperature-induced soil CO 2 emission and CH 4 uptake/emission presented a clear alternative variation, while soil thaw only slightly increased (<15 μg N m −2 h −1 ) in the steppes and sand dunes and was significantly higher in the wetland (11–96 μg N m −2 h −1 ). Long-term grazing significantly inhibited soil respiration rates at all grassland types, significantly decreased CH 4 uptake by the Leymus chinensis steppes, and did not show significant influence on N 2 O emission due to large spatial variations for all types. Compared to the ungrazed Leymus steppes, Stipa steppes, sand dune and wetland, continuously grazed sites were significantly reduced by 22%, 38%, 48% and 47% in total GHG emissions, respectively. Our results indicate that the potential of the steppe soil CH 4 sink function can be offset by N 2 O emission, especially in over-grazed plots. Furthermore, N 2 O emissions should be considered in wetland rangelands with significantly higher N 2 O emission potential (range: 0–343 μg N m −2 h −1 ) more than steppes (range: 0–132 μg N m −2 h −1 ) and sand dunes (range: 0–49 μg N m −2 h −1 ). Nevertheless, comprehensive evaluation of the grazing effect on ecosystem GHG emissions merits consider in both field observation and incubation experiments because soil properties and environmental factors could be changed by vegetation growth in different grazing practices.

Origin of warm springs in Banks Peninsula, New Zealand

Thermal springs present rare opportunities to locate and interpret the geological drivers of upper-crustal fluid flow. Interpreting the conditions through which crustal fluids are heated and released to the surface is important for advancing our understanding of crustal deformation and geothermal resource potential across tectonic contexts. In New Zealand, the majority of thermal springs are associated with magmatic-hydrothermal systems in the central North Island or with the rapidly uplifting bedrock in the South Island's convergent fault systems. However, low enthalpy systems outside of these areas represent attractive targets for potential geothermal resource development. The low enthalpy warm springs of Banks Peninsula, located immediately adjacent to Christchurch, represent a highly understudied but potentially significant resource to the South Island's most densely populated metropolitan area. Hosted within the eroded 11–5.8 Ma volcanic complex of Banks Peninsula, these warm springs (14.5–33.6 °C) represent an anomalous hydrothermal system that has been perturbed by the 2010–2016 Canterbury Earthquake Sequence (CES). The February 22, 2011, Mw 6.2 earthquake induced observable changes to the Banks Peninsula warm springs system, including the appearance of new warm springs within the peninsula's north-western Hillsborough Valley. We assess the origins of the volcanically-hosted Banks Peninsula warm springs post-CES using an integrated isotopic, geochemical, and soil gas flux approach. Additionally, we elucidate the tectonic context and geological drivers of upper-crustal fluid flow in the Banks Peninsula warm spring system. Aqueous phase emissions from the springs predominantly plot within the Na+ + K+/HCO3− type waters and exhibit δ18O, δD, and δ13C values of −8.30 to −9.26‰ V-SMOW, −60.15 to −64.19‰ V-SMOW, and −12.37 to −15.06‰ V-PDB, respectively. Soil gas flux surveys of the springs at Rapaki Bay revealed CO2 fluxes that average 6.93 ± 10 gm-2 day−1, with an average δ13C value of −19.81 ± 5‰ V-PDB, and CH4 fluxes that average 5.58 ± 12 gm-2 day−1, with an average δ13C value of −59.52 ± 1‰ V-PDB. Our results suggest that the Banks Peninsula warm springs are a structurally controlled, upper-crustal metamorphic hydrothermal heated system, sourced from high-altitude Southern Alps derived meteoric waters. Carbon isotope compositions of gaseous emissions associated with the Banks Peninsula thermal springs are consistent with an upper-crustal metamorphic decarbonation and decarboxylation carbon source. Based on their geochemistry, we propose that the Banks Peninsula warm springs should be considered an outboard extension of the South Island's plate-boundary hydrothermal system. Such connectivity implies that long-lived low-enthalpy geothermal resources may be associated with permeable and distributed fault networks in the periphery of convergent margins.

Biochar influences on soil CO2 and CH4 fluxes in response to wetting and drying cycles for a forest soil

Biochar has been the focus of significant research efforts in agriculture, but little research has been conducted in forested ecosystems. Here, we assess CO2 and CH4 fluxes from a forest soil in response to biochar additions using a before-after-control-intervention experimental design. Soil CO2 and CH4 fluxes were measured over a series of wetting cycles by coupling soil mesocosms equipped with auto-chambers to a laser-based spectrometer for high-frequency measurements of gas fluxes and related soil processes. We found that soil CO2 fluxes were higher and CH4 fluxes were less negative (e.g. reduced CH4 uptake) for the biochar-amended soil compared to the no biochar condition. Furthermore, biochar improved soil infiltrability under wet conditions, and enhanced soil moisture levels under dry conditions. Biochar additions shifted the point of maximum soil respiration (i.e. soil CO2 efflux) to a slightly wetter soil moisture level. The point of maximum CH4 uptake was also shifted to a slightly wetter moisture level for soil with biochar. Overall differences in soil gas fluxes were found to be minor compared to the increase in soil carbon resulting from the biochar addition. Biochar may thus contribute to improved forest management through increases to soil carbon stocks and improved soil moisture levels.

