Measured Soil CO2 Research Articles

Autotrophic and heterotrophic contributions to short-term soil CO2efflux following simulated summer precipitation pulses in a Mediterranean dehesa

[1] Autotrophic and heterotrophic components of soil CO2 efflux may have differential responses to environmental factors, so estimating the relative contribution of each component during summer precipitation pulses is essential to predict C balance in soils experiencing regular drought conditions. As even small summer rains induced high instantaneous soil respiration rates in Mediterranean wooded grasslands, we hypothesized that standing dead mass, surface litter, and topsoil layer could play a dominant role in the initial flush of CO2 produced immediately after soil rewetting; in contrast, soil CO2 effluxes during drought periods should be mostly derived from tree root activity. In a grazed dehesa, we simulated four summer rain events and measured soil CO2 efflux discontinuously, estimating its δ13C through a Keeling plot nonsteady state static chamber approach. In addition, we estimated litter contribution to soil CO2 efflux and extracted soil available C fractions (K2SO4-extracted C and chloroform-fumigated extracted C). The δ13C-CO2 from in-tube incubated excised tree roots and rewetted root-free soil was −25.0‰ (±0.2) and −28.4‰ (±0.2), respectively. Assuming those values as end-members' sources, the autotrophic component of soil CO2 efflux was dominant during the severe drought, whereas the heterotrophic contribution dominated from the very beginning of precipitation pulses. As standing dead mass and fresh litter contribution was low (<25%) in the first day and negligible after, we concluded that CO2 efflux after rewetting was mostly derived from microbial mineralization of available soil organic C fractions.

Seasonal variation and fire effects on CH4, N2O and CO2 exchange in savanna soils of northern Australia

Tropical savanna ecosystems are a major contributor to global CO2, CH4 and N2O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N2O and CH4 exchange. We measured soil CO2, CH4 and N2O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1–2 years). There was no seasonal change and no fire effect upon soil N2O exchange. Soil N2O fluxes were very low, generally between −1.0 and 1.0μg Nm−2h−1, and often below the minimum detection limit. There was an increase in soil NH4+ in the months after the 2008 fire event, but no change in soil NO3−. There was considerable nitrification in the early wet season but minimal nitrification at all other times.Savanna soil was generally a net CH4 sink that equated to between −2.0 and −1.6kg CH4ha−1y−1 with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH4 uptake. There were short periods of soil CH4 emission, up to 20μg Cm−2h−1, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO2 fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH4 fluxes, but a much stronger relationship with soil CO2 fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N2O source, and possibly even a sink. Annual soil CH4 flux measurements suggest that the 1.9million km2 of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO2-e per year. This sink estimate would offset potentially 10% of Australian transport related CO2-e emissions. This CH4 sink estimate does not include concurrent CH4 emissions from termite mounds or ephemeral wetlands in Australian savannas.

Soil CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; efflux of a larch forest in northern Japan

Abstract. We had continuously measured soil CO2 efflux (Rs) in a larch forest in northern Japan at hourly intervals for the snow-free period in 2003 with an automated chamber system and partitioned Rs into heterotrophic respiration (Rh) and autotrophic respiration (Rr) by using the trench method. In addition, we applied the soil CO2 concentration gradients method to continuously measure soil CO2 profiles under snowpack in the snowy period and to partition Rs into topsoil (Oa and A horizons) CO2 efflux (Ft) with a depth of 0.13 m and sub-soil (C horizon) CO2 efflux (Fc). We found that soil CO2 effluxes were strongly affected by the seasonal variation of soil temperature but weakly correlated with soil moisture, probably because the volumetric soil moisture (30–40% at 95% confidence interval) was within a plateau region for root and microbial activities. The soil CO2 effluxes changed seasonally in parallel with soil temperature in topsoil with the peak in late summer. On the other hand, the contribution of Rr to Rs was the largest at about 50% in early summer, when canopy photosynthesis and plant growth were more active. The temperature sensitivity (Q10) of Rr peaked in June. Under snowpack, Rs was stable until mid-March and then gradually increased with snow melting. Rs summed up to 79 gC m−2 during the snowy season for 4 months. The annual Rs was determined at 934 gC m−2 y−1 in 2003, which accounted for 63% of ecosystem respiration. The annual contributions of Rh and Rs to Rs were 57% and 43%, respectively. Based on the gradient approach, Rs was partitioned vertically into litter (Oi and Oe horizons) with a depth of 0.01–0.02 m, topsoil and sub-soil respirations with proportions of 6, 72 and 22%, respectively, on an annual basis. The vertical distribution of CO2 efflux was consistent with those of soil carbon and root biomass.

