CO2 Flux Rates Research Articles

Contribution of winter soil respiration to annual soil CO2 emission in a Mollisol under different tillage practices in northeast China

Winter soil CO2 emission is a very important component of the annual carbon budgets, however, almost no information on winter CO2 emission is available from the cropland soil in northeast China. In this study, soil CO2flux was measured for a 2‐year period from an ongoing tillage trial on Black soil in northeast China to quantify seasonal patterns in soil CO2 flux rate and wintertime contribution to annual soil respiration. Average soil CO2 flux rates in the winter (November to March) were between 0.64 to 1.22 g CO2 m−2 d−1, in the non‐growing season (October and April) were 2.09–3.56 g CO2 m−2 d−1, whereas in the growing season (May to September) they were between 10.9 to 12.7 g CO2 m−2 d−1, with no significant differences among tillage treatments. Total winter, non‐growing and growing season soil CO2 emissions were 0.28–0.45 Mg C ha−1, 0.36–0.53 Mg C ha−1, and 4.52–5.55 Mg C ha−1, respectively, among tillage treatments. The contributions of winter soil respiration to annual soil CO2emission ranged from 5.1 to 7.1%, and the non‐growing season emission ranged from 11.4 to 15.2% among tillage treatments. Our results indicate that in northeast China, cropland Black soil continuously emits CO2throughout the non‐growing season, and the wintertime soil respiration plays a significant role in annual soil carbon budgets. Hence winter soil CO2 emission must be taken into consideration when the role of the soil ecosystem is assessed as either a sink or source of CO2 to the atmosphere.

Simultaneous measurement of the TCA cycle and respiration in isolated mitochondria and intact cells with the XF24‐3 Analyzer

Measuring mitochondrial dysfunction is increasingly important in the study of neurodegenerative and cardiovascular disease, metabolic syndrome, diabetes, cancer, and aging. Mitochondrial function is usually measured by oxygen consumption rates using respirometry methods. However, a critical aspect of mitochondrial function may be overlooked when only O2 consumption is employed: the tricarboxylic acid (TCA) or Krebs cycle, a central metabolic pathway. Measuring the TCA cycle function requires monitoring the flux of an additional analyte, such as carbon dioxide (CO2). The reported method describes simultaneous monitoring of carbon dioxide evolution rates (CDER) and oxygen consumption rates (OCR) using isolated mitochondria and intact cells, in real‐time, using the XF24‐3 Analyzer. This technology employs fluorescent sensors specific for CO2 and O2, which operate reversibly, and reveal details of mitochondrial respiration and TCA cycle activities. Results indicate that O2 consumption and CO2 evolution may be monitored simultaneously, and that data agrees with attributes of TCA cycle and mitochondrial function obtained by other methods. Differential rates of O2 and CO2 flux can be identified relative to substrate utilization and interdependency among the TCA cycle, electron transport, and oxidative phosphorylation systems. Support for this research is provided by Seahorse Bioscience.

SOIL CO2 FLUX IN GRASSLAND, AFFORESTED LAND AND RECLAIMED COALMINE OVERBURDEN DUMPS: A CASE STUDY

ABSTRACTThe aim of this study was to measure the in situ soil CO2 flux from grassland, afforested land and reclaimed coalmine overburden dumps by using the automated soil CO2 flux system (LICOR‐8100® infrared gas analyzer, LICOR Inc., Lincoln, NE). The highest soil CO2 flux was observed in natural grassland (11·16 µmol CO2 m−2s−1), whereas the flux was reduced by 38 and 59 per cent in mowed site and at 15‐cm depth, respectively. The flux from afforested area was found 5·70 µmol CO2 m−2s−1, which is 50 per cent lower than natural grassland. In the reclaimed coalmine overburden dumps, the average flux under tree plantation was found to be lowest in winter and summer (0·89–1·12 µmol CO2 m−2s−1) and highest during late monsoon (3–3·5 µmol CO2 m−2s−1). During late monsoon, the moisture content was found to be higher (6–7·5 per cent), which leads to higher microbial activity and decomposition. In the same area under grass cover, soil CO2 flux was found to be higher (8·94 µmol CO2 m−2s−1) compared with tree plantation areas because of higher root respiration and microbial activity. The rate of CO2 flux was found to be determined predominantly by soil moisture and soil temperature. Our study indicates that the forest ecosystem plays a crucial role in combating global warming than grassland; however, to reduce CO2 flux from grassland, mowing is necessary. Copyright © 2012 John Wiley & Sons, Ltd.

