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.     

Uncoupling of microbial CO2 production and release in frozen soil and its implications for field studies of arctic C cycling

Knowledge of seasonal trends and controls of soil CO₂ emissions to the atmosphere is important for simulating atmospheric CO₂ concentrations and for understanding and predicting the global carbon cycle. This is particularly the case for high arctic soils subject to extreme fluctuating environmental conditions. Based on field measurements of soil CO₂ efflux, temperature, water content, pore gas composition in soil and frozen cores as well as detailed temperature experiments performed in the laboratory, we evaluated seasonal controls of CO₂ effluxes from a well-drained tundra heath site in NE-Greenland. During the growing season, near-surface temperatures correlated well with observed CO₂ effluxes (r²>0.9). However, during intensive thawing of near-surface layers we observed up to 1.5-fold higher effluxes than expected due to temperature alone. These high rates were consistent with high CO₂ concentrations in frozen soil (>10% CO₂) and suggested a spring burst event during soil thawing and a corresponding trapping of produced CO₂ during winter. Laboratory experiments revealed that microbial soil respiration continued down to a least −18 °C and that up to 80% of the produced CO₂ was trapped in soil at temperatures between 0 and −9 °C. The trapping of CO₂ in frozen soil was positively correlated with soil moisture (r²=0.85) and led to an abrupt change of the temperature sensitivity (Q₁₀) observed for soil CO₂ release at 0 °C with Q₁₀ values below 0 °C being up to 100-fold higher than above 0 °C. The results of sub-zero CO₂ production allowed us to predict the microbial soil respiration throughout the year and to evaluate to what extent burst events during thawing can be explained by the release of CO₂ being produced and trapped during winter. Taking only the upper 20 cm of the soil into account, winter soil respiration accounted for about 40% of the annual soil respiration. At least 14% of the winter CO₂ production was trapped during the winter 2000-2001 and observed to be released upon thawing. Thus, the site-specific winter soil respiration is an important part of the annual C cycle and CO₂ trapping should be accounted for in future field and modelling studies of soil respiration dynamics in arctic ecosystems. In conclusion, we have discovered a soil moisture dependent uncoupling of CO₂ production and release in frozen soils with important implications for future field studies of Arctic C cycling.     

A new technique to measure soil CO2 efflux at constant CO2 concentration

A new principle for measuring soil CO₂ efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO₂ within the system to achieve a constant CO₂ concentration inside the soil chamber at ambient level, thus balancing the amount of CO₂ entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO₂ efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO₂ within the soil, while allowing for continuous capture of the CO₂ released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO₂ efflux and its δ¹³C, since both variables show sensitivity to a distortion of the soil CO₂ profile commonly found in conventional chamber techniques.     

Measurement of soil CO2 efflux using soda lime absorption: both quantitative and reliable

Measurement of soil CO₂ efflux using a non-flow-through steady-state (NFT-SS) chamber with alkali absorption of CO₂ 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 CO₂ efflux were evaluated: (i) absorption capacity of the soda lime, (ii) additional and thus artifactual absorption of CO₂ by soda lime during the experimental procedure, (iii) variation in the CO₂ concentration inside the chamber headspace, and (iv) effects of chamber closure on soil CO₂ efflux. Soil CO₂ efflux, as measured using soda lime (with a range of quantities: 50, 100, and 200 g per 0.082 m² 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 CO₂ 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 CO₂ efflux from field soils.     

Factors controlling soil CO2 effluxes and the effects of rewetting on effluxes in adjacent deciduous, coniferous, and mixed forests in Korea