2D profiles of CO2, CH4, N2O and gas diffusivity in a well aerated soil: measurement and Finite Element Modeling

Soil gas fluxes depend on soil gas concentrations and physical properties of a soil. Taking soil samples for physical analysis into the laboratory strongly modifies soil gas concentrations and also cuts roots that sustain the activity in the rhizosphere. Since microbial processes interact with gas concentrations in soil, we need to study gas transport and production in situ.We developed a method to monitor the transport and production and consumption of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in soils in situ in a two dimensional (2D) profile using tetrafluoromethane (CF4) and sulfur hexafluoride (SF6) as tracer gases and Finite Element Modeling of soil gas transport. Continuous injection of the inert tracer gases and 2D gas sampling in a soil profile allowed for inverse modeling of the 2D profile of soil gas diffusivity. In a second step, the 2D profiles of the production and consumption of CO2, CH4, and N2O were inversely determined.Soil gas concentrations were monitored in a Scots pine stand in South-West Germany during a rain-free week in the fall. The 2D relative (so as to be independent of gas species) soil gas diffusivity profile showed large horizontal variability. Relative soil gas diffusivity was found to be anisotropic with the vertical direction greater by a factor of 1.26. Topsoil moisture decreased slowly over time resulting in an increase in relative soil gas diffusivity. The soil was found to be a source of CO2, and a net sink of CH4 and N2O, with the highest production (CO2) and consumption (CH4, N2O) occurring in the topsoil. The gas concentration and production profiles of CO2 were nearly horizontally homogenous, while those for CH4 showed larger horizontal differences. Net consumption of CH4 and net production of CO2 both increased as the soil dried. This occurred despite reverse trends for these variables in the topsoil (0–8cm depth) which were more than offset by the underlying soil becoming more active. Sensitivity tests showed that the determination of 2D profiles of soil gas diffusivity and production and consumption of CO2 and CH4 were more reliable than the estimates for N2O because the magnitudes of these for N2O were very low. Our method represents a useful tool for the analyses of soil gas flux heterogeneities and associated microbial processes within soil profiles.

What may be the Consequences of a CO2 Leakage? Insights from Soil Gas Measurements in an Urban Area – Clermont-Ferrand, French Massif Central

This study presents the results of soil gas concentrations and flux monitoring in an urban area with emphasis on CO2 gas. First goal is to establish if gas hazard may exist at city scale and, if not, to locate areas where the presence of gas emissions is likely to occur. Second objective is to evaluate the ways risk management can be undertaken in such populated areas considering learnings from other countries. As CO2 leakage is also of concern for Carbon Capture and Storage applications, comparisons are made between the present study and storage sites.

Drivers of Plot-Scale Variability of CH4 Consumption in a Well-Aerated Pine Forest Soil

While differences in greenhouse gas (GHG) fluxes between ecosystems can be explained to a certain degree, variability of the same at the plot scale is still challenging. We investigated the spatial variability in soil-atmosphere fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) to find out what drives spatial variability on the plot scale. Measurements were carried out in a Scots pine (Pinus sylvestris L.) forest in a former floodplain on a 250 m2 plot, divided in homogenous strata of vegetation and soil texture. Soil gas fluxes were measured consecutively at 60 points along transects to cover the spatial variability. One permanent chamber was measured repeatedly to monitor temporal changes to soil gas fluxes. The observed patterns at this control chamber were used to standardize the gas fluxes to disentangle temporal variability from the spatial variability of measured GHG fluxes. Concurrent measurements of soil gas diffusivity allowed deriving in situ methanotrophic activity from the CH4 flux measurements. The soil emitted CO2 and consumed CH4 and N2O. Significantly different fluxes of CH4 and CO2 were found for the different soil-vegetation strata, but not for N2O. Soil CH4 consumption increased with soil gas diffusivity within similar strata supporting the hypothesis that CH4 consumption by soils is limited by the supply with atmospheric CH4. Methane consumption in the vegetation strata with dominant silty texture was higher at a given soil gas diffusivity than in the strata with sandy texture. The same pattern was observed for methanotrophic activity, indicating better habitats for methantrophs in silt. Methane consumption increased with soil respiration in all strata. Similarly, methanotrophic activity increased with soil respiration when the individual measurement locations were categorized into silt and sand based on the dominant soil texture, irrespective of the vegetation stratum. Thus, we suggest the rhizosphere and decomposing organic litter might represent or facilitate a preferred habitat for methanotrophic microbes, since rhizosphere and decomposing organic are the source of most of the soil respiration.