Comparison of soil CO 2 flux between uncleared and cleared windthrow areas in Estonia and Latvia

Storms can turn a great proportion of forests’ assimilation capacity into dead organic matter because of windthrow and thus its role as a carbon sink will be diminished for some time. However, little is known about the magnitude or extent to which storms affect carbon efflux. We compared soil CO 2 fluxes in wind-thrown forest stands with different time periods since a storm event, and with different management practices (deadwood cleared or left on-site). This study examined changes in soil CO 2 efflux in two windthrow areas in north-eastern Estonia and one area in north-western Latvia, which experienced severe wind storms in the summers of 2001, 2002 and 1967, respectively. We measured soil CO 2 fluxes in stands formerly dominated by Norway spruce ( Picea abies L. Karst.) with total and partial canopy destruction (all trees or roughly half of the trees in stand damaged by storm), in harvested areas (material removed after the wind storm) and in control areas (no damage by wind). Removal of wind-damaged material decreased instantaneous CO 2 flux from the soil surface. The highest instantaneous fluxes were measured in areas with total and partial canopy destruction (0.67 g CO 2 m −2 h −1 in both cases) compared with fluxes in the control areas (0.51 g CO 2 m −2 h −1), in the new storm-damaged areas where the material was removed (0.57 g CO 2 m −2 h −1) and in the old storm-damaged area where wood was left on site (0.55 g CO 2 m −2 h −1). The only factor affecting soil CO 2 flux was location of the measuring collar (plastic collar with diameter 100 mm, height 50 mm) – either on undamaged forest ground or on the uprooted tree pit, where the mineral soil was exposed after disturbance. New wind-thrown stands where residues are left on site would most likely turn to sources of CO 2 for several years until forest regeneration reaches to substantial assimilation rates. New wind-thrown stands where residues are left on site would most likely tend to have elevated CO 2 fluxes for several years until forest regeneration reaches to substantial assimilation rates. However, forest managers might be concerned about the amounts of CO 2 immediately released into the atmosphere if the harvested logs are burned.

Forest floor CO 2 flux estimated from soil CO 2 and radon concentrations

Having a quantitative understanding of the carbon cycle in forests is of great importance for predicting global warming issues. Carbon dioxide production in soil is the largest CO 2 source in forests, and exhibits large temporal and spatial variations. Continuous observation of soil CO 2 flux at many sites over a forest is therefore necessary to obtain representative soil CO 2 fluxes for the forest. In this study, a gradient method to measure soil CO 2 flux indirectly from soil radon and CO 2 measurements was theoretically modified to conveniently measure the soil CO 2 flux from soil radon and CO 2 concentrations measured at one soil depth. To experimentally test the modified method, a field observation was conducted continuously in a forest over a 31-day period. Since changes in the soil water content near the soil surface were small throughout the observation, a constant effective diffusivity for CO 2 was assumed for the soil CO 2 flux estimation. The soil CO 2 flux was then calculated as the product of the effective diffusivity and the gradient of the soil CO 2 concentration, each calculated from soil radon and CO 2 concentrations. The estimated flux ranged from 1.9 to 5.8 μmol m −2 s −1, and, correlating well with the reference value, measured with a conventional ventilated-chamber method. We therefore conclude that the modified gradient method based on the measurement of soil CO 2 and radon concentration at one depth is reliable, at least under conditions where the change in the soil water content is small.

