Measured Soil CO2 Research Articles

Diel patterns of soil respiration in a tropical forest after Hurricane Wilma

Hurricanes are infrequent disturbances that affect tropical ecosystems and cause obvious damage on aboveground variables, but their impact on soil respiration is unclear. This study is the first attempt to continuously measure soil CO2concentrations in a seasonally dry tropical forest to describe diel patterns of soil CO2and respiration. Here we used solid‐state CO2sensors and the gradient flux method to calculate soil respiration following the unexpected pass of hurricane Wilma at the Yucatán Peninsula, Mexico. Mean annual soil respiration for the year following the hurricane was 10.4μmol CO2m−2s−1with a cumulative soil respiration of 3821 g CO2m−2a−1. Soil CO2concentrations returned to prehurricane levels after 6 months at 16 cm depth, however, soil CO2remained higher at 2 and 16 cm depth 1 year after the event. Contrary to previous studies, our results showed that soil respiration was 18% higher 1 year after the hurricane than during the same dates before the event. Diel patterns of soil respiration were decoupled from soil temperature with higher values at night between 2400 and 0600 h. Between 67 and 70% of total soil respiration was produced in the 2–8 cm layer, and this contribution was relatively constant across the sampled year and before the hurricane. We found seasonal shifts in the amplitude and shape and of diel hysteresis effects of soil respiration suggesting that other biophysical mechanisms regulate daily patterns of soil respiration other than soil temperature alone in this tropical forest.

Comparison of eddy covariance, chamber, and gradient methods of measuring soil CO 2 efflux in an annual semi-arid grass, Bromus tectorum

Eddy covariance measures net ecosystem exchange of CO 2 (NEE) at a scale between chamber-based measurements of CO 2 exchange processes and large-scale models of CO 2 flux dynamics. As the intermediate, it represents a link between small and large-scale estimates of NEE. Accuracy is therefore critical. However, estimates of nighttime ecosystem respiration based on scaled-up measurements of soil and leaf CO 2 exchange are most often larger than from eddy covariance. Identifying the source of the discrepancy is difficult due to large measurement uncertainties associated with high variability of fluxes in complex ecosystems. This study compared measurements in a simple system that allowed for minimal uncertainty. We compared measurements of soil efflux using (1) soil chambers, (2) the soil CO 2 gradient technique and ecosystem respiration using the (3) the eddy covariance method from a surface that was covered with living vegetation, straw, and snow in turn through a year. Results showed general agreement among measurements in a range of conditions during canopy absence indicating that each measurement technique is theoretically sound. However, we found disagreement among measurements in specific conditions that indicated certain limitations with each method. Nighttime eddy covariance measurements of ecosystem respiration were below the uncertainty limits of soil respiration measurements during the period of active canopy growth (leaf area index from approx. 0.3–1.0 m 2 m −2. This raises questions about the accuracy of nocturnal eddy covariance measurements over more complex surfaces. There was indication that the chamber method estimates were unrepresentative of the footprint in certain conditions due to within collar surface treatment and due to collar interaction with the environment. Lastly, the gradient method failed to represent surface fluxes during summer rain. To measure soil efflux in all conditions typical of this site, a combination of all three methods is recommended. A combined NEE estimate for 2005 for soil efflux was 406 ± 73 g C m −2.

Soil CO2 respiration: Comparison of chemical titration, CO2 IRGA analysis and the Solvita gel system