The Carbon Cycle and Climate Change: Memoirs of my 60 years in Science

This manuscript presents the development of our current understanding of how the oceans operate and how past climates are reconstructed from the personal viewpoint of the author. It begins with the application of radiocarbon dating of carbonate rocks to deduce past climatic conditions. Age dating of carbonate sediments and microfossils provides a history of sea level rise and fall that closely matches fluctuations in temperature, which in turn correspond with cyclic changes in the Earth’s orbit. The origins of our understanding of how ocean circulation can affect global climate are reviewed and the essential role of atmospheric carbon-dioxide content is explored. Central to this climate link is the rate of CO2 flux between the various terrestrial reservoirs including seawater, glacial ice and the terrestrial biota. The manuscript concludes by describing how the collected knowledge about the carbon cycle provides insight into capture of CO2 from the atmosphere with the aim of storing it and attenuating the potentially devastating effects of fossil fuel burning on global climate change.

Long-term surface carbon dioxide flux monitoring at the Ketzin carbon dioxide storage test site

Subsurface geologic storage of carbon dioxide calls for sophisticated monitoring tools with respect to long-term safety and environmental impact issues. Despite extensive research, many factors governing the fate of injected carbon dioxide (CO2) remain unclear. To identify possible risks through leakage of the CO2 storage reservoir, a program for monitoring of the CO2 flux at the surface was started at the Ketzin test site, which allows to distinguish between natural temporal and spatial flux variations and a potential leakage. To gain adequate long-term baseline data on the local background CO2 flux variations, CO2 soil gas flux, soil moisture, and temperature measurements were conducted once a month during a 6-yr period. Furthermore, soil samples were analyzed for their organic carbon and total nitrogen contents. The mean flux of all sampling sites before the CO2 injection (2005–2007) was 2.8 mol m2 s1 (ranging from 2.4 to 3.5), with a Q10 factor of 2.4, and in the years after commencing injection (2009–2010), 2.4 mol m2 s1 (ranging from 2.2 to 2.5), with the same Q10 factor. The CO2 flux rate is mainly controlled by the soil temperature. A significant influence of diurnal temperature variation and soil moisture was not detected. The spatial variability of the CO2 flux among the 20 sampling locations ranges from 1.0 to 4.5 mol m2 s1, depending on the organic carbon and total nitrogen content of the soil. Through comparison with the long-term measurements, unusual high CO2 fluxes can theoretically be distinguished from natural variations.

Soil respiration rates and in natural beech forest (Fagus sylvatica L.) in relation to stand structure

Soil respiration rates were studied as a function of soil type, texture and light intensity at five selected natural beech forest stands with contrasting geology: stands on silicate bedrock at Kladje and Brička in Pohorje, a stand on quartz sandstone at Vrhovo and two stands on a carbonate bedrock in the Karstic-Dinaric area in Kočevski Rog, Snežna jama and Rajhenav, Slovenia, during the growing season in 2005–2006. Soil respiration exhibited pronounced seasonal and spatial variations in the studied forest ecosystem plots. The CO2 flux rates ranged from minimum averages of 2.3 μmol CO2 m−2 s−1 (winter) to maximum averages of about 7 μmol CO2 m−2 s−1 (summer) at all the investigated locations. An empirical model describing the relationship between soil respiration and soil temperature predicted seasonal variations in soil respiration reasonably well during 2006. Nevertheless, there were also some indications that soil moisture in relation to soil texture could influence the soil CO2 efflux rates in both sampling seasons. It was shown that spatial variability of mean soil respiration at the investigated sites was high and strongly related to root biomass. Based on the data, it was shown that new photoassimilates could account for a major part of the total soil respiration under canopy conditions in forest ecosystems where no carbonate rocks are present, indicating that microbial respiration could not always dominate bulk soil CO2 fluxes. At Snežna jama and Rajhenav, the abiotic CO2 derived from carbonate dissolution had a pronounced influence on CO2 efflux accounting, on average, to ∼17%. Further spatial heterogeneity of soil respiration was clearly affected by management practice. Higher respiration rates as well as higher variability in respiration rates were observed in the virgin forest (Rajhenav) than in the management forest (Snežna jama) and could be related to a higher amount of detritus and consequently to less pronounced influence of inorganic pool to CO2 efflux, lower mixing with atmospheric CO2 and higher sensitivity to environmental changes. Major differences in soil carbon dynamics among these five forest ecosystems can be explained by the influence of bedrock geology (particularly, the presence or absence of carbonate minerals) and soil texture (affecting gas exchange with overlying air and soil moisture).