Zea mays rhizosphere respiration, but not soil organic matter decomposition was stable across a temperature gradient

In a greenhouse experiment, we grew maize plants at different densities. We added fertilizer to half of the pots and created a temperature gradient. After 10 weeks of plant growth, we measured soil CO2 efflux (SCE) and determined rhizosphere respiration (Rrhizo) and the decomposition rate of soil organic matter (RSOM) using the different δ13C of the C3 soil and C4 plants. Whereas Rrhizo remained stable across the temperature gradient, RSOM significantly increased with growth temperature. Neither plant density, nor the fertilizer treatment affected the relation between Rrhizo or RSOM and growth temperature. Although Rrhizo might still increase with temperature in the short term, long term exposure to higher temperatures revealed full thermal acclimation of Rrhizo, but not of RSOM.

Evaluation of methods for estimating soil carbon dioxide efflux across a gradient of forest disturbance

AbstractBetter understanding of variation in soil carbon dioxide (CO2) efflux caused by measurement techniques is needed, especially over gradients of site disturbance, to accurately estimate the global carbon cycle. We present soil CO2 efflux data from a gradient of disturbance to ponderosa pine (Pinus ponderosa C. Lawson var. scopulorum Engelm.) forests in northern Arizona, USA that were obtained using four different techniques: vented static chambers, a Licor 6400‐09, and soil CO2 diffusion profiles using two different models (Moldrup, Millington–Quirk) to estimate soil gas diffusivity. We also compared soil CO2 efflux measured by the Moldrup and Millington–Quirk diffusion profile methods to nighttime total ecosystem respiration (TER) data from an eddy covariance tower. We addressed four questions: (1) Does the use of a given method to measure soil CO2 efflux bias results across a disturbance gradient? (2) Does the magnitude of difference between observed and modeled estimates of soil CO2 differ between methods and across sites? (3) What is the spatial variability of each method at each site? (4) Which method is closest to the estimate of TER measured by the eddy covariance tower? Although soil CO2 efflux varied significantly among methods the differences were consistent among sites. Measured and modeled total growing season fluxes were generally higher for the Licor 6400‐09 and Millington–Quirk diffusion gradient methods compared with static chamber and the Moldrup diffusion gradient methods. A power analysis showed that the larger static chamber was the most efficient method at sampling spatial variation in soil CO2 efflux. Nighttime measurements of soil CO2 efflux from the Moldrup diffusion gradient method were most strongly related to nighttime TER assessed with eddy covariance. The use of a single, well‐implemented method to measure soil CO2 efflux is unlikely to create bias in comparisons across a gradient of forest disturbance.

Permanent shallow subsoil CO 2 flux chambers for monitoring of onshore CO 2 geological storage sites

Public concern over the possibility of migration of stored CO 2 to the surface with resulting damage to vegetation or hazard to humans and animals is a matter which will need to be addressed to be able to satisfy likely regulatory requirements for onshore CO 2 storage in a number of jurisdictions. While soil CO 2 concentration is readily measured continuously and in situ with current technology, the measurement of CO 2 flux at depths below the soil A horizon may be a more sensitive and meaningful technique for early detection of a near surface CO 2 plume. We describe a system for the continuous measurement of soil CO 2 flux at a depth of approximately 1.3 m and present results from three instruments deployed at the Otway Basin Pilot Project in Victoria, Australia and one development system deployed at Sutton, near the Australian Capital Canberra.

Spatiotemporal changes in CO2 emissions during the second ZERT injection, August–September 2008

This study reports the first field test of a multi-channel, auto-dilution, steady-state, soil–CO2 flux monitoring system being developed to help understand the pathways by which fugitive CO2 from a geologic sequestration site migrates to the surface. The test was conducted from late August through mid-October 2008 at the Zero Emissions Research and Technology project site located in Bozeman, MT. Twenty steady-state and five non-steady-state flux chambers were installed in a 10 × 15 m area, one boundary of which was directly above a shallow (2-m depth) horizontal injection well located 0.5 m below the water table. A total flux of 52 kg CO2 day−1 was injected into the well for 13 days and the efflux from the soil was monitored by the chambers before, during, and for 33 days after the injection. The results showed a rapid increase in soil efflux once injection started, with maximal values reached within 3–7 days in most chambers. Efflux returned to background levels within a similar time period after injection ceased. A radial efflux pattern was observed to at least 2 m from the injection well, and evidence for movement of the CO2 plume during the injection, presumably due to groundwater flow, was seen. The steady-state chambers yielded very stable data, but threefold to fivefold higher fluxes than the non-steady-state chambers. The higher fluxes were attributed to vacuum induced in the steady-state chambers by narrow vent tubes. High winds resulted in significant decreases in measured soil CO2 efflux, presumably by enhancing efflux from soil outside the chambers.

