Annual soil CO2 effluxes in the High Arctic: The role of snow thickness and vegetation type

Little work has been done to quantify annual soil CO₂ effluxes in the High Arctic region because of the difficulty in taking winter measurements. Since the effects of climate change are expected to be higher in Arctic than in temperate ecosystems, it is important that summer measurements are extended to cover the entire year. This study evaluates the quantity and quality of soil organic C (SOC) and seasonal controls of soil CO₂ effluxes in three soils under three dominating types of vegetation (Dryas, Cassiope, and Salix) at Svalbard. Measurements included soil CO₂ effluxes in the field and the laboratory, temperature, water content, and snow thickness. About 90% of the variation in soil respiration throughout 1 year was due to near-surface soil temperatures which ranged from −12 to +12 °C. Total annual soil CO₂ effluxes varied from 103 g C m⁻² at soils under Cassiope, 152 g C m⁻² under Dryas sites, and 176 g C m⁻² under Salix, with 20%, 14%, and 30%, respectively, being released during a 6-month winter period. The sensitivity of soil respiration with respect to soil temperature was the same year round and differences in winter CO₂ effluxes at the three vegetation types were mainly related to subsurface soil temperatures controlled by snow depth. The quantity and quality of soil organic matter varied under the different vegetation types. Soils under Salix had the largest and most labile pool of SOC and were characterized by a long period of snow cover. In contrast, soils under Cassiope were more nutrient-poor, more acidic and held the smallest amount of total and labile SOC, whereas soils under Dryas remained snow-free most of the winter and therefore had the coldest winter conditions. Thus, winter soil respiration rates under Dryas and Cassiope were significantly lower than those under Salix; under Dryas this was mainly due to snow depth, under Cassiope this was a combination of snow depth and poor litter quality. It is concluded that winter respiration is highly variable across Arctic landscapes and depends on the spatial distribution of snow, which acts as a direct control on soil temperatures and indirect on vegetation types and thereby, the amount and quality of soil organic matter, which serve as additional important drivers of soil respiration.     

Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative winter soil CO₂ fluxes are reported to be a significant component of their annual carbon budgets. However, little information on winter soil CO₂ efflux is known from mid-latitude ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of ecosystem carbon budgets and the response of soil CO₂ efflux to climate changes. In this study we measured winter soil CO₂ efflux and its contribution to annual soil respiration for seven ecosystems (three forests: Pinus sylvestris var. mongolica plantation, Larix principis-rupprechtii plantation and Betula platyphylla forest; two shrubs: Rosa bella and Malus baccata; and two meadow grasslands) in a forest-steppe ecotone, north China. Overall mean winter and growing season soil CO₂ effluxes were 0.15-0.26 μmol m⁻² s⁻¹ and 2.65-4.61 μmol m⁻² s⁻¹, respectively, with significant differences in the growing season among the different ecosystems. Annual Q₁₀ (increased soil respiration rate per 10 °C increase in temperature) was generally higher than the growing season Q₁₀. Soil water content accounted for 84% of the variations in growing season Q₁₀ and soil temperature range explained 88% of the variation in annual Q₁₀. Soil organic carbon density to 30 cm depth was a good surrogate for SR₁₀ (basal soil respiration at a reference temperature of 10 °C). Annual soil CO₂ efflux ranged from 394.76 g C m⁻² to 973.18 g C m⁻² using observed ecosystem-specific response equations between soil respiration and soil temperature. Estimates ranged from 424.90 g C m⁻² to 784.73 g C m⁻² by interpolating measured soil respiration between sampling dates for every day of the year and then computing the sum to obtain the annual value. The contributions of winter soil CO₂ efflux to annual soil respiration were 3.48-7.30% and 4.92-7.83% using interpolated and modeled methods, respectively. Our results indicate that in mid-latitude ecosystems, soil CO₂ efflux continues throughout the winter and winter soil respiration is an important component of annual CO₂ efflux.     

