CO2 transported in xylem sap affects CO2 efflux from Liquidambar styraciflua and Platanus occidentalis stems, and contributes to observed wound respiration phenomena

The [CO₂] in the xylem of tree stems is typically two to three orders of magnitude greater than atmospheric [CO₂]. In this study, xylem [CO₂] was experimentally manipulated in saplings of sycamore (Platanus occidentalis L.) and sweetgum (Liquidambar styraciflua L.) by allowing shoots severed from their root systems to absorb water containing [CO₂] ranging from 0.04% to 14%. The effect of xylem [CO₂] on CO₂ efflux to the atmosphere from uninjured and mechanically injured, i.e., wounded, stems was examined. In both wounded and unwounded stems, and in both species, CO₂ efflux was directly proportional to xylem [CO₂], and increased 5-fold across the range of xylem [CO₂] produced by the [CO₂] treatment. Xylem [CO₂] explained 76–77% of the variation in pre-wound efflux. After wounding, CO₂ efflux increased substantially but remained directly proportional to internal stem [CO₂]. These experiments substantiated our previous finding that stem CO₂ efflux was directly related to internal xylem [CO₂] and expanded our observations to two new species. We conclude that CO₂ transported in the xylem may confound measurements of respiration based on CO₂ efflux to the atmosphere. This study also provided evidence that the rapid increase in CO₂ efflux observed after tissues are excised or injured is likely the result of the rapid diffusion of CO₂ from the xylem, rather than an actual increase in the rate of respiration of wounded tissues.     

Soil CO2 efflux in a beech forest: comparison of two closed dynamic systems

The aim of this study was to understand why two closed dynamic systems with a very similar design gave large differences in soil CO₂ efflux measurements (PP systems and LI-COR). Both in the field (forest beech stand) and in the laboratory, the PPsystems gave higher estimations of soil CO₂ efflux than the LI-COR system (ranging from 30% to 50%). The difference in wind speed occurring within the soil respiration chambers (0.9 m s⁻¹ within the SRC-1 and 0.4 m s⁻¹ within the LI-6000-09 chambers) may account for the discrepancy between the two systems. An excessive air movement inside the respiration chamber is thought to disrupt the high laminar boundary layer over the forest floor. This would promote an exhaust of the CO₂ accumulated into the upper soil layers into the chamber and a lateral diffusion of CO₂ in the soil towards the respiration chamber. The discrepancy between the two systems was reduced (i) by decreasing fan speed within the SRC-1, (ii) by increasing wind speed over the soil surface outside the respiration chamber, or (iii) by using an artificial soil design without high CO₂ concentration in soil pores. We show that wind speed is an important component of soil CO₂ diffusion which must be taken into account when measuring soil CO₂ efflux, even on very fine textured soil like silt-loam soil. Proper measurement can be achieved by maintaining wind speed inside the chamber below 0.4 m s⁻¹ since low wind speed conditions predominate under forest canopies. However, more accurate measurements will be obtained by regulating wind speeds within the chamber at a velocity representative of the wind speed recorded simultaneously at the floor surface. This revised version was published online in June 2006 with corrections to the Cover Date.     

Spatial and temporal variation in soil CO2 efflux in an old-growth neotropical rain forest, La Selva, Costa Rica

Our objectives were to quantify and compare soil CO₂ efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO₂ release. We measured soil CO₂ efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). At the same time we measured soil CO₂ concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep). Between old alluvium sites, the two-year average CO₂ flux rates ranged from 117.3 to 128.9 mg C m^–2 h^–1. Significantly higher soil CO₂ flux occurred on the residual sites (141.1 to 184.2 mg C m^–2 h^–1). Spatial differences in CO₂ efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content. Spatial variability in CO₂ storage was high and the amount of CO₂ stored in the upper and lower soil profile was different between old alluvial and residual sites. The major factor identified for explaining temporal variations in soil CO₂ efflux was soil water content. During periods of high soil water content CO₂ emission decreased, probably due to lower diffusion and CO₂ production rates. During the 2-year study period inter-annual variation in soil CO₂ efflux was not detected.     

Dynamics of CO2 efflux from the substrate root system of container-grown plants associated with irrigation cycles

In intensive horticultural crops, the choice of growing media and the adequate management of irrigation must ensure an optimal trade-off between aeration and water supply to roots. The proportion of gas-filled pores and their composition can be strongly affected by the water status and hence by irrigation. In this context, continuous measurement of gas exchange and water status of the growing medium could bring out some insights into how irrigation events affect root activity and aeration in a time scale of minutes to several hours. For this purpose, a measuring system was developed that measured the CO₂ efflux rate from the entire substrate root system of pot plants while their shoots were kept outside, undisturbed. It was able to monitor four plants at a time for several weeks at a rate of one measurement per plant every 10 min, thus tracing the dynamics of CO₂ efflux through a great many irrigation cycles. The results showed a marked pattern of CO₂ efflux around each irrigation event, consisting mainly of a sharp, conspicuous peak followed by a depression until a threshold in substrate water potential was reached. Analysis of these data suggests that the pattern is imposed mainly by the effects of irrigation and water content on the mobility of gases in the growing medium. The peak can be explained by the CO₂-enriched air being displaced by the water added to the growing medium in the pot, and the following depression can be the result of the reduced mobility of gases when substrate water content is high. In spite of the great variation in the instantaneous efflux rate of CO₂, the integration of these CO₂ values for the entire day provides a rather predictable value given the root biomass and does not seem to be affected by the number of irrigation events that occur in a given day.     