Factors controlling soil CO 2 effluxes and the effects of rewetting on effluxes in adjacent deciduous, coniferous, and mixed forests in Korea

To better understand the factors that control forest soil CO 2 efflux and the effects of rewetting on efflux, we measured soil CO 2 efflux in adjacent deciduous, coniferous, and mixed forests in the central part of the Korean Peninsula over the course of one year. We also conducted laboratory rewetting experiments with soil collected from the three sites using three different incubation temperatures (4 °C, 10 °C, and 20 °C). Soil moisture (SM), soil organic matter (SOM), and total root mass values of the three sites were significantly different from one another; however, soil temperature (ST), observed soil CO 2 efflux and sensitivity of soil CO 2 efflux to ST (i.e., Q 10 = 3.7 ± 0.1) were not significantly different among the three sites. Soil temperature was a dominant control factor regulating soil CO 2 efflux during most of the year. We infer that soil CO 2 efflux was not significantly different among the sites due to similar ST and Q 10. Though a significant increase in soil CO 2 efflux following rewetting of dry soil was observed both in the field observations (60–170%) and laboratory incubation experiments (100–1000%), both the increased rates of soil CO 2 efflux and the magnitude of change in SM were not significantly different among the sites. The increased rates of soil CO 2 efflux following rewetting depended on the initial SM before rewetting. During drying phase after rewetting, a significant correlation between SM and soil CO 2 efflux was found, but the effect of ST on increased soil CO 2 efflux was not clear. Cumulative peak soil CO 2 efflux (11.3 ± 0.7 g CO 2 m −2) following rewetting in the field was not significantly different among the sites. Those evidences indicate that the observed similar rewetting effects on soil CO 2 efflux can be explained by the similar magnitude of change in SM after rewetting at the sites. We conclude that regardless of vegetation type, soil CO 2 efflux and the effect of rewetting on soil CO 2 efflux do not differ among the sites, and ST is a primary control factor for soil CO 2 efflux while SM modulates the effect of rewetting on soil CO 2 efflux. Further studies are needed to quantify and incorporate relationship of initial dryness of the soil and the frequency of the dry–wet cycle on soil CO 2 efflux into models describing carbon (C) processes in forested ecosystems.

Assessment of the soil CO2 gradient method for soil CO2 efflux measurements: comparison of six models in the calculation of the relative gas diffusion coefficient

This paper uses a refined soil gradient method to estimate soil CO2 efflux. Six different models are used to determine the relative gas diffusion coefficient (ξ ). A weighted harmonic averaging is used to estimate the soil CO2 diffusion coefficient, yielding a better estimate of soil CO2 efflux. The resulting soil CO2 efflux results are then compared to the soil CO2 efflux measured with a soil chamber. Depending on the choice of ξ model used, the estimated soil CO2 efflux using the gradient method reasonably approximates the efflux obtained using the soil chamber method. In addition, the estimated soil CO2 efflux obtained by this improved method is well described by an exponential function of soil temperature at a depth of 0.05 m with the temperature sensitivity (Q10) of 1.81 and a linear function of soil moisture at a depth of 0.12 m, in general agreement with previous findings. These results suggest that the gradient method is a practical cost-effective means to measure soil CO2 emissions. Results from the present study suggest that the gradient method can be used successfully to measure soil CO2 efflux provided that proper attention is paid to the judicious use of the proper diffusion coefficient.