Long-term effects of seasonal and diurnal temperature fluctuations on carbon dioxide efflux from a forest soil

Temperature fluctuations are a fundamental entity of the soil environment in the temperate zone and show fast (diurnal) and slow (seasonal) dynamics. Responses of soil respiration to temperature fluctuations were investigated in a root-free soil of a mid-European beech-oak forest. First, in laboratory we analysed the efflux of CO₂ from soil microcosms exposed to seasonal (±5 °C of the annual mean) and diurnal fluctuations (±5 °C of the seasonal levels) in a two-factorial design. Second, in field microcosms we investigated effects of smoothing diurnal temperature fluctuations in soil (simulating a possible global trend) on CO₂ efflux. Third, the natural temperature regime was simulated in laboratory microcosms and their CO₂ efflux was compared to the one in the field. The experiments lasted for 1 year to differentiate seasonal and annual responses.Dynamics of CO₂ efflux, microbial basal respiration, biomass and qO₂ varied with seasonal temperature regime. However, in the laboratory the annual cumulative CO₂-C production did not differ between treatments and varied between 10.9% and 11.7% of the total microcosm C, disregarding seasonal and/or diurnal fluctuations. The similarity of cumulative C production suggests that the availability of microbially mobilisable carbon pools rather than the temperature regime limited soil respiration. Diurnal fluctuations generally did not affect CO₂ efflux and microbial activity, though winter Q₁₀ values were increased in their absence. Simulation of the natural temperature regime in the laboratory resulted in CO₂ efflux similar to field microcosms. In the field, rates of CO₂ efflux and microbial activity, seasonal and annual cumulative CO₂-C production were significantly higher at smoothed than at natural temperature conditions (annually 13.1% and 11.0% of total C was respired, respectively). Facing global climate changes the mechanisms regulating responses of soil respiration to temperature fluctuations need further investigation.     

Strong temperature dependence and no moss photosynthesis in winter CO2 flux for a Kobresia meadow on the Qinghai-Tibetan plateau

We examined the CO₂ exchange of a Kobresia meadow ecosystem on the Qinghai-Tibetan plateau using a chamber system. CO₂ efflux from the ecosystem was strongly dependence on soil surface temperature. The CO₂ efflux-temperature relationship was identical under both light and dark conditions, indicating that no photosynthesis could be detected under light conditions during the measurement period. The temperature sensitivity (Q₁₀) of the CO₂ efflux showed a marked transition around −1.0 °C; Q₁₀ was 2.14 at soil surface temperatures above and equal to −1.0 °C but was 15.3 at temperatures below −1.0 °C. Our findings suggest that soil surface temperature was the major factor controlling winter CO₂ flux for the alpine meadow ecosystem and that freeze-thaw cycles at the soil surface layer play an important role in the temperature dependence of winter CO₂ flux.     

Microbial communities, biomass, and activities in soils as affected by freeze thaw cycles

Two Finnish agricultural soils (peat soil and loamy sand) were exposed to four freeze-thaw cycles (FTC), with a temperature change from −17.3±0.4 °C to +4.1±0.4 °C. Control cores from both soils were kept at constant temperature (+6.6±2.0 °C) without FTCs. Soil N₂O and CO₂ emissions were monitored during soil thawing, and the effects of FTCs on soil microbes were studied. N₂O emissions were extremely low in peat soil, possibly due to low soil water content. Loamy sand had high N₂O emission, with the highest emission after the second FTC. Soil freeze-thaw increased anaerobic respiration in both soil types during the first 3-4 FTCs, and this increase was higher in the peat soil. The microbial community structure and biomass analysed with lipid biomarkers (phospholipid fatty acids, 3- and 2- hydroxy fatty acids) were not affected by freezing-thawing cycles, nor was soil microbial biomass carbon (MIB-C). Molecular analysis of the microbial community structure with temperature gradient gel electrophoresis (TGGE) also showed no changes due the FTCs. These results show that freezing and thawing of boreal soils does not have a strong effect on microbial biomass or community structure.     

Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

To better understand the biotic and abiotic factors that control soil CO₂ efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO₂ effluxes and soil CO₂ concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO₂ sensors for long-term continuous real-time measurement of CO₂ concentrations at different depths, and measured half-hourly soil CO₂ effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO₂ diffusivity in undisturbed soil cores. The method accounts for the CO₂ production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO₂ concentration at all depths increased with increase in soil temperature, likely due to a rise in CO₂ production, and with increase in soil water content due to decreased diffusivity or increased CO₂ production or both. It also increased with soil depth reaching almost 10 mmol mol⁻¹ at the 50-cm depth. Annually, soil CO₂ efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C (F₁₀) of 2.6 μmol m⁻² s⁻¹ and the Q₁₀ of 3.7. No evidence of displacement of CO₂-rich soil air with rain was observed.Effluxes calculated from soil CO₂ concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO₂ efflux originated in the top 20 cm soil. Calculated CO₂ production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO₂ efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO₂ storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO₂ efflux was proportional to CO₂ concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO₂ in the soil. The latter proved successful in calculating effluxes during 2004.     

Soil CO2 efflux in a temperate deciduous forest: Environmental drivers and component contributions

Soil CO₂ efflux is a large component of total respiration in many ecosystems. It is important to understand the environmental controls on soil CO₂ efflux, in order to evaluate potential responses of ecosystems to climate change. This study investigated the relationship between total soil CO₂ efflux and soil temperature, soil moisture and solar radiation on an interannual basis for a plot of temperate deciduous ancient semi-natural woodland at Wytham Woods in central southern England. We also aimed to quantify the contribution of soil organic matter decomposition (SOM), root-and-rhizosphere respiration, and mycorrhizal respiration components to total soil CO₂ efflux, and determine their environmental correlates. Total soil CO₂ efflux was measured regularly from April 2006 to December 2008 and found to average 4.1 Mg C ha⁻¹ yr⁻¹ in both 2007 and 2008. In addition, we applied a recently developed approach to partition the efflux into SOM, root-and-rhizosphere, and mycorrhizal components in situ using mesh bags. SOM decomposition, root-and-rhizosphere, and mycorrhizal respiration were estimated to contribute 70 ± 6%, 22 ± 6% and 8 ± 3% of total soil CO₂ efflux respectively, equating to 3.0 ± 0.3, 0.9 ± 0.2 and 0.3 ± 0.1 Mg C ha⁻¹ yr⁻¹. In order to avoid the effect of temporal correlation between variables caused by seasonality, we investigated interannual variability by examining the relationship between CO₂ flux anomalies and anomalies in environmental variables. Variation in soil temperature explained 50% of the interannual variance in soil CO₂ efflux, and soil moisture a further 18% of the residual variance. Solar radiation, as a proxy for plant photosynthesis, had no significant effect on total soil CO₂ efflux, but was positively correlated with root-and-rhizosphere respiration, and mycorrhizal respiration. The relationship between anomalies in soil CO₂ efflux and soil temperature was highly significant, with a sensitivity of 0.164 ± 0.023 μmol CO₂ m⁻² s⁻¹ °C⁻¹. For mean peak summer efflux rates (2.03 μmol CO₂ m² s⁻¹), this is equivalent to 8% per °C, or a Q₁₀ temperature sensitivity of 2.2 ± 0.2. We demonstrate the utility of an anomaly analysis approach and conclude that soil temperature is the key driver of total soil CO₂ efflux primarily through its positive relationship with SOM-decomposition rate.     

Nitrous oxide emissions from agricultural soils at low temperatures: a laboratory microcosm study

We studied in laboratory microcosms (intact soil cores) N₂O and CO₂ emissions from four different agricultural soil types (organic soil, clay, silt and loam) at low temperatures with or without freezing-thawing events. When the temperature of the frozen soil cores was increased stepwise from −8 °C the N₂O emissions began to increase at −0.5 °C, and peaked at −0.1 °C in the organic, clay and silt soils, and at +1.6 °C in the loam soils. However, a stepwise decrease in soil temperature from +15 °C also induced an increase in the N₂O emissions close to the 0 °C. These emissions peaked between −0.4 and +2.5 °C depending on the soil type and water content. However, the emission maxima were from 2 to 14.3% of those encountered in the experiments where frozen soils were thawed. Our results show that in addition to the well-documented thawing peak, soils also can have a maximum in their N₂O emission near 0 °C when soil temperature decrease. These emissions, however, are less than those emitted from thawing soils. The correlations between the N₂O and CO₂ emissions were weak. Our results suggest that N₂O is produced in soils down to a temperature of −6 °C.     