AbstractThe measurement of soil carbon dioxide respiration is a means to gauge biological soil fertility. Test methods for respiration employed in the laboratory vary somewhat, and to date the equipment and labor required have somewhat limited more widespread adoption of such methodologies. The purpose of this research is to compare the results of measured soil CO2 respiration using three methods: (1) titration method; (2) infrared gas analysis (IRGA); and (3) the Solvita gel system for soil CO2 analysis. We acquired 36 soil samples from across the USA for comparison, which ranged in pH from 4.5 to 8.5, organic C from 0.8 to 4.6% and the clay content from 6 to 62%. All three methods were highly correlated with each other after 24-h of incubation (titration and Solvita r2=0.82, respirometer and Solvita r2=0.79 and titration versus respirometer r2=0.95). The 24-h (1-day) CO2 release from all three methods was also highly correlated to both basal soil respiration (7–28 days) and cumulative 28-day CO2 respiration. An additional 24 soil samples were acquired and added to the original 36, for a total of 60 soil samples. These samples were used for calibration of the Solvita gel digital color reader results using CO2-titration results and regression analysis. Regression analysis resulted in the equation y=20.6∗(Solvita number)−16.5 with an r2 of 0.83. The data suggest that the Solvita gel system for soil CO2 analysis could be a simple and easily used method to quantify soil microbial activity. Applications may also exist for the gel system for in situ measurements in surface gas chambers. Once standardized soil sampling and laboratory analysis protocols are established, the Solvita method could be easily adapted to commercial soil testing labs as an index of soil microbial activity.

Temporal Variability in Soil CO 2 Emission in an Orchard Forest Ecosystem

Temporal variability in soil CO 2 emission from an orchard was measured using a dynamic open-chamber system for measuring soil CO 2 efflux in Heshan Guangdong Province, in the lower subtropical area of China. Intensive measurements were conducted for a period of 12 months. Soil CO 2 emissions were also modeled by multiple regression analysis from daily air temperature, dry-bulb saturated vapor pressure, relative humidity, atmospheric pressure, soil moisture, and soil temperature. Data was analyzed based on soil moisture levels and air temperature with annual data being grouped into either hot-humid season or relatively cool season based on the precipitation patterns. This was essential in order to acquire simplifled exponential models for parameter estimation. Minimum and maximum daily mean soil CO 2 efflux rates were observed in November and July, with respective rates of 1.98 ± 0.66 and 11.04 ± 0.96 μmol m −2 s −1 being recorded. Annual average soil CO 2 emission ( F CO 2) was 5.92 μmol m −2 s −1. Including all the weather variables into the model helped to explain 73.9% of temporal variability in soil CO 2 emission during the measurement period. Soil CO 2 efflux increased with increasing soil temperature and soil moisture. Preliminary results showed that Q 10, which is deflned as the difierence in respiration rates over a 10 °C interval, was partly explained by flne root biomass. Soil temperature and soil moisture were the dominant factors controlling soil CO 2 efflux and were regarded as the driving variables for CO 2 production in the soil. Including these two variables in regression models could provide a useful tool for predicting the variation of CO 2 emission in the commercial forest soils of South China.

Significance of soil temperature and moisture for soil respiration in a Chinese mountain area