To better understand the factors that control forest soil CO₂ efflux and the effects of rewetting on efflux, we measured soil CO₂ efflux in adjacent deciduous, coniferous, and mixed forests in the central part of the Korean Peninsula over the course of one year. We also conducted laboratory rewetting experiments with soil collected from the three sites using three different incubation temperatures (4 °C, 10 °C, and 20 °C). Soil moisture (SM), soil organic matter (SOM), and total root mass values of the three sites were significantly different from one another; however, soil temperature (ST), observed soil CO₂ efflux and sensitivity of soil CO₂ efflux to ST (i.e., Q₁₀ = 3.7 ± 0.1) were not significantly different among the three sites. Soil temperature was a dominant control factor regulating soil CO₂ efflux during most of the year. We infer that soil CO₂ efflux was not significantly different among the sites due to similar ST and Q₁₀. Though a significant increase in soil CO₂ efflux following rewetting of dry soil was observed both in the field observations (60-170%) and laboratory incubation experiments (100-1000%), both the increased rates of soil CO₂ efflux and the magnitude of change in SM were not significantly different among the sites. The increased rates of soil CO₂ efflux following rewetting depended on the initial SM before rewetting. During drying phase after rewetting, a significant correlation between SM and soil CO₂ efflux was found, but the effect of ST on increased soil CO₂ efflux was not clear. Cumulative peak soil CO₂ efflux (11.3 ± 0.7 g CO₂ m⁻²) following rewetting in the field was not significantly different among the sites. Those evidences indicate that the observed similar rewetting effects on soil CO₂ efflux can be explained by the similar magnitude of change in SM after rewetting at the sites. We conclude that regardless of vegetation type, soil CO₂ efflux and the effect of rewetting on soil CO₂ efflux do not differ among the sites, and ST is a primary control factor for soil CO₂ efflux while SM modulates the effect of rewetting on soil CO₂ efflux. Further studies are needed to quantify and incorporate relationship of initial dryness of the soil and the frequency of the dry-wet cycle on soil CO₂ efflux into models describing carbon (C) processes in forested ecosystems.     

Soil CO2 efflux vs. soil respiration: Implications for flux models

In the long term, all CO₂ produced in the soil must be emitted by the surface and soil CO₂ efflux (FCO₂) must correspond to soil respiration (R_soil). In the short term, however, the efflux can deviate from the instantaneous soil respiration, if the amount of CO₂ stored in the soil pore-space (SCO₂) is changing. We measured FCO₂ continuously for one year using an automated chamber system. Simultaneously, vertical soil profiles of CO₂ concentration, moisture, and temperature were measured in order to assess the changes in the amount of CO₂ stored in the soil. R_soil was calculated as the sum of the rate of change of the CO₂ storage over time and FCO₂. The experiment was split into a warm and a cold season. The dependency of soil respiration and soil efflux on soil temperature and on soil moisture was analyzed separately. Only the moisture-driven model of the warm season was significantly different for FCO₂ and R_soil. At our site, a moisture-driven soil-respiration model derived from CO₂ efflux data would underestimate the importance of soil moisture. This effect can be attributed to a temporary storage of CO₂ in the soil pore-space after rainfalls where up to 40% of the respired CO₂ were stored.     

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.     

Effects of elevated CO2 concentration on rhizodeposition from Lolium perenne grown on soil exposed to 9 years of CO2 enrichment

The effects of enriched CO₂ atmosphere on partitioning of recently assimilated carbon were investigated in a plant-soil-microorganism system in which Lolium perenne seedlings were planted into cores inserted into the resident soil within a sward that had been treated with elevated CO₂ for 9 consecutive years, under two N fertilisation levels (Swiss FACE experiment). The planted cores were excavated from the ambient (35 Pa pCO₂) and enriched (60 Pa pCO₂) rings at two dates, in spring and autumn, during the growing season. The cores were brought back to the laboratory for ¹⁴C labelling of shoots in order to trace the transfer of recently assimilated C both within the plant and to the soil and microbial biomass. At the spring sampling, high N supply stimulated shoot and total dry matter production. Consistently, high N enhanced the allocation of recently fixed C to shoots, and reduced it to belowground compartments. Elevated CO₂ had no consequences for DM or the pattern of C allocation. At the autumn sampling, at high N plot, yield of L. perenne was stimulated by elevated CO₂. Consistently, ¹⁴C was preferentially allocated aboveground and, consequently belowground recent C allocation was depressed and rhizodeposition reduced. At both experimental periods, total soil C content was similar in all treatments, providing no evidence for soil carbon sequestration in the Swiss Free Air CO₂ Enrichment experiment (FACE) after 9 years of enrichment. Recently assimilated C and soil C were mineralised faster in soils from enriched rings, suggesting a CO₂-induced shift in the microbial biomass characteristics (structure, diversity, activity) and/or in the quality of the root-released organic compounds.     