Microbial activity in soils frozen to below −39 °C

Recent research on life in extreme environments has shown that some microorganisms metabolize at extremely low temperatures in Arctic and Antarctic ice and permafrost. Here, we present kinetic data on CO₂ and ¹⁴CO₂ release from intact and ¹⁴C-glucose amended tundra soils (Barrow, Alaska) incubated for up to a year at 0 to −39°C. The rate of CO₂ production declined exponentially with temperature but it remained positive and measurable, e.g. 2-7 ng CO₂-C cm⁻³ soil d⁻¹, at −39 °C. The variation of CO₂ release rate (v) was adequately explained by the double exponential dependence on temperature (T) and unfrozen water content (W) (r²>0.98): v=A exp(λT+kW) and where A, λ and k are constants. The rate of ¹⁴CO₂ release from added glucose declined more steeply with cooling as compared with the release of total CO₂, indicating that (a) there could be some abiotic component in the measured flux of CO₂ or (b) endogenous respiration is more cold-resistant than substrate-induced respiration. The respiration activity was completely eliminated by soil sterilization (1 h, 121 °C), stimulated by the addition of oxidizable substrate (glucose, yeast extract), and reduced by the addition of acetate, which inhibits microbial processes in acidic soils (pH 3-5). The tundra soil from Barrow displayed higher below-zero activity than boreal soils from West Siberia and Sweden. The permafrost soils (20-30 cm) were more active than the samples from seasonally frozen topsoil (0-10 cm, Barrow). Finding measurable respiration to −39 °C is significant for determining, understanding, and predicting current and future CO₂ emission to the atmosphere and for understanding the low temperature limits of microbial activity on the Earth and on other planets.     

Grazing intensity alters soil respiration in an alpine meadow on the Tibetan plateau

Grazing intensity may alter the soil respiration rate in grassland ecosystems. The objectives of our study were to (1) determine the influence of grazing intensity on temporal variations in soil respiration of an alpine meadow on the northeastern Tibetan Plateau; and (2) characterise the temperature response of soil respiration under different grazing intensities. Diurnal and seasonal soil respiration rates were measured for two alpine meadow sites with different grazing intensities. The light grazing (LG) meadow site had a grazing intensity of 2.55 sheep ha⁻¹, while the grazing intensity of the heavy grazing (HG) meadow site, 5.35 sheep ha⁻¹, was approximately twice that of the LG site. Soil respiration measurements showed that CO₂ efflux was almost twice as great at the LG site as at the HG site during the growing season, but the diurnal and seasonal patterns of soil respiration rate were similar for the two sites. Both exhibited the highest annual soil respiration rate in mid-August and the lowest in January. Soil respiration rate was highly dependent on soil temperature. The Q₁₀ value for annual soil respiration was lower for the HG site (2.75) than for the LG site (3.22). Estimates of net ecosystem CO₂ exchange from monthly measurements of biomass and soil respiration revealed that during the period from May 1998 to April 1999, the LG site released 2040 g CO₂ m⁻² y⁻¹ to the atmosphere, which was about one third more than the 1530 g CO₂ m⁻² y⁻¹ released at the HG site. The results suggest that (1) grazing intensity alters not only soil respiration rate, but also the temperature dependence of soil CO₂ efflux; and (2) soil temperature is the major environmental factor controlling the temporal variation of soil respiration rate in the alpine meadow ecosystem.     