Springtime CO2 exchange over seasonal sea ice in the Canadian Arctic Archipelago

AbstractSpringtime measurements of CO2 exchange over seasonal sea ice in the Canadian Arctic Archipelago using eddy covariance show that CO2 was generally released to the atmosphere during the cold (ice surface temperatures less than about –6˚C) early part of the season, but was absorbed from the atmosphere as warming advanced. Hourly maximum efflux and uptake rates approached 1.0 and –3.0 μmol m–2 s–1, respectively. These CO2 flux rates are far greater than previously reported over sea ice and are comparable in magnitude to exchanges observed within other systems (terrestrial and marine). Uptake generally occurred for wind speeds in excess of 6 m s–1 and corresponded to local maxima in temperature at the snow–ice interface and net radiation. Efflux, on the other hand, occurred under weaker wind speeds and periods of local minima in temperature and net radiation. the wind speeds associated with uptake are above a critical threshold for drifting and blowing snow, suggesting that ventilation of the snowpack and turbulent exchange with the brine-wetted grains are an important part of the process. Both the uptake and release fluxes may be at least partially driven by the temperature sensitivity of the carbonate system speciation in the brine-wetted snow base and upper sea ice. the period of maximum springtime CO2 uptake occurred as the sea-ice permeability increased, passing a critical threshold allowing vertical brine movement throughout the sea-ice sheet. At this point, atmospheric CO2 would have been available to the under-ice sea-water carbonate system, with ramifications for carbon cycling in sea-ice-dominated polar waters.

Development of Methods for Gaseous Phase Geochemical Monitoring on the Surface and in the Intermediate Overburden Strata of Geological CO2Storage Sites

The developments and results presented in this paper are taken from the work carried out as part of the GeoCarbon-Monitoring project, which was partly funded by the French National Research Agency (ANR). An important part of this project covers methods for gas monitoring on the surface as well as within the cap rock of geological CO2 storage sites. The work undertaken by INERIS was targeted at two specific approaches which are often recommended as essential for the monitoring of future storage sites : early detection (pre-alert), based on the sampling and analysis of gas at the bottom of the dedicated boreholes which are drilled from the surface into the intermediate cap rock strata ; the detection and quantification of the gaseous flux of CO2 released from the ground into the atmosphere. These two approaches were developed in the laboratory successively and then applied and tested in-situ, under conditions that are as close as possible to those of the future storage sites. They offer the advantage of ensuring a direct measurement as well as providing real-time information on the presence or, on the contrary, the absence of CO2 leaks. The tests undertaken on a 200 meter deep borehole have shown that the detection of CO2 leaks passing through the intermediate overburden strata was possible thanks to the continuous sampling and analysis of the composition of the gas which accumulated at the bottom of the borehole. In particular, the detection of small releases of gas emanating from the surrounding rock gave rise to a number of good results. These releases may be a precursor to a larger leak. Likewise, it has been possible to take a sample and ensure the transit of gas over long distances, up to 1000 meters from the sampling point. This was done without causing any significant deformation or dilution of the initial gaseous signal, even for low amplitude leaks. These results allow us to envisage the implementation of a relatively simple system for detecting and monitoring gas leaks through intermediate cap rock strata. This system will largely comprise conventional industrial gas sensors which are available off the shelf. The direct measurement of gas flows emanating from the ground is one of the most effective ways to monitor a storage site. The INERIS accumulation chamber method has been improved to measure low and very low CO2 flux rates. It can now be used to measure a wide range of CO2 flux rates, from very low emission levels of 0.05 to 0.2 cm3.min-1.m-2 up to extremely high flux rates of some 3000 cm3.min-1.m-2. The accuracy and operational characteristics of chamber method have been checked and validated by tests performed in a laboratory and on a test rig, as well as through field measurements taken under real conditions at sites that naturally release CO2. These tests have shown that the method has reached full technical maturity and that it can be applied on a practical level to detect and monitor CO2 and methane emissions on the ground's surface. The two methods which have been tested are now operational and ready for integration into the surveillance strategy applied at future CO2 storage sites. They can be used at every stage of a storage site's life : site reconnaissance, definition of the initial state, injection, post-injection phase, and residual monitoring after the site has been abandoned.