We measured soil CO 2 efflux rate at 11 sites with different vegetation types, elevations and soil textures in a mountain area near Taiyuan city, China, over a period of 1 year. The aim was to understand the seasonal and spatial changes of soil respiration ( R s) and its responses to soil temperature ( T s) and soil water content ( W s). During the experimental period the mean T s of the sites at 10 cm depth ranged from about 0 to 26 °C, and the mean W s of the surface 0–10 cm soil layer fluctuated between the levels of field water holding capacity (WHC) and less than 1/3 of WHC. The temporal course of R s and T s could be fitted with a three-parameter Gaussian equation, with the higher values in August and the lower values in March and December. The annual mean R s (based on daily-weighted monthly mean R s) was 3.08 ± 2.12 (mean ± S.D.), 3.85 ± 2.92, 3.62 ± 2.71, 2.47 ± 2.12, 3.45 ± 3.35, 3.56 ± 2.80, 3.65 ± 3.02, 4.27 ± 3.69, 4.63 ± 4.05, 3.79 ± 2.66 and 2.18 ± 1.47 μmol CO 2 m −2 s −1 from site 1F (forest) to 11B (bare), respectively, and 3.51 ± 0.71 μmol CO 2 m −2 s −1 across the 11 sites. Cumulative annual R s (March to December) varied from 692.1 to 1472.0 g C m −2 yr −1, with an average of 1114.6 g C m −2 yr −1 across all 11 sites. The spatial variations (between-site and within-site) of R s were significant, but there was no clear evidence for which factor mainly affected the spatial variation. Temporal variations of R s were dominantly controlled by T s during most days of the year. However, during early summer, when W s was limiting, R s decreased dramatically and W s exerted control over R s. At all sites, T s was the primary factor driving temporal variations in R s. The functional relationships of R s to T s could be described well by exponential and Lloyd and Taylor equations. The coefficients of determination R 2 of T s to R s in 11 sites varied from 0.55 to 0.84 for the exponential equation, and from 0.51 to 0.86 for the Lloyd and Taylor equation when the drought-affected data were excluded. The relationships of W s to R s could be described well by linear and power equations. The R 2 of W s to R s in 11 sites ranged from 0.27 to 0.73, which were smaller than those of T s to R s, when the R s was normalized using the fit of the Q 10 function with T s at 10 °C. Both the Q 10 and R 10 increased when dry-affected data were removed from the data sets. The Q 10 ranged from 2.37 to 5.53 in 11 sites, and the R 10 of the exponential equations varied from 1.27 to 2.90 μmol CO 2 m −2 s −1, slightly lower than those of Lloyd and Taylor equations ranging from 1.34 to 4.34 μmol CO 2 m −2 s −1. The calculated Q 10 and R 10 of each site at the seasonal time scale were negatively correlated with T s and positively correlated with W s. For all data sets, four two-variable equations including linear and non-linear ones could be used to model relationships of R s to both T s and W s together, with the R 2 ranging from a minimum of 0.58 to a maximum of 0.86 for individual site. Our research results can bear important implications for the study of CO 2 efflux in similar semiarid regions.

Effects of an induced drought on soil carbon dioxide (CO2) efflux and soil CO2 production in an Eastern Amazonian rainforest, Brazil

AbstractIn the next few decades, climate of the Amazon basin is expected to change, as a result of deforestation and rising temperatures, which may lead to feedback mechanisms in carbon (C) cycling that are presently unknown. Here, we report how a throughfall exclusion (TFE) experiment affected soil carbon dioxide (CO2) production in a deeply weathered sandy Oxisol of Caxiuanã (Eastern Amazon). Over the course of 2 years, we measured soil CO2 efflux and soil CO2 concentrations, soil temperature and moisture in pits down to 3 m depth. Over a period of 2 years, TFE reduced on average soil CO2 efflux from 4.3±0.1 μmol CO2 m−2 s−1 (control) to 3.2±0.1 μmol CO2 m−2 s−1 (TFE). The contribution of the subsoil (below 0.5 m depth) to the total soil CO2 production was higher in the TFE plot (28%) compared with the control plot (17%), and it did not differ between years. We distinguished three phases of drying after the TFE was started. The first phase was characterized by a translocation of water uptake (and accompanying root activity) to deeper layers and not enough water stress to affect microbial activity and/or total root respiration. During the second phase a reduction in total soil CO2 efflux in the TFE plot was related to a reduction of soil and litter decomposers activity. The third phase of drying, characterized by a continuing decrease in soil CO2 production was dominated by a water stress‐induced decrease in total root respiration. Our results contrast to results of a drought experiment on clay Oxisols, which may be related to differences in soil water retention characteristics and depth of rooting zone. These results show that large differences exist in drought sensitivity among Amazonian forest ecosystems, which primarily seem to be affected by the combined effects of texture (affecting water holding capacity) and depth of rooting zone.

Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO 2 efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration ( R s) on soil temperature ( T s) and water content ( θ). Daily mean R s varied from a minimum of 0.1 μmol m −2 s −1 in February to a maximum of 9.2 μmol m −2 s −1 in mid-July. Daily mean T s at the 2-cm depth was the primary variable accounting for the temporal variation of R s and no differences between Arrhenius and Q 10 response functions were found to describe the seasonal relationship. R s at 10 °C ( R s10) and the temperature sensitivity of R s ( Q 10Rs) calculated at the seasonal time scale were 3.8 μmol m −2 s −1 and 3.8, respectively. Temperature normalization of daily mean R s ( R sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R s during the growing season. Daily R sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer ( θ fc, ∼0.30 m 3 m −3): (1) R sN was strongly reduced when θ decreased below θ fc, which reflected a reduction in microbial decomposition, and (2) R sN slightly decreased when θ increased above θ fc, which reflected a restriction of CO 2 or O 2 transport in the soil profile. Diurnal variations of half-hourly R s were usually out of phase with T s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R s10 and Q 10Rs parameters calculated from half-hourly nighttime measurements of R s and T s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m −2 s −1 and 5.5 to 1.3, respectively. On average, daily nighttime R s10 (4.5 μmol m −2 s −1) and Q 10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R s10 and Q 10Rs values at low θ. Overall, the use of seasonal R s10 and Q 10Rs parameters led to an overestimation of daily ranges of half-hourly R s (Δ R s) during drought conditions, which supported findings that the short-term temperature sensitivity of R s was lower during periods of low θ. The use of daily nighttime R s10 and Q 10Rs parameters greatly helped at simulating Δ R s during these periods but did not improve the estimation of half-hourly R s throughout the year as it could not account for the diurnal hysteresis effect.

Long‐term CO2 production from deeply weathered soils of a tropical rain forest: evidence for a potential positive feedback to climate warming

AbstractCurrently, it is unknown what role tropical forest soils will play in the future global carbon cycle under higher temperatures. Many tropical forests grow on deeply weathered soils and although it is generally accepted that soil carbon decomposition increases with higher temperatures, it is not known whether subsurface carbon pools are particularly responsive to increasing soil temperatures. Carbon dioxide (CO2) diffusing out of soils is an important flux in the global carbon. Although soil CO2 efflux has been the subject of many studies in recent years, it remains difficult to deduct controls of this flux because of the different sources that produce CO2 and because potential environmental controls like soil temperature and soil moisture often covary. Here, we report results of a 5‐year study in which we measured soil CO2 production on two deeply weathered soil types at different depths in an old‐growth tropical wet forest in Costa Rica. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). Annual soil CO2 efflux varied between 2.8–3.6 μmol CO2‐C m−2 s−1 (old alluvium) and 3.4–3.9 μmol CO2‐C m−2 s−1 (residual). More than 75% of the CO2 was produced in the upper 0.5 m (including litter layer) and less than 7% originated from the soil below 1 m depth. This low contribution was explained by the lack of water stress in this tropical wet forest which has resulted in very low root biomass below 2 m depth. In the top 0.5 m CO2 production was positively correlated with both temperature and soil moisture; between 0.6 and 2 m depth CO2 production correlated negatively with soil moisture in one soil and positively with photosynthetically active radiation in the other soil type. Below 2 m soil CO2 production strongly increased with increasing temperature. In combination with reduced tree growth that has been shown for this ecosystem, this would be a strong positive feedback to ecosystem warming.

Indirect partitioning of soil respiration in a series of evergreen forest ecosystems

A simple estimation of heterotrophic respiration can be obtained analytically as the y-intercept of the linear regression between soil-surface CO2 efflux and root biomass. In the present study, a development of this indirect methodology is presented by taking into consideration both the temporal variation and the spatial heterogeneity of heterotrophic respiration. For this purpose, soil CO2 efflux, soil carbon content and main stand characteristics were estimated in seven evergreen forest ecosystems along an elevation gradient ranging from 250 to 1740 m. For each site and for each sampling date the measured soil CO2 efflux (R S) was predicted with the model R S = a × S C + b × R D ± e, where S C is soil carbon content per unit area to a depth of 30 cm and R D is the root density of the 2–5 mm root class. Regressions with statistically significant a and b coefficients allowed the indirect separation of the two components of soil CO2 efflux. Considering that the different sampling dates were characterized by different soil temperature, it was possible to investigate the temporal and thermal dependency of autotrophic and heterotrophic respiration. It was estimated that annual autotrophic respiration accounts for 16–58% of total soil CO2 efflux in the seven different evergreen ecosystems. In addition, our observations show a decrease of annual autotrophic respiration at increasing availability of soil nitrogen.