Performance of Solid‐State Sensors for Continuous, Real‐Time Measurement of Soil CO2 Concentrations

Recent advances in sensor technology provide a robust capability for continuous measurement of soil gases. The performance of solid‐state CO2 sensors (Model GMM220 series, Vaisala, Inc., Helsinki, Finland) was evaluated in laboratory, greenhouse, and irrigated winter wheat (Triticum aestivum L.). In ambient CO2 concentration, the GMM222 sensor averaged 427 ± 8.3 μL L−1. Under variable CO2 concentrations, the sensor was slightly lower than concentrations measured with an infrared gas analyzer (IRGA). In greenhouse pots planted with triticale (Triticale hexaploide Lart.) and an agricultural field of irrigated winter wheat, soil CO2 concentration exceeded the 10,000 μL L−1 limit of the GMM222. Alternatively, the GMM221 sensor, designed to measure between 0 and 20,000 μL L−1, showed soil CO2 concentrations were between 14,000 and 16,000 μL L−1. The GMM222 accurately measures real‐time soil CO2 concentrations under field conditions that were within the sensor detection limit. However, periods of high biological soil activity require the GMM221 sensor with a higher detection limit.

CO 2 evolution from a ridge tilled and a mouldboard ploughed Luvisol in the field

This field study was initiated to measure soil CO 2 evolution of the Turiel ridge tillage system under faba bean ( Vicia faba L.) compared with mouldboard ploughing. The CO 2–C measurements were done in situ with a transportable infrared gas analyzer at different positions on the ridges and furrows and on mouldboard plough plots with and without vegetation, respectively. The CO 2 evolution at different ridge positions reflected a ridge pattern with a CO 2–C evolution in the order crown > shoulder > furrow. On a hectare basis the mouldboard ploughed soil evolved 0.11 t more CO 2–C ha −1 than ridges, and the plots with vegetation evolved 0.58 t more CO 2–C ha −1 than the plots without vegetation during the investigation period of 57 days. At higher air temperatures the soil temperature tended to be higher on ploughed plots compared to ridges. The CO 2 evolution on the planted plots decreased with time, reflecting the decreasing physiological activity of the plant roots. Overall, despite a high degree of soil disturbance, the amount of CO 2 evolved from the ridge tilled soil was lower compared to mouldboard plough tillage, probably reflecting less favorable conditions for soil microorganisms in the ridges compared to the ploughed soil.

The effects of elevated atmospheric CO2 and nitrogen amendments on subsurface CO2 production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO2 concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO2 enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO2 concentration levels (ambient [CO2]a, ambient + 200 ppmv, [CO2]e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 gN m−2 year−1). The combination of these measurements permitted indirect estimation of belowground CO2 production and flux profiles in the mineral soil. Adjacent to the soil CO2 profiles, direct (chamber-based) measurements of CO2 fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO2 profiles, neither [CO2]e nor N fertilization had a statistically significant effect on seasonal soil CO2, CO2 production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO2 concentration depending on the depth. Variations in CO2 were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO2 pulses were induced by rainfall events. The soil effluxes were equal to the CO2 production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO2 profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO2 diffusivity is required for these soil moisture extremes.

Interactions between physical and biotic factors influence CO 2 flux in Antarctic dry valley soils