Characteristics of soil CO2 efflux variability in an aseasonal tropical rainforest in Borneo Island

Although soil carbon dioxide (CO2) efflux from tropical forests may play an important role in global carbon (C) balance, our knowledge of the fluctuations and factors controlling soil CO2 efflux in the Asian tropics is still poor. This study characterizes the temporal and spatial variability in soil CO2 efflux in relation to temperature/moisture content and estimates annual efflux from the forest floor in an aseasonal intact tropical rainforest in Sarawak, Malaysia. Soil CO2 efflux varied widely in space; the range of variation averaged 17.4 μmol m−2 s−1 in total. While most CO2 flux rates were under 10 μmol m−2 s−1, exceptionally high fluxes were observed sporadically at several sampling points. Semivariogram analysis revealed little spatial dependence in soil CO2 efflux. Temperature explained nearly half of the spatial heterogeneity, but the effect varied with time. Seasonal variation in CO2 efflux had no fixed pattern, but was significantly correlated with soil moisture content. The correlation coefficient with soil moisture content (SMC) at 30 and 60 cm depth was higher than at 10 cm depths. The annual soil CO2 efflux, estimated from the relationship between CO2 efflux and SMC at 30 cm depth, was 165 mol m−2 year−1 (1,986 g C m−2 year−1). As this area is known to suffer severe drought every 4–5 years caused by the El Nino-Southern Oscillation, the results suggest that an unpredictable dry period might affect soil CO2 efflux, leading an annual variation in soil C balance.

Factors Affecting on Response of Broad Bean and Corn to Air Quality and Soil CO2 Flux Rates in Egypt

In response to worldwide increases in the burning of fossil fuels to meet energy demands for electric power generation and transportation, atmospheric CO2 concentrations are currently rising at approximately 0.5% per year and ground-level O3 values are increasing at a rate of 0.32% per year. Some plants showed positive increases in response to elevated atmospheric CO2 concentrations, but are depressed when exposed to enhanced O3 air pollution. The objective of this research was to examine relationships between alterations in leaf plant characteristics in response to air quality treatments and soil CO2 flux activities during the growing season. Field studies were conducted in 2-m diameter × 2-m height open-top chambers (OTC’s) at Sharkia Province during 2004 and 2005 involving the growth of broad bean (Vicia faba L. cv. Giza 40) and corn (Zea mays L. cv. 30 K8) in rotations using no-till management while being subjected full-season to five air quality treatments: charcoal-filtered (CF) air; CF + 150 µL CO2 L−1; non-filtered (NF) air; NF + 150 µL CO2 L−1 and ambient air (AA). Leaf photosynthesis (Ps), leaf area index (LAI), and vegetative carbohydrate contents were determined during pre- and post-anthesis in the two crops and soil CO2 flux rates were monitored monthly during two growing seasons (2004–2005). Multiple and stepwise regression analyses were performed to establish linkages between plant canopy characteristics and soil CO2 flux rates with results combined over growth stages and year for each crop. Increasing the atmospheric CO2 concentration typically stimulated leaf Ps, soluble and total leaf carbohydrate contents, LAI values, and soil CO2 flux rates throughout the growing season in both crop; however, the elevated O3 treatments in NF air tended to lower these values compared to CF air. Soil CO2 flux rates were significantly correlated with LAI, soluble and total sugar contents at P ≤ 0.01 and with Ps rates at P ≤ 0.05 in broad bean leaves, but with soluble and total sugar contents of leaves in corns at P ≤ 0.01 only. Results of this study provided solid evidences linking the impact of changing air quality on plants factors processes and possible indirect effects on soil CO2 flux activities throughout the growing season.