Carbon monoxide uptake kinetics in unamended and long-term nitrogen-amended temperate forest soils

The effect of nitrogen (N) additions on the dynamics of carbon monoxide consumption in temperate forest soils is poorly understood. We measured soil CO profiles, potential rates of CO consumption and uptake kinetics in temperate hardwood and pine control plots and plots amended with 50 and 150 kg N ha-1 year-1 for more than 15 years. Soil profiles of CO concentrations were above atmospheric levels in the high-N plots of both stands, suggesting that in these forest soils the balance between consumption and production may be shifted so that either production is increased or consumption decreased. Highest rates of CO consumption were measured in the organic horizon and decreased with soil depth. In the N-amended plots, CO consumption increased in all but one soil depth of the hardwood stand, but decreased in all soil depths of the pine stand. CO enzyme affinities increased with soil depth in the control plots. However, enzyme affinities in the most active soil depths (organic and 0-5 cm mineral) decreased in response to low levels of N in both stands. In the high-N plots, affinities dramatically-increased in the hardwood stand, but decreased in the organic horizon and increased slightly in the 0-5 cm mineral soil in the pine stand. These findings indicate that long-term N addition either by fertilization or deposition may alter the size, composition and/or physiology of the community of CO consumers so that their ability to act as a sink for atmospheric CO has changed. This change could have a substantial effect on the lifetime of greenhouse gases such as CH4 and therefore the future of Earth's climate.

Comparison of soil respiration methods in a mid-latitude deciduous forest

In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO2 efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO2 efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO2 effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates (Rsoil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO2 efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO2 efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO2 efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating Rsoil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization.

On maintaining pressure equilibrium between a soil CO2 flux chamber and the ambient air

Pressure equilibrium between inside a soil CO2 flux chamber and the surrounding air outside the chamber must be maintained during a measurement if measured soil CO2 flux (FCO2) is to accurately represent the rate occurring naturally outside the chamber. In previous studies a simple vent tube connecting to the chamber has often been used to maintain pressure equilibrium. This approach, however, can be effective only under calm conditions. Under windy conditions, negative pressure excursions will occur inside the chamber that are artifacts resulting from wind passing over the vent tube's external open end, a phenomenon known as the Venturi effect. This causes anomalous mass flow of CO2‐rich air from the soil into the chamber, leading to a significant overestimation of FCO2. In this present study, we found that negative chamber pressure excursions due to the Venturi effect cannot be observed unless the differential pressure measurement is made with the chamber resting on an impermeable base. Making pressure measurements with a chamber resting on porous soil can lead to the erroneous conclusion that an anomalous mass flow is not a problem precisely when it is causing serious artifacts. We also present a new vent design for a soil CO2 flux chamber capable of maintaining pressure equilibrium between inside the chamber and the ambient air outside the chamber under both calm and windy conditions. Differential pressure measurements from field experiments show that the pressures inside our newly designed vented chamber equal those outside the chamber when wind speed at a height of 0.5 m is up to 7 m s−1, thus virtually eliminating artifacts due to the Venturi effect. Our field data show that the problem of overestimation in measured FCO2 by a chamber with older vent designs under windy conditions can be avoided with our newly designed vented chamber.