Influence of warming and enchytraeid activities on soil CO2 and CH4 fluxes

To determine the sum of ‘direct’ and ‘indirect’ effects of climatic change on enchytraeid activity and C fluxes from an organic soil we assessed the influence of temperature (4, 10 and 15 °C incubations) on enchytraeid populations and soil CO₂ and CH₄ fluxes over 116 days. Moisture was maintained at 60% of soil dry weight during the experimental period and measurements of enchytraeid biomass and numbers, and CO₂ and CH₄ fluxes were made after 3, 16, 33, 44, 65, 86 and 116 days. Enchytraeid population numbers and biomass increased in all temperature treatments with the greatest increase produced at 15 °C (to over threefold initial values by day 86). Results also showed that enchytraeid activity increased CO₂ fluxes by 10.7±4.5, 3.4±4.0 and 26.8±2.6% in 4, 10 and 15 °C treatments, respectively, with the greatest CO₂ production observed at 15 °C for the entire 116 day incubation period (P<0.05). The soil respiratory quotient analyses at lower temperatures (i.e. 4-10 °C) gave a Q₁₀ of 1.7 and 1.9 with and without enchytraeids, respectively. At temperatures above 10 °C (i.e. 10-15 °C) Q₁₀ significantly increased (P<0.01) and was 25% greater in the presence of enchytraeids (Q₁₀=3.4) than without (Q₁₀=2.6). In contrast to CO₂ production, no significant relationships were observed between net CH₄ fluxes and temperature and only time showed a significant effect on CH₄ production (P<0.01).Total soil CO₂ production was positively linked with enchytraeid biomass and mean soil CO₂-C production was 77.01±6.05 CO₂-C μg mg enchytraeid tissue⁻¹ day⁻¹ irrespective of temperature treatment. This positive relationship was used to build a two step regression model to estimate the effects of temperature on enchytraeid biomass and soil CO₂ respiration in the field. Predictions of potential CO₂ production were made using enchytraeid biomass data obtained in the field from two upland grassland sites (Sourhope and Great Dun Fell at the Moor House Nature Reserve, both in the UK). The findings of this work suggest that a 5 °C increase in atmospheric temperature above mean ambient temperature could have the potential to produce a significant increase in enchytraeid biomass resulting in a near twofold increase in soil CO₂ release from both soil types. The interaction between temperature and soil biology will clearly be an important determinant of soil respiration responses to global warming.     

Microbial decomposition of skeletal muscle tissue (Ovis aries) in a sandy loam soil at different temperatures

A laboratory experiment was conducted to determine the effect of temperature (2, 12, 22 °C) on the rate of aerobic decomposition of skeletal muscle tissue (Ovis aries) in a sandy loam soil incubated for a period of 42 days. Measurements of decomposition processes included skeletal muscle tissue mass loss, carbon dioxide (CO₂) evolution, microbial biomass, soil pH, skeletal muscle tissue carbon (C) and nitrogen (N) content and the calculation of metabolic quotient (qCO₂). Incubation temperature and skeletal muscle tissue quality had a significant effect on all of the measured process rates with 2 °C usually much lower than 12 and 22 °C. Cumulative CO₂ evolution at 2, 12 and 22 °C equaled 252, 619 and 905 mg CO₂, respectively. A significant correlation (P<0.001) was detected between cumulative CO₂ evolution and tissue mass loss at all temperatures. Q₁₀s for mass loss and CO₂ evolution, which ranged from 1.19 to 3.95, were higher for the lower temperature range (Q₁₀(2-12 °C)>Q₁₀(12-22 °C)) in the Ovis samples and lower for the low temperature range (Q₁₀(2-12 °C)<Q₁₀(12-22 °C)) in the control samples. Metabolic quotient and the positive relationship between skeletal muscle tissue mass loss and cumulative CO₂ evolution suggest that tissue decomposition was most efficient at 2 °C. These phenomena may be due to lower microbial catabolic requirements at lower temperature.     