Impact of atmospheric CO2 and plant life forms on soil microbial activities

From the global change perspective, increase of atmospheric CO₂ and land cover transformation are among the major impacts caused by human activities. In this study, we are addressing the combined issues of the effect of CO₂ concentration increase and plant type on soil microbial activities by asking how annual and perennial plant groups affect soil microbial processes under elevated CO₂. The experimental design used a mix of species of different growth forms for both annuals and perennials. Our objective was: (1) to determine how two years of annual or perennial plant cover and CO₂ enrichment could affect Mediterranean soil microbial processes; (2) to test the resistance and the resilience of these soil functional processes after a natural perturbation. We determined the effects of 2 years atmospheric CO₂ enrichment on soil potential respiration (SIR), denitrification (DEA) and nitrification (NEA) activities. We could not find any significant effect of CO₂ increase on SIR, DEA and NEA. However, we found a strong effect of the plant cover type, i.e. annuals versus perennials, on the potential microbial activity related to N cycling. DEA and NEA were significantly higher in soil under annual plants while SIR was not significantly different. To determine whether these changes would survive a natural perturbation, we carried out a rain event experiment once the experimental treatments (i.e. different plant cover and atmospheric CO₂ concentration) were stopped. The soil potential respiration, as expressed by the SIR, was not affected and remained stable. DEA rates converged rapidly under annuals and perennials after the rain event. Under both annuals and perennials NEA increased significantly after the rain event but remained significantly higher in the soil with annual plants. The relative change of the soil microbial processes induced by annual and perennial plants was inversely related to the density and the diversity of the corresponding microbial functional groups.     

Theoretical model of the abiotic component of soil CO2 tracer efflux in C pulse-labeling experiments on plant-soil systems

For measurement of the time lag between photosynthesis and CO₂ efflux from soil, the carbon isotope pulse-labeling technique is considered as the most suitable. However, an interference from the abiotic tracer CO₂ component is identified as a key difficulty for obtaining accurate results with this technique. Guidelines on how to reduce this interference are therefore urgently needed. The flux of abiotic ¹³CO₂ tracer into soil during the labeling stage, and its return to atmosphere during the monitoring stage was modeled numerically, and the labeling stage also analytically. The controls of the abiotic interference were investigated using these models. The amount of the abiotic tracer component and the time distribution of its rate of return to the atmosphere, were predicted by these models. The main model parameters were D_m (=the ratio between the soil ¹³CO₂ diffusivity and the retardation factor), and the ¹³CO₂ concentration at the soil-atmosphere interface during the labeling stage (S₁₃), while background ¹³CO₂ soil production parameters were unnecessary. The presented models guide the selection of experimental parameters for minimization of the abiotic interference. With parameterization for a particular case, the present numerical model provides a preliminary order-of-magnitude estimate of the abiotic component, which would indicate if this interference is of significance.     

Short-term effects of clearfelling on soil CO2, CH4, and N2O fluxes in a Sitka spruce plantation

We examined the effects of forest clearfelling on the fluxes of soil CO₂, CH₄, and N₂O in a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation on an organic-rich peaty gley soil, in Northern England. Soil CO₂, CH₄, N₂O as well as environmental factors such as soil temperature, soil water content, and depth to the water table were recorded in two mature stands for one growing season, at the end of which one of the two stands was felled and one was left as control. Monitoring of the same parameters continued thereafter for a second growing season. For the first 10 months after clearfelling, there was a significant decrease in soil CO₂ efflux, with an average efflux rate of 4.0 g m⁻² d⁻¹ in the mature stand (40-year) and 2.7 g m⁻² d⁻¹ in clearfelled site (CF). Clearfelling turned the soil from a sink (−0.37 mg m⁻² d⁻¹) for CH₄ to a net source (2.01 mg m⁻² d⁻¹). For the same period, soil N₂O fluxes averaged 0.57 mg m⁻² d⁻¹ in the CF and 0.23 mg m⁻² d⁻¹ in the 40-year stand. Clearfelling affected environmental factors and lead to higher daily soil temperatures during the summer period, while it caused an increase in the soil water content and a rise in the water table depth. Despite clearfelling, CO₂ remained the dominant greenhouse gas in terms of its greenhouse warming potential.     