Some characteristics of soil respiration in hybrid poplar plantations in northern Alberta

Fast-growing hybrid poplars are being planted in the Canadian prairies to meet the increasing demand for fibre and environmental services of trees and forests; however, the impact of hybrid poplars on C dynamics and storage on previously farmed land is largely unknown for the boreal region. We measured soil CO2 efflux along a chronosequence (3-, 9-, and 11-yr-old stands) of hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) plantations and a control agricultural field from June to August 2004. Measurements were made between 0800 and 1800 with a portable Li-Cor 6400-09 system and were based on 4–5 min averaging. We also measured the response to simulated rainfall and the diurnal fluctuation of soil CO2 efflux. Soil CO2 efflux ranged from 1.30 µmol CO2 m-2 s-1 in the 3-yr old plantation to 5.41 µmol CO2 m-2 s-1 in the agricultural control field, or from 0.17 ìmol CO2 m-2 s-1 kg-1 C (based on soil organic C content to a 0.4 m depth) in the 3-yr-old plantation to 1.09 µmol CO2 m-2 s-1 kg-1 C in the 11-yr-old plantation. Simulated rainfall applied in the 3-yr-old plantation and a newly planted site resulted in an immediate pulse of CO2 efflux, 2.90 and 2.54 µmol CO2 m-2 s-1, respectively, followed by an efflux rate sustained slightly above pre-irrigation levels. No secondary pulse of soil respiration was observed in the 2-h period following water application. Diurnal variation of soil respiration was found to be small between 0600 and 1900 in the agricultural control field, with values that varied from 2.66 to 3.17 µmol CO2 m-2 s-1. Continued monitoring of soil respiration and other C cycling processes in the chronosequence will improve our understanding of the potential for C sequestration in hybrid poplar plantations in northern Alberta. Key words: Carbon sequestration, greenhouse gas emissions, biomass, boreal forest, land-use change, hybrid poplar

Measurement of soil CO 2 efflux using soda lime absorption: both quantitative and reliable

Measurement of soil CO 2 efflux using a non-flow-through steady-state (NFT-SS) chamber with alkali absorption of CO 2 by soda lime was tested and compared with a flow-through non-steady-state (FT-NSS) IRGA method to assess suitability of using soda lime for field monitoring over large spatial scales and integrated over a day. Potential errors and artifacts associated with the soda lime chamber method were investigated and improvements made. The following issues relating to quantification and reliable measurement of soil CO 2 efflux were evaluated: (i) absorption capacity of the soda lime, (ii) additional and thus artifactual absorption of CO 2 by soda lime during the experimental procedure, (iii) variation in the CO 2 concentration inside the chamber headspace, and (iv) effects of chamber closure on soil CO 2 efflux. Soil CO 2 efflux, as measured using soda lime (with a range of quantities: 50, 100, and 200 g per 0.082 m 2 ground area enclosed in chamber), was compared with transient IRGA measurements as a reference method that is based on well-established physical principles, using several forms of spatial and temporal comparisons. Natural variation in efflux rates ranged from 2 to 5.5 g C m −2 day −1 between different chambers and over different days. A comparison of the IRGA-based assay with measurement based on soda lime yielded an overall correlation coefficient of 0.82. The slope of the regression line was not significantly different from the 1:1 line, and the intercept was not significantly different from the origin. This result indicated that measurement of CO 2 efflux by soda lime absorption was quantitatively similar and unbiased in relation to the reference method. The soda lime method can be a highly practical method for field measurements if implemented with due care (in terms of drying and weighing soda lime, and in minimizing leakages), and validated for specific field conditions. A detailed protocol is presented for use of the soda lime method for measurement of CO 2 efflux from field soils.

Small-scale spatial variation in soil CO 2 concentration in a natural carbon dioxide spring and some related plant responses