Soil carbon dioxide (CO 2) flux is an integrative measure of ecosystem functioning representing both biotic and physical controls over carbon (C) balance. In the McMurdo Dry Valleys of Antarctica, soil CO 2 fluxes (approximately −0.1–0.15 μmol m −2 s −1) are generally low, and negative fluxes (uptake of CO 2) are sometimes observed. A combination of biological respiration and physical mechanisms, driven by temperature and mediated by soil moisture and mineralogy, determine CO 2 flux and, therefore, soil organic C balance. The physical factors important to CO 2 flux are being altered with climate variability in many ecosystems including arid forms such as the Antarctic terrestrial ecosystems, making it critical to understand how climate factors interact with biotic drivers to control soil CO 2 fluxes and C balances. We measured soil CO 2 flux in experimental field manipulations, microcosm incubations and across natural environmental gradients of soil moisture to estimate biotic soil respiration and abiotic sources of CO 2 flux in soils over a range of physical and biotic conditions. We determined that temperature fluctuations were the most important factor influencing diel variation in CO 2 flux. Variation within these diel CO 2 cycles was explained by differences in soil moisture. Increased temperature (as opposed to temperature fluctuations) had little or no effect on CO 2 flux if moisture was not also increased. We conclude that CO 2 flux in dry valley soils is driven primarily by physical factors such as soil temperature and moisture, indicating that future climate change may alter the dry valley soil C cycle. Negative CO 2 fluxes in arid soils have recently been identified as potential net C sinks. We demonstrate the potential for arid polar soils to take up CO 2, driven largely by abiotic factors associated with climate change. The low levels of CO 2 absorption into soils we observed may not constitute a significant sink of atmospheric CO 2, but will influence the interpretation of CO 2 flux for the dry valley soil C cycle and possibly other arid environments where biotic controls over C cycling are secondary to physical drivers.

Soil CO2 flux and photoautotrophic community composition in high‐elevation, ‘barren’ soil

Soil-dominated ecosystems, with little or no plant cover (i.e. deserts, polar regions, high-elevation areas and zones of glacial retreat), are often described as 'barren', despite their potential to host photoautotrophic microbial communities. In high-elevation, subnival zone soil (i.e. elevations higher than the zone of continuous vegetation), the structure and function of these photoautotrophic microbial communities remains essentially unknown. We measured soil CO(2) flux at three sites (above 3600 m) and used molecular techniques to determine the composition and distribution of soil photoautotrophs in the Colorado Front Range. Soil CO(2) flux data from 2002 and 2007 indicate that light-driven CO(2) uptake occurred on most dates. A diverse community of Cyanobacteria, Chloroflexi and eukaryotic algae was present in the top 2 cm of the soil, whereas these clades were nearly absent in deeper soils (2-4 cm). Cyanobacterial communities were composed of lineages most closely related to Microcoleus vaginatus and Phormidium murrayi, eukaryotic photoautotrophs were dominated by green algae, and three novel clades of Chloroflexi were also abundant in the surface soil. During the light hours of the 2007 snow-free measurement period, CO(2) uptake was conservatively estimated to be 23.7 g C m(-2) season(-1). Our study reveals that photoautotrophic microbial communities play an important role in the biogeochemical cycling of subnival zone soil.

Temperature controls temporal variation in soil CO2 efflux in a secondary beech forest in Appi Highlands, Japan

Forest soil is a huge reserve of carbon in the biosphere. Therefore to understand the carbon cycle in forest ecosystems, it is important to determine the dynamics of soil CO2 efflux. This study was conducted to describe temporal variations in soil CO2 efflux and identify the environmental factors that affect it. We measured soil CO2 efflux continuously in a beech secondary forest in the Appi Highlands in Iwate Prefecture for two years (except when there was snow cover) using four dynamic closed chambers that automatically open after taking measurements. Temporal changes in soil temperature and volumetric soil water content were also measured at a depth of 5 cm. The soil CO2 efflux ranged from 14 mg CO2 m−2 h−1 to 2,329 mg CO2 m−2 h−1, the peak occurring at the beginning of August. The relationship between soil temperature and soil CO2 efflux was well represented by an exponential function. Most of temporal variation in soil CO2 efflux was explained by soil temperature rather than volumetric soil water content. The Q10 values were 3.7 ± 0.8 and estimated annual carbon emissions were 837 ± 210 g C m−2 year−1. These results provide a foundation for further development of models for prediction of soil CO2 efflux driven by environmental factors.