Pulsing hydrology, methane emissions and carbon dioxide fluxes in created marshes: A 2-year ecosystem study

Short-term methane and carbon dioxide flux rates were measured in two created, experimental marshes in the Midwestern U.S. over a two-year period (2004–2005) in which hydrologic conditions were manipulated to simulate flood-pulse and steady-flow conditions. Gas flux rates were measured in three distinct wetland zones: continuously inundated areas; edge zones with emergent macrophytes; and edge zones in which emergent macrophytes were removed. Methane fluxes between years were not significantly different in edge zones with and without emergent vegetation, but were twice as high in continuously inundated zones during the steady-flow year compared to the flood pulsed year. There was no apparent relationship between emergent vegetation and methane flux, as mean flux rates were not significantly different in either year in edge zones where emergent vegetation was removed, compared to edge zones containing emergent vegetation. Continuously inundated wetland zones emitted methane from summer through fall, while in edge zones methane fluxes were only substantial in spring and summer. Neither daytime rates of carbon dioxide uptake or nighttime rates of respiration were significantly different between the years for any wetland zone. When CO2 flux rates (daytime uptake plus nighttime respiration) were normalized for solar radiation and day length, solar efficiency was found to be comparable between the steady flow and pulsed years. Methane fluxes were more strongly affected than carbon dioxide fluxes by the differences in hydrology, but only in the deeper areas of the wetlands.

Greenhouse gas fluxes during rewetting of peatlands by use of effluents – a lysimeter study

The present study aims at assessing the effect of using the effluent of a wastewater treatment plant as an alternative measure for rewetting nutrient-rich fen soils over the growing season on the emission of greenhouse gas (GHG) and at discussing possible changes in the greenhouse potential as a result of this practice. In order to allow a discussion on GHG based on integrated CH4, N2O, and CO2 flux rates, fluxes were measured in our lysimeter study using the chamber methodology from May to December in 2003 and 2004. The study compares the gaseous fluxes of fen soils in lysimeters treated with the effluent and/or freshwater for rewetting. Only freshwater was applied to the control lysimeter. The source of water hardly had any statistically significant effect on trace gas fluxes. However, there was a trend towards higher CH4 emissions at the effluent lysimeters compared to the control lysimeter. Effluent usage did not decrease the greenhouse effect at the same rate, which could be observed at the control. Nevertheless, regarding gaseous emissions the use of effluents could prove to be a solution to the current problem of today's major peat oxidation and fen soil loss by drainage.

Lateral CO2 diffusion inside dicotyledonous leaves can be substantial: quantification in different light intensities.

Substantial lateral CO(2) diffusion rates into leaf areas where stomata were blocked by grease patches were quantified by gas exchange and chlorophyll a fluorescence imaging in different species across the full range of photosynthetic photon flux densities (PPFD). The lateral CO(2) flux rate over short distances was substantial and very similar in five dicotyledonous species with different vascular anatomies (two species with bundle sheath extensions, sunflower [Helianthus annuus] and dwarf bean [Phaseolus vulgaris]; and three species without bundle sheath extensions, faba bean [Vicia faba], petunia [Petunia hybrida], and tobacco [Nicotiana tabacum]). Only in the monocot maize (Zea mays) was there little or no evident lateral CO(2) flux. Lateral diffusion rates were low when PPFD <300 micromol m(-2) s(-1) but approached saturation in moderate PPFD (300 micromol m(-2) s(-1)) when lateral CO(2) diffusion represented 15% to 24% of the normal CO(2) assimilation rate. Smaller patches and higher ambient CO(2) concentration increased lateral CO(2) diffusion rates. Calculations with a two-dimensional diffusion model supported these observations that lateral CO(2) diffusion over short distances inside dicotyledonous leaves can be important to photosynthesis. The results emphasize that supply of CO(2) from nearby stomata usually dominates assimilation, but that lateral supply over distances up to approximately 1 mm can be important if stomata are blocked, particularly when assimilation rate is low.

Interannual variability of the oceanic CO2 sink in the subtropical gyre of the North Atlantic Ocean over the last 2 decades

Between 1983 and 2005, continuous oceanic CO2 observations at two time series sites in the North Atlantic Ocean near Bermuda indicate that surface seawater dissolved inorganic carbon (DIC) and pCO2 increased annually at rates similar to that expected from oceanic equilibration with increasing CO2 in the atmosphere. In addition, seawater pH, CO32− ion concentrations, and CaCO3 saturation states have also decreased over time. There was considerable seasonal asymmetry in the oceanic CO2 sink or source rates, with wintertime air‐to‐sea CO2 influx greater than the summertime sea‐to‐air CO2 efflux. On an annual basis, the region was an oceanic sink for CO2, with a mean net annual air‐sea CO2 flux rate of −815 ± 251 and −1295 ± 294 mmol CO2 m−2 yr−1, respectively, estimated using different synoptic and data assimilation model wind speed data sets. Peak‐to‐peak variability of ∼850–1950 mmol CO2 m−2 yr−1 represented an interannual variability of ∼0.2–0.3 Pg C yr−1 in the oceanic CO2 sink scaled to the subtropical gyre of North Atlantic Ocean. The long‐term trend over the 1983–2005 period was a slight increase in the oceanic CO2 sink, associated primarily with a gradual increase in wind speed over the same period. Interannual variability of summertime (June–September) and fall (October–December) air‐sea CO2 flux rates were correlated to the North Atlantic Oscillation (NAO) and strongly influenced by wind events such as hurricanes. Wintertime (January–May) air‐sea CO2 flux rates were poorly correlated with the NAO and Arctic Oscillation (AO), although gas exchange rates were ∼11–40% higher during concurrent El Niño periods compared to La Niña periods.