Small-scale spatial and temporal distribution of soil CO 2 within the natural CO 2 spring (mofette) field Stavešinci (NE Slovenia) was studied. By using an infrared gas-analyser, gas measurements were performed measuring soil CO 2 in a regular sampling grid (depth 20 cm, resolution 0.5 m) in an area of 60 m 2. The measurements were performed at different times of the year, in May (20) 2002 and in November (10 and 17) and December (4) 2003. To illustrate the small-scale spatial variability of soil CO 2 concentration, the data were interpolated by the inverse distance third power moving average method. The main geothermal exhalation spots were marked and the gaseous environment of different parts of the mofette area was described. One third of the plot was strongly enriched in CO 2 with median CO 2 concentration ranging from 21.3% to 30.9% in four samplings. The measurements revealed a clear patchy pattern of soil CO 2 enrichment. Drastic changes of soil CO 2 concentrations over a short distance indicate limited lateral diffusion in the silty-clay mofette soil. The spatial patterns of soil gases can be regarded as fairly constant over longer periods of time as revealed by a comparison of consecutive samplings. The growth of Solidago gigantea in the studied area was followed and clear negative correlations have been proved when plant height and leaf chlorophyll content were compared to soil CO 2 concentration. The Spearman rank correlation coefficient for average plant height and average soil [CO 2] was r = − 0.69, and for leaf chlorophyll content and soil [CO 2] was r = − 0.76. Our study showed that the relation between the gaseous environment and plant response at the natural CO 2 springs could be efficiently explained on the basis of soil [CO 2].

Management effects on soil CO 2 efflux in northern semiarid grassland and cropland

Soil respiration is a process influenced by land use, management practices, and environmental conditions. Our objectives were to evaluate relationships between management-induced differences in soil organic carbon (SOC) and soil CO 2 efflux from continuous no-till spring wheat ( Triticum aestivum L.), spring wheat-fallow under no-till, and a native mixed-grass prairie with grazing near Mandan, ND. A Werner–Sen–Chama soil complex (Entic Haplustoll, Typic Haplustoll, and Typic Calciustoll) was present at the grassland site and a Wilton silt loam (Pachic Haplustoll) at the cropping sites. Soil chambers were used to measure soil CO 2 effluxes about every 21 days starting 14 May 2001 to 1 April 2003. Soil water and soil temperature were measured at time of CO 2 efflux measurements. Soil organic carbon, microbial biomass carbon (MBC), and above and belowground plant biomass were measured in mid-July each year. Root biomass to 0.3 m depth of the undisturbed grassland was significantly greater (12.3 Mg ha −1) than under continuous wheat (1.3 Mg ha −1) and wheat-fallow (0.3 Mg ha −1). Grassland SOC content of 84 Mg ha −1 to 0.3 m soil depth was 1.2 times greater than continuous wheat and 1.3 times greater than wheat-fallow. The MBC of the grassland was 2.2 Mg ha −1, or 3.6 times greater than continuous wheat and 7.2 times greater than wheat-fallow treatments. Soil CO 2 efflux averaged 2.8 g CO 2–C m −2 day −1 for grassland, compared to 1.9 g CO 2–C m −2 day −1 for wheat fallow and 1.6 g CO 2–C m −2 day −1 for continuous wheat treatments. Although these CO 2 efflux rates were based on measurements made at intervals of about 21 days, the differences among treatments with time were rather consistent. Differences in soil CO 2 efflux among treatments could be attributed to differences in SOC and MBC, suggesting that land use plays a significant role in soil CO 2 efflux from respiration.

Using measurements of soil CO2 efflux and concentrations to infer the depth distribution of CO2 production in a forest soil

Carbon dioxide (CO2) from soil respiration is the focus of many ecosystem-atmosphere studies. However, due to the difficulty in obtaining measurements, there is a relative lack of information on the behavior of CO2 in the soil. This paper describes an accurate method that can be used in the field to measure CO2 concentration in small (10-cm) samples of soil air using an infrared gas analyzer. Measurements on samples drawn by syringe from different depths are compared to those calculated using a simple steady-state model derived from the conservation equation for soil CO2 and Fick’s law of diffusion. The study site is a second growth coastal temperate Douglas-fir forest plantation (53 yr old) located on the eastern slope of Vancouver Island, Canada. Measurements of CO2 concentrations were obtained from two locations at six soil depths between 0.02 and 1 m at various times of the year in 2000 and 2001. The vertical profiles of CO2 concentrations generally had similar shapes throughout the year although there was considerably more short-term variability in the shallow layers. Below-ground CO2 concentrations were higher during summer and decreased during the winter. Non-zero CO2 concentration gradients between the 0.5- and 1-m depths suggested some CO2 production below the 1-m depth. By matching modelled CO2 concentrations with measured values and using measured CO2 efflux, we used the model to determine the CO2 production profile. Calculations of CO2 production profiles indicated that more than 85% of the CO2 efflux from this forest soil originates at depths shallower than 30 cm. Key words: Soil carbon dioxide, CO2, soil respiration, steady-state model