Soil CO2 flux in six monospecific forest plantations in Southern Rwanda

Forest soils contain the largest carbon stock of all terrestrial biomes and are probably the most important source of carbon dioxide (CO₂) to atmosphere. Soil CO₂ fluxes from 54 to 72-year-old monospecific stands in Rwanda were quantified from March 2006 to December 2007. The influences of soil temperature, soil water content, soil carbon (C) and nitrogen (N) stocks, soil pH, and stand characteristics on soil CO₂ flux were investigated. The mean annual soil CO₂ flux was highest under Eucalyptus saligna (3.92 μmol m⁻² s⁻¹) and lowest under Entandrophragma excelsum (3.13 μmol m⁻² s⁻¹). The seasonal variation in soil CO₂ flux from all stands followed the same trend and was highest in rainy seasons and lowest in dry seasons. Soil CO₂ flux was mainly correlated to soil water content (R² = 0.36-0.77), stand age (R² = 0.45), soil C stock (R² = 0.33), basal area (R² = 0.21), and soil temperature (R² = 0.06-0.17). The results contribute to the understanding of factors that influence soil CO₂ flux in monocultural plantations grown under the same microclimatic and soil conditions. The results can be used to construct models that predict soil CO₂ emissions in the tropics.     

Rice root-derived carbon input and its effect on decomposition of old soil carbon pool under elevated CO2

Rice (Oryza sativa) was grown in sunlit, semi-closed growth chambers (4×3×2 m, L×W×H) at 650 μl l⁻¹ CO₂ (elevated CO₂) to determine: (1) rice root-derived carbon (C) input into the soil under elevated CO₂ in one growing season, and (2) the effect of the newly input C on decomposition of the more recalcitrant native soil organic C. The initial δ¹³C value of the experimental soil was −25.8‰, which was 6‰ less depleted in ¹³C than the plants grown under elevated CO₂. Significant changes in δ¹³C of the soil organic C were detected after one growing season. The amount of new soil C input was estimated to be 0.9 t ha⁻¹ (or 2.1%) at 30 kg N ha⁻¹ and 1.8 t ha⁻¹ (4.1%) at 90 kg N ha⁻¹. Changes in soil δ¹³C suggested that the surface 5 cm of soil received more C input from plants than soils below. Laboratory incubation (25 °C) of soils from different horizons indicated that increased availability of the labile plant-derived C in the soil reduced decomposition of the native soil organic C. Provided the retardant effect of the new C on old soil organic C holds in the field in the longer-term, paddy soils will likely sequester more C from the atmosphere if more plant C enters the soil under elevated atmospheric CO₂.     

Quantification of priming and CO2 emission sources following the application of different slurry particle size fractions to a grassland soil

The highest emissions of CO₂ from soils and most pronounced priming effect (PE) from soils generally occur immediately after slurry application. However, the influence of different particle size slurry fractions on net soil C respiration dynamics and PE has not been studied. Therefore, a slurry separation technique based on particle sizes was used in the present study. Six distinct fractions (>2000, 425-2000, 250-425, 150-250, 45-150, <45 μm) were generated from two dairy slurries (one from cows fed a predominantly maize silage diet and the other from cows fed a grass silage diet) were applied to soil. During the first days of the 332 days experiment, all slurry fraction amendments significantly increased soil CO₂ effluxes (by 2-8 times) compared to the non-amended control. The increased CO₂ emission rates had a negative relationship with slurry particle size, but its duration was positively correlated with slurry particle size. The percentage of the cumulative CO₂ emitted was only higher in the first 8 days in the finest slurry particle sizes (<150 μm). The proportion of slurry-derived C emitted as CO₂ 2 h after addition to soil varied between 29% and 100% of total emitted CO₂-C. Generally, the proportion of slurry-derived C emitted initially decreased rapidly in the <250 μm fractions, but decreased more slowly or even increased in the >250 μm fractions. The overall contribution of slurry C to total CO₂ emissions was higher in smaller slurry particle size treatments in the first days after application. The addition of the various slurry fractions to soil caused both significant positive and negative PEs on the soil organic matter mineralization. The timing and type (positive or negative) of PE depended on the slurry particle size. Clearly, farm based separation pre-treatment leading to two or more fractions with different particle sizes has also the potential to reduce or modify short-term CO₂ emissions immediately after slurry application to soil.     