Explaining temporal variation in soil CO2 efflux in a mature spruce forest in Southern Germany

An open dynamic chamber system was used to measure the soil CO₂ efflux intensively and continuously throughout a growing season in a mature spruce forest (Picea abies) in Southern Germany. The resulting data set contained a large amount of temporally highly resolved information on the variation in soil CO₂ efflux together with environmental variables. Based on this background, the dependencies of the soil CO₂ efflux rate on the controlling environmental factors were analysed in-depth. Of the abiotic factors, soil temperature alone explained 72% of the variation in the efflux rate, and including soil water content (SWC) as an additional variable increased the explained variance to about 83%. Between April and December, average rates ranged from 0.43 to 5.15 μmol CO₂ m⁻² s⁻¹ (in November and July, respectively) with diurnal variations of up to 50% throughout the experiment. The variability in wind speed above the forest floor influenced the CO₂ efflux rates for measuring locations with a litter layer of relatively low bulk density (and hence relatively high proportions of pore spaces). For the temporal integration of flux rates for time scales of hours to days, however, wind velocities were of no effect, reflecting the fact that wind forcing acts on the transport, but not the production of CO₂ in the soil. The variation in both the magnitude of the basal respiration rate and the temperature sensitivity throughout the growing season was only moderate (coefficient of variation of 15 and 25%, respectively). Soil water limitation of the CO₂ production in the soil could be best explained by a reduction in the temperature-insensitive basal respiration rate, with no discernible effect on the temperature sensitivity. Using a soil CO₂ efflux model with soil temperature and SWC as driving variables, it was possible to calculate the annual soil CO₂ efflux for four consecutive years for which meteorological data were available. These simulations indicate an average efflux sum of 560 g C m⁻² yr⁻¹ (SE=22 g C m⁻² yr⁻¹). An alternative model derived from the same data but using temperature alone as a driver over-estimated the annual flux sum by about 7% and showed less inter-annual variability. Given a likely shift in precipitation patterns alongside temperature changes under projected global change scenarios, these results demonstrate the necessity to include soil moisture in models that calculate the evolution of CO₂ from temperate forest soils.     

Effects of elevated CO2 and O3 on soil respiration under ponderosa pine

Soil respiration represents the integrated response of plant roots and soil organisms to environmental conditions and the availability of C in the soil. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing a reconstructed ponderosa pine/soil-litter system. The study used a 2×2 factorial design with two levels of CO₂ and two levels of O₃ and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO₂ and O₃, singly and in combination, on soil respiration, fine root growth and soil organisms. Fine root growth and soil organisms were included in the study as indicators of the autotrophic and heterotrophic components of soil respiration. The study evaluated three hypotheses: (1) elevated CO₂ will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O₃ will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO₂ and O₃ have opposing effects on C assimilation and allocation, elevated CO₂ will eliminate or reduce the negative effects of elevated O₃ on soil respiration. A mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting when testing for the effects of CO₂ and O₃ on soil respiration. The covariance analysis showed that elevated CO₂ significantly reduced the soil respiration while elevated O₃ had no significant effect. Despite the lack of a direct CO₂ stimulation of soil respiration, there were significant interactions between CO₂ and soil temperature, soil moisture and days from planting indicating that elevated CO₂ altered soil respiration indirectly. In elevated CO₂, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO₂. Soil respiration increased more with days from planting in elevated than in ambient CO₂. Elevated CO₂ had no effect on fine root biomass but increased abundance of culturable bacteria and fungi suggesting that these increases were associated with increased C allocation belowground. Elevated CO₂ had no significant effect on microarthropod and nematode abundance. Elevated O₃ had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature.     