Seasonal and diurnal variations in topsoil and subsoil respiration under snowpack in a temperate deciduous forest

We measured soil CO2 concentration at half‐hour intervals with infrared gas analyzers buried in soil at four depths throughout the snow cover season extending from early December to early April in a deciduous temperate forest. We evaluated soil CO2 efflux or total soil respiration, topsoil (the A‐horizon) respiration, and subsoil (the C‐horizon) respiration using a modified flux gradient method. Thereby, we investigated seasonal and diurnal variations in these soil respirations under snowpack. Soil CO2 concentration and soil respiration changed dynamically under the condition of constant soil temperature. Topsoil respiration decreased rapidly in late autumn and relatively constant until mid‐winter, whereas it increased in late winter when snowmelt progressed. On the other hand, subsoil respiration decreased gradually until mid‐winter and increased slightly in late winter. Both topsoil and subsoil respirations showed similar diurnal variations with a peak in early or mid‐afternoon, respectively, independently of soil temperature. These seasonal and diurnal variations in soil respiration were inferred to result from the supply of labile carbon compounds, which were respiratory substrates for microorganisms, into soil from litter with meltwater. The seasonal sum of topsoil, subsoil, and total respirations for the snow cover period of 4 months were 21.4, 48.0, and 69.4 gC m−2, respectively; these accounted for 3.0, 27.3, and 7.7% of the annual sum, respectively. The ratio of topsoil respiration to total soil respiration was 0.31 on average during the snow period, which was considerably lower than that in the summer season.

Coupling of canopy gas exchange with root and rhizosphere respiration in a semi-arid forest

The strength of coupling between canopy gas exchange and root respiration was examined in ~15-yr-old ponderosa pine (Pinus ponderosa Doug. Ex Laws.) growing under seasonally drought stressed conditions. By regularly watering part of the root system to reduce tree water stress and measuring soil CO2 efflux on the dry, distant side of the tree, we were able to determine the strength of the relationship between soil autotrophic (root and rhizosphere) respiration and changes in canopy carbon uptake and water loss by comparison with control trees (no watering). After ~40 days the soil CO2 efflux rate, relative to pre-treatment conditions, was twice that of the controls. This difference, attributable to root and rhizosphere respiration, was strongly correlated with differences in transpiration rates between treatments (r2 = 0.73, p 0.99) for both treatments. At the ecosystem scale, daily soil CO2 efflux rate was linearly related to gross primary productivity (GPP) as measured by eddy-covariance technique (r2 = 0.55, p<0.01), suggesting patterns of soil CO2 release appear strongly correlated to recent carbon assimilation in this young pine stand. Collectively the observed relationships suggest some consideration should be given to the inclusion of canopy processes in future models of soil respiration.

Seasonal patterns in soil surface CO2 flux under snow cover in 50 and 300 year old subalpine forests

Soil CO2 flux can contribute as much as 60-80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO2 efflux through snow in 50 and � 300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO2 flux; determine if differences in soil CO2 flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO2 flux during the 2002-2003 and 2003-2004 snow season averaged 0.31 and 0.35 lmols m � 2 s � 1 for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO2 flux during the winter with fluxes averaging 0.22 lmols m � 2 s � 1 in December and January and increasing to an average of 0.61 lmols m � 2 s � 1 in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO2 concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ±10% of the mean. Variability in CO2 flux estimates among plots (size = 400 m 2 ) was similar to that within plot and was also low (CV = � 28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for a = 0.05.