Carbon dynamics in a temperate grassland soil after 9 years exposure to elevated CO2 (Swiss FACE)

Elevated pCO₂ increases the net primary production, C/N ratio, and C input to the soil and hence provides opportunities to sequester CO₂-C in soils to mitigate anthropogenic CO₂. The Swiss 9 y grassland FACE (free air carbon-dioxide enrichment) experiment enabled us to explore the potential of elevated pCO₂ (60 Pa), plant species (Lolium perenne L. and Trifolium repens L.) and nitrogen fertilization (140 and 540 kg ha⁻¹ y⁻¹) on carbon sequestration and mineralization by a temperate grassland soil. Use of ¹³C in combination with respired CO₂ enabled the identification of the origins of active fractions of soil organic carbon. Elevated pCO₂ had no significant effect on total soil carbon, and total soil carbon was also independent of plant species and nitrogen fertilization. However, new (FACE-derived depleted ¹³C) input of carbon into the soil in the elevated pCO₂ treatments was dependent on nitrogen fertilization and plant species. New carbon input into the top 15 cm of soil from L. perennne high nitrogen (LPH), L. perenne low nitrogen (LPL) and T. repens low nitrogen (TRL) treatments during the 9 y elevated pCO₂ experiment was 9.3±2.0, 12.1±1.8 and 6.8±2.7 Mg C ha⁻¹, respectively. Fractions of FACE-derived carbon in less protected soil particles >53 μm in size were higher than in <53 μm particles. In addition, elevated pCO₂ increased CO₂ emission over the 118 d incubation by 55, 61 and 13% from undisturbed soil from LPH, LPL and TRL treatments, respectively; but only by 13, 36, and 18%, respectively, from disturbed soil (without roots). Higher input of new carbon led to increased decomposition of older soil organic matter (priming effect), which was driven by the quantity (mainly roots) of newly input carbon (L. perenne) as well as the quality of old soil carbon (e.g. higher recalcitrance in T. repens). Based on these results, the potential of well managed and established temperate grassland soils to sequester carbon under continued increasing concentrations of atmospheric CO₂ appears to be rather limited.     

Availability of CO2 as a factor affecting the rate of nitrification in soil

A laboratory incubation experiment was conducted to demonstrate that reduced availability of CO₂ in soil may be an important factor limiting nitrification. Soil samples were incubated at 30±2 °C for 20 days using vessels with or without the arrangement for trapping CO₂ in sodium hydroxide. This arrangement led to a decrease of ca. 96% in the CO₂ concentration of the headspace, with a range of 95.7-97.5 at different sampling intervals. In the absence of trapping arrangement, CO₂ concentration of the headspace varied from 580 to 859 ppm, i.e. 62-140% higher than that of the outside atmosphere (358 ppm). The nitrification process was significantly retarded under conditions of reduced CO₂ concentration; reduction varied from 8 to 62% at different incubation intervals. The results of the study led to the inference that decreased availability of CO₂ in closed vessels (with arrangement for trapping CO₂) will have a significant bearing on the process of nitrification and hence on the overall dynamics of N transformations.     

Interannual and interseasonal soil CO2 efflux and VOC exchange rates in a Mediterranean holm oak forest in response to experimental drought

Climate models predict drier conditions in the next decades in the Mediterranean basin. Given the importance of soil CO₂ efflux in the global carbon balance and the important role of soil monoterpene and volatile organic compounds (VOCs) in soil ecology, we aimed to study the effects of the predicted drought on soil CO₂, monoterpenes and other VOC exchange rates and their seasonal and interannual variations. We decreased soil water availability in a Mediterranean holm oak forest soil by means of an experimental drought system performed since 1999 to the present. Measurements of soil gas exchange were carried out with IRGA, GC and PTR-MS techniques during two annual campaigns of contrasting precipitation. Soil respiration was twice higher the wet year than the dry year (2.27±0.26 and 1.05±0.15, respectively), and varied seasonally from 3.76±0.85 μmol m⁻² s⁻¹ in spring, to 0.13±0.01 μmol m⁻² s⁻¹ in summer. These results highlight the strong interannual and interseasonal variation in CO₂ efflux in Mediterranean ecosystems. The drought treatment produced a significant soil respiration reduction in drought plots in the wet sampling period. This reduction was even higher in wet springs (43% average reduction). These results show (1) that soil moisture is the main factor driving seasonal and interannual variations in soil respiration and (2) that the response of soil respiration to increased temperature is constrained by soil moisture. The results also show an additional control of soil CO₂ efflux by physiology and phenology of trees and animals. Soil monoterpene exchange rates ranged from −0.01 to 0.004 nmol m⁻² s⁻¹, thus the contribution of this Mediterranean holm oak forest soil to the total monoterpenes atmospheric budget seems to be very low. Responses of individual monoterpenes and VOCs to the drought treatment were different depending on the compound. This suggests that the effect of soil moisture reduction in the monoterpenes and VOC exchange rates seems to be dependent on monoterpene and VOC type. In general, soil monoterpene and other VOC exchange rates were not correlated with soil CO₂ efflux. In all cases, only a low proportion of variance was explained by the soil moisture changes, since almost all VOCs increased their emission rates in summer 2005, probably due to the effect of high soil temperature. Results indicate thus that physical and biological processes in soil are controlling soil VOC exchange but further research is needed on how these factors interact to produce the observed VOCs exchange responses.     