Dynamics of organic and inorganic carbon across contiguous mangrove and seagrass systems (Gazi Bay, Kenya)

We report on the water column biogeochemistry in adjacent mangrove and seagrass systems in Gazi Bay (Kenya), with a focus on assessing the sources and cycling of organic and inorganic carbon. Mangrove and seagrass‐derived material was found to be the dominant organic carbon sources in the water column, and could be distinguished on the basis of their δ13C signatures and particulate organic carbon:total suspended matter (POC/TSM) ratios. Spatially, a distinct boundary existed whereby the dominance of mangrove‐derived material decreased sharply close to the interface between the mangrove forest and the dense seagrass beds. The latter is consistent with the reported export of mangrove‐derived material, which is efficiently trapped in the adjacent seagrass beds. There were significant net inputs of POC and dissolved organic carbon (DOC) along the Kidogoweni salinity gradient, for which the δ13CPOC signatures were consistent with those of mangroves. DOC was the dominant form of organic carbon in both mangrove and seagrass beds, with DOC/POC ratios typically between 3 and 15. Dynamics of dissolved inorganic carbon in the creeks were strongly influenced by diagenetic C degradation in the intertidal mangrove areas, resulting in significant CO2 emission from the water column to the atmosphere. Although highest partial pressure of CO2 (pCO2) values and areal CO2 flux rates were observed in the mangrove creeks, and the water column above the seagrass beds was in some locations a net sink of CO2, most of the ecosystems' emission of CO2 to the atmosphere occurred in the seagrass beds adjacent to the mangrove forest. The presence of dense seagrass beds thus had a strong effect on the aquatic biogeochemistry, and resulted in trapping and further mineralization of mangrove‐derived POC, intense O2 production and CO2 uptake. The adjacent seagrass beds provide a large area with conditions favorable to exchange of CO2 with the atmosphere, thereby limiting export of mangrove‐derived organic and inorganic carbon toward the coastal ocean.

Fluxes and production of N2O, CO2 and CH4 in boreal agricultural soil during winter as affected by snow cover

Agricultural soils are important source of atmospheric nitrous oxide (N2O) and a considerable part of annual N2O release occurs during the cold season in the boreal region. According to recent studies N2O can be produced in soils at low temperatures, below 0°C. We studied if removal of the snowcover lowers soil temperatures and whether this would affect flux rates of N2O, carbon dioxide (CO2) and methane (CH4) from an agricultural soil in eastern Finland. Gas flux rates and concentrations in soil were measured from study plots with undisturbed snow cover and from plots with snow removed. This experiment simulates changes in the soil thermal conditions with less snowfall. Plots without snow had even 15°C lower temperature at the depth of 5 cm and they had higher N2O emissions during soil freezing and thawing. However, there were only minor changes in CH4 or CO2 flux rates after removal of snow over the cold season. N2O and CO2 accumulated in the soil during winter and were then released rapidly during thawing in spring. CH4 concentrations in the soil remained lower than the atmospheric levels during winter and subsequently increased to the ambient levels after thawing. Future climate scenarios suggest possible decline in snowfall in northern Europe resulting in lower soil temperatures. This could lead to higher N2O emissions from boreal agricultural soils.