Development of a CO2 gas analyzer for monitoring soil CO2 concentrations

Measurement of soil CO2 concentrations is important for investigating the dynamics and diffusion of CO2 in soil. In this study, we developed a small CO2 analyzer for measuring in situ-soil CO2 concentrations. The CO2 analyzer consists of a module containing an infrared CO2 gas sensor, a temperature sensor, and a relative humidity sensor. These sensors are installed in a protective box with an air vent, which is suitable for burying in the soil. The output response time of the CO2 analyzer was 349 s, as evaluated from the phase lag after input of known CO2 concentrations. This response time is short enough to measure soil CO2 concentrations, because variations in concentration are slower than the response time of the analyzer. In a field test, we used the CO2 analyzer to measure soil CO2 concentrations at five depths (0–50 cm) over 2.5 months. While the CO2 concentration generally increased with depth, the amplitude of the variation in CO2 concentration decreased with depth. The phase lag of the variations in soil CO2 concentration also increased with depth, as did soil temperature. The tests confirm that the CO2 analyzer is applicable to continuous monitoring of soil CO2 concentrations.

Carbon isotopic composition of soil CO 2 efflux, a powerful method to discriminate different sources feeding soil CO 2 degassing in volcanic-hydrothermal areas

A new method combining measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux was developed in order to qualitatively and quantitatively characterise the CO 2 source feeding the soil CO 2 diffuse degassing. The method was tested in March 2007 at the Solfatara of Pozzuoli volcano degassing area (Naples, Italy) where more than 300 measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux were performed, surveying Solfatara crater and its surroundings. The wide range of CO 2 flux and CO 2 isotopic composition values (from 8.4 g m − 2 d − 1 to 28,834 g m − 2 d − 1 , and from 0.73‰ to − 33.54‰, respectively), together with their statistical distributions suggests the occurrence of multiple CO 2 sources feeding soil degassing. The combined interpretation of flux and isotopic data allows us to identify and characterise two distinct gas sources: a hydrothermal and a biogenic source. The soil CO 2 from the hydrothermal source is characterised by a mean δ 13C CO 2 of − 2.3‰ ± 0.9‰, hence close to the isotopic composition of the fumarolic CO 2 ( δ 13C CO 2 = − 1.48‰ ± 0.22‰) and by a mean CO 2 flux of 2875 g m − 2 d − 1 . The CO 2 from the biogenic source is characterised by a mean δ 13C CO 2 of − 19.4‰ ± 2.1‰, and by a mean CO 2 flux of 26 g m − 2 d − 1 , which are both in the range of the typical values for biologic CO 2 soil degassing. This reliable characterisation of the biogenic CO 2 flux would not have been possible by solely applying a statistical analysis of the CO 2 flux values, which is commonly applied in volcanological studies for the partitioning between background fluxes and anomalous CO 2 fluxes. A map of the Solfatara diffuse degassing structure was derived from the estimated threshold for the biogenic CO 2 flux, highlighting that soil degassing of hydrothermal CO 2 mixed in different proportion with biogenic CO 2 occurs over a large area (~ 0.8 km 2), which extends over the inner part of the Solfatara crater as well as the eastern periphery, corresponding with a NW–SE fault system. The presented method and data analysis are important means of surveillance of the volcanic activity.

Continuous monitoring of soil CO2 flux on Mt. Etna: The 2004–2005 eruption and the role of regional tectonics and volcano tectonics

The soil CO2 flux on Mt. Etna as recorded by the ETNAGAS network (an automatic system for measuring soil CO2 flux and meteorological parameters) started to increase strongly about 5 months prior to the onset of the 2004–2005 eruption and decreased a few months before the end of the eruption. Time delays in the occurrences of anomalies in soil CO2 flux at different sites in the geochemical network constrain the relationship between soil CO2 flux distributions and the tectonic framework of Etna volcano. The anomalies observed before the 2004–2005 eruption support the intrusion of new undegassed magma into the upper feeding system of the volcano (&lt;20 km below sea level). Magma subsequently rose slowly in the volcano conduits, thereby triggering the onset of the 2004–2005 eruption. The time delays in the occurrences of anomalies in combination with spectral analysis indicate the importance of tectonic and volcanotectonic structures in driving the ascent of deep gases within the crust. Moreover, greatest amplitude pulsations of the low‐frequency components of the CO2 flux signals were correlated with the paroxystic activities of the 2004–2005 eruption. This study confirms that CO2 flux variation is a useful indicator for volcanic activity in the surveillance of the Mt. Etna and similar basaltic volcanoes.