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.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

CO2浓度倍增对冬小麦生长发育产量形成及发芽率的影响

利用OTC-1型开顶式气室进行了CO₂浓度倍增对冬小麦影响的诊断试验，结果表明，CO₂浓度倍增对冬小麦生长发育、叶面积变化、生物量及产量形成等影响显著，且均为正效应。     

几种主要CO2施肥肥源性能的比较与评价

在空闲拱棚和黄瓜日光温室内，分别研究了化学反应法(H₂SO₄+NH₄HCO₃）、煤球燃烧法和颗粒CO₂气肥3种肥源的性能，并与液体CO₂进行成本比较，结果表明：化学反应法产气迅速，设备折旧成本较低；煤球燃烧法产气速度中等，原料成本最低；颗粒CO₂气肥产气速度较慢且不易调控，原料成本最高。考虑化学反应产物的再利用因素，化学反应法、煤球燃烧法和液体CO₂ 3种肥源总成本接近，但从生态、节能、成本和效果等方面综合评价，煤球燃烧法原料丰富、成本低廉，较符合我国目前的设施、经济、资源和技术条件。     

Elevated CO2 stimulates N2O emissions in permanent grassland

To evaluate climate forcing under increasing atmospheric CO₂ concentrations, feedback effects on greenhouse gases such as nitrous oxide (N₂O) with a high global warming potential should be taken into account. This requires long-term N₂O flux measurements because responses to elevated CO₂ may vary throughout annual courses. Here, we present an almost 9 year long continuous N₂O flux data set from a free air carbon dioxide enrichment (FACE) study on an old, N-limited temperate grassland. Prior to the FACE start, N₂O emissions were not different between plots that were later under ambient (A) and elevated (E) CO₂ treatments, respectively. However, over the entire experimental period (May 1998–December 2006), N₂O emissions more than doubled under elevated CO₂ (0.90 vs. 2.07 kg N₂O-N ha⁻¹ y⁻¹ under A and E, respectively). The strongest stimulation occurred during vegetative growth periods in the summer when soil mineral N concentrations were low. This was surprising because based on literature we had expected the highest stimulation of N₂O emissions due to elevated CO₂ when mineral N concentrations were above background values (e.g. shortly after N application in spring). N₂O emissions under elevated CO₂ were moderately stimulated during late autumn–winter, including freeze–thaw cycles which occurred in the 8th winter of the experiment. Averaged over the entire experiment, the additional N₂O emissions caused by elevated CO₂ equaled 4738 kg CO₂-equivalents ha⁻¹, corresponding to more than half a ton (546 kg) of CO₂ ha⁻¹ which has to be sequestered annually to balance the CO₂-induced N₂O emissions. Without a concomitant increase in C sequestration under rising atmospheric CO₂ concentrations, temperate grasslands may be converted into greenhouse gas sources by a positive feedback on N₂O emissions. Our results underline the need to include continuous N₂O flux measurements in ecosystem-scale CO₂ enrichment experiments.     

Effects of root and litter exclusion on soil CO2 efflux and microbial biomass in wet tropical forests

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass at a soil depth of 25 cm in a secondary forest (dominated by Tabebuia heterophylla) and a pine (Pinus caribaea) plantation in the Luquillo Experimental Forest in Puerto Rico. The experimental plots were initially established in 1990, when root, forest floor mass and new litterfall were excluded for 7 y since then. Soil respiration was significantly reduced in the litter and root exclusion plots in both the secondary forest and the pine plantation compared with the control. Root exclusion had a greater effect on soil CO₂ efflux than the litter exclusion in the plantation, whereas a reversed pattern was observed in the secondary forest. The reduction of microbial biomass in the root exclusion plot was greater in the secondary forest (59%) than in the plantation (31%), while there was no difference of the reduction in the litter exclusion plots between these forests. Our results suggest that above-ground input and roots (root litter and exudates) differentially affect soil CO₂ efflux under different vegetation types.     

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.