14C – a tool for separation of autotrophic and heterotrophic soil respiration

AbstractWe assessed the potential of using 14C contents of soil respired CO2 to calculate the contributions of heterotrophic and autotrophic respiration to total soil respiration. The partitioning of these fluxes is of utmost importance to evaluate implications of environmental change on soil carbon cycling and sequestration. At three girdled forest stands in Sweden and Germany, where the tree root (autotrophic) respiration had been eliminated, we measured both flux rates and 14C contents of soil respired CO2 in girdled and control plots in the summers of 2001 or 2002. At all stands, CO2 flux rates were slightly higher in the control plots, whereas the 14C contents of respired CO2 tended to be higher in the girdled plots. This was expected and confirmed that heterotrophically respired CO2 cycles more slowly through the forest ecosystem than autotrophically respired CO2. On the basis of these data, the contributions of hetero‐ and autotrophic respiration to total soil respiration were calculated using two separate approaches (i.e. based on flux rates or 14C). Fractions of heterotrophic respiration ranged from 53% to 87%. Values calculated by both approaches did not differ significantly from each other. Finally, we compared the 14C contents of soil respired CO2 in the girdled plots with the 14C contents of heterotrophically respired CO2 calculated by three different 14C models. None of the models matched the measured data sufficiently. In addition, we suspect that inherent effects of girdling may cause the 14C content of CO2 respired in the girdled plots to be lower than ‘true’ heterotrophically respired CO2 in an undisturbed plot. Nevertheless, we argue that measurements and modeling of 14C can be developed into a valuable tool for separating heterotrophic and autotrophic soil respiration (e.g. when girdling cannot be performed).

Production of CO2 in Soil Profiles of a California Annual Grassland

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO2 to the atmosphere. Although a large number of studies have focused on the CO2 flux at the soil–air interface, relatively few studies have examined the rates of CO2 production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO2 in the soil air, but the sources of this CO2 and the spatiotemporal dynamics of CO2 production throughout the soil profile are poorly understood. We studied CO2 dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO2 concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO2 production with soil profile depth. The CO2 diffusion constants were checked for accuracy using measured soil air 222Rn activities. The modeled net CO2 production rates were compared with CO2 fluxes measured at the soil surface. In general, the modeled and measured net CO2 fluxes were very similar although the model consistently underestimated CO2 production rates in the surficial soil horizons when the soils were moist. Profile CO2 production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO2 production in both the surface and subsurface soil horizons. Whole profile CO2 production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO2 flux from the soil profiles are relatively low (4–11 mg C– CO2 m−2 h−1), 20%–50% of the CO2 diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance 14C signatures of the CO2 and soil organic C suggest that the subsurface CO2 is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.

Modeling soil CO2 emissions from ecosystems

We present a new soil respiration model, describe a formal model testing procedure, and compare our model with five alternative models using an extensive data set of observed soil respiration. Gas flux data from rangeland soils that included a large number of measurements at low temperatures were used to model soil CO2 emissions as a function of soil temperature and water content. Our arctangent temperature function predicts that Q10 values vary inversely with temperature and that CO2 fluxes are significant below 0 °C. Independent data representing a broad range of ecosystems and temperature values were used for model testing. The effects of plant phenology, differences in substrate availability among sites, and water limitation were accounted for so that the temperature equations could be fairly evaluated. Four of the six tested models did equally well at simulating the observed soil CO2 respiration rates. However, the arctangent variable Q10 model agreed closely with observed Q10 values over a wide range of temperatures (r2 = 0.94) and was superior to published variable Q10 equations using the Akaike information criterion (AIC). The arctangent temperature equation explained 16–85% of the observed intra-site variability in CO2 flux rates. Including a water stress factor yielded a stronger correlation than temperature alone only in the dryland soils. The observed change in Q10 with increasing temperature was the same for data sets that included only heterotrophic respiration and data sets that included both heterotrophic and autotrophic respiration.

Radiocarbon – a low‐impact tool to study nutrient transport by soil fungi under field conditions

Here, we present a new in-situ method to study the uptake of amino acids by soil fungi. We injected 14C-labeled glycine into a marshland soil and measured the rate and the 14C signature of CO2 respired from sporocarps of Pholiota terrestris over 53.5 h and 2 m. We also determined the incorporation of glycine-C into sporocarp tissue. The 14C signature of the CO2 and tissue was quantified by accelerator mass spectrometry. After the label application, the rate of CO2 flux and its 14C signature from chambers with sporocarps were significantly higher than from chambers without sporocarps, and then declined with time. Postlabel, the 14C signature of the sporocarp tissue increased by 35 per thousand. We show that this approach can be used to study below-ground food webs on an hourly time-scale while minimizing the perturbation of competitive relationships among soil microorganisms and between plants and soil microorganisms. Additionally we show that care must be taken to avoid confounding effects of sporocarp senescence on rates and radiocarbon signatures of respired CO2.