Soil carbon dioxide fluxes in established switchgrass land managed for biomass production

Switchgrass (Panicum virgatum L.) grown for biomass feedstock production has the potential to increase soil C sequestration, and soil CO₂ flux in grassland is an important component in the global C budget. The objectives of this study were to: (1) determine the effects of N fertilization and harvest frequency on soil CO₂ flux, soil microbial biomass carbon (SMBC), and potentially mineralizable carbon (PMC); and (2) evaluate the relationship of soil CO₂ flux with soil temperature, soil moisture, SMBC, and PMC. Two N rates (0 and 224 kg ha⁻¹) were applied as NH₄NO₃ and cattle (Bos Taurus L.) manure. Switchgrass was harvested every year at anthesis or alternate years at anthesis. The data were collected during growing season (May-October) 2001-2004 on switchgrass-dominated Conservation Reserve Program (CRP) land in east-central South Dakota, USA. Manure application increased soil CO₂ flux, SMBC, and PMC during the early portion of the growing season compared with the control, but NH₄NO₃ application did not affect soil CO₂ flux, SMBC, and PMC. However, seasonal variability of soil CO₂ flux was not related to SMBC and PMC. Estimated average soil CO₂ fluxes during the growing periods were 472, 488, and 706 g CO₂-C m⁻² for control, NH₄NO₃-N, and manure-N plots, respectively. Switchgrass land with manure application emitted more CO₂, and approximately 45% of the C added with manure was respired to the atmosphere. Switchgrass harvested at anthesis decreased soil CO₂ flux during the latter part of the growing season, and flux was lower under every year harvest treatment than under alternate years harvest. Soil temperature was the most significant single variable to explain the variability in soil CO₂ flux. Soil water content was not a limiting factor in controlling seasonal CO₂ flux.     

Temporal change in spatial variability of soil respiration on a slope of Japanese cedar (Cryptomeria japonica D. Don) forest

Although information regarding the spatial variability of soil respiration is important for understanding carbon cycling and developing a suitable sampling design for estimating average soil respiration, it remains relatively understudied compared to temporal changes. In this study, soil respiration was measured at 35 locations by season on a slope of Japanese cedar forest in order to examine temporal changes in the spatial distribution of soil respiration. Spatial variability of soil respiration varied between seasons, with the highest coefficient variation in winter (42%) and lowest in summer (26%). Semivariogram analysis and kriged maps revealed different patterns of spatial distribution in each season. Factors affecting the spatial variability were relief index (autumn), soil hardness of the A layer (winter), soil hardness at 50 cm depth (spring) and the altitude and relief index (summer). Annual soil respiration (average: 39 mol m⁻² y⁻¹) varied from 26 mol m⁻² y⁻¹ to 55 mol m⁻² y⁻¹ between the 35 locations and was higher in the upper part of the slope and lower in the lower part. The average Q₁₀ value was 2.3, varying from 1.3 to 3.0 among the locations. These findings suggest that insufficient information on the spatial variability of soil respiration and imbalanced sampling could bias estimates of current and future carbon budgets.