CO2 efflux from a Mediterranean semi-arid forest soil. II. Effects of soil fauna and surface stoniness

Many forest soils in the Mediterranean basin areshallow and contain high amounts of gravel in theorganic layers. Recent studies on soil organic matteraccumulation have shown high amounts of organic matteroccurring mainly in soils with high levels ofstoniness at the soil surface. The gravel layer mayaffect the microclimatic conditions of the soilsurface and probably the distribution and activity ofsoil fauna.In order to quantify the combined effects soil fauna(epigeic macrofauna and earthworms) and stoniness onthe release of soil CO₂, we performed a threefactor field experiment by using a series ofreconstructed soil profiles. Factors 1 and 2 consistedof the exclusion/presence of soil epigeic macrofaunaand earthworms, and factor 3 of the presence/absenceof a gravel layer intermingled with the H horizon. Weincubated ¹⁴C straw in the H horizon and carriedout three 40 mm rainfall simulations.Soil respiration primarily depended on the season. Theeffects of soil fauna were generally small and did notcoincide with periods of high faunal activity. Thelargest effects of both earthworms and soil epigeicfauna were found after wetting the soil in summer. Theeffects of the earthworms were concentrated in themineral soil while the effects of the epigeic faunawere concentrated in the H horizon and mainly arosetowards the end of the experiment. This suggests thatthe effects of epigeic fauna may have beenunderestimated due to the length of the experiment.The gravel layer increased the effect of faunaprobably by creating more favorable microclimaticconditions. The accumulation of organic matter insoils with high levels of stoniness cannot beexplained by the effect of gravel on soil microclimatenor by its effect on the activity of soil fauna.     

Response of soil CO2 efflux to water manipulation in a tallgrass prairie ecosystem

Although CO₂ efflux plays a critical role in carbon exchange between the biosphere and atmosphere, our understanding of its regulation by soil moisture is rather limited. This study was designed to examine the relationship between soil CO₂ efflux and soil moisture in a natural ecosystem by taking advantage of the historically long drought period from 29 July to 21 September 2000 in the southern Central Great Plain, USA. At the end of August when soil moisture content at the top 50 mm was reduced to less than 50 g kg^–1 gravimetrically, we applied 8 levels of water treatments (simulated to rainfall of 0, 10, 25, 50, 100, 150, 200, and 300 mm) with three replicates to 24 plots in a Tallgrass Prairie ecosystem in Central Oklahoma, USA. In order to quantify root-free soil CO₂ efflux, we applied the same 8 levels of water treatments to 24 500-mm soil columns using soil from field adjacent to the experimental plots. We characterized dynamic patterns of soil moisture and soil CO₂ efflux over the experimental period of 21 days. Both soil moisture content and CO₂ efflux showed dramatic increases immediately after the water addition, followed by a gradual decline. The time courses in response to water treatments are well described by Y=Y₀+ate^–bt, where Y is either soil moisture or CO₂ efflux, t is time, Y ₀, a, and b are coefficients. Among the 8 water treatments, the maximal soil CO₂ efflux rate occurred at the 50 mm water level in the field and 100 mm in the root-free soil 1 day after the treatment. The maximal soil CO₂ efflux gradually shifted to higher water levels as the experiment continued. We found the relationship between soil CO₂ efflux and soil moisture using the data from the 21-day experiment was highly scattered, suggesting complex mechanisms determining soil CO₂ efflux by soil moisture.     

杉木人工林不同深度土壤CO2通量

土壤CO₂通量具有明显的时间和空间变异性。土壤温度和含水量是影响土壤CO₂通量的重要因素,同时,不同深度的土壤CO₂通量对温度和含水量变化的响应差异较大,因此,研究土壤CO₂通量和影响因素随土壤深度的变化,对于准确评估土壤碳排放具有重要意义。选择福建三明杉木人工林(Cunninghamia lanceolata)作为研究对象,利用非散射红外CO₂浓度探头和Li-8100开路式土壤碳通量系统,并使用Fick扩散法计算了0-60cm深度土壤CO₂的通量,结果表明:(1)5种扩散模型计算的表层(5cm)CO₂通量与Li-8100测量结果均具有显著相关性(P<0.01),Moldrup气体扩散模型计算结果较好。(2)土壤CO₂浓度随深度的增加而升高,但60cm深度以下土壤CO₂浓度开始降低;不同深度土壤CO₂浓度的日变化均呈现单峰型;0-60cm土壤CO₂通量日通量均值变化范围为0.54-2.17μmol m^-2 s^-1;(3)指数拟合分析显示,5、10cm和60cm深度处土壤CO₂通量与温度具有显著相关性,Q₁₀值分别为1.35、2.01和4.95。不同深度土壤含水量与CO₂通量的相关性不显著。     

Effect of elevated atmospheric CO2 concentration on soil and root respiration in winter wheat by using a respiration partitioning chamber

Soil respiration in a cropland is the sum of heterotrophic (mainly microorganisms) and autotrophic (root) respiration. The contribution of both these types to soil respiration needs to be understood to evaluate the effects of environmental change on soil carbon cycling and sequestration. In this paper, the effects of free-air CO₂ enrichment (FACE) on hetero- and autotrophic respiration in a wheat field were differentiated and evaluated by a novel split-root growth and gas collection system. Elevated atmospheric pCO₂ of approximately 200 μmol mol⁻¹ above the ambient pCO₂ significantly increased soil respiration by 15.1 and 14.8% at high nitrogen (HN) and low nitrogen (LN) application rates, respectively. The effect of elevated atmospheric pCO₂ on root respiration was not consistent across the wheat growth stages. Elevated pCO₂ significantly increased and decreased root respiration at the booting-heading stage (middle stage) and the late-filling stage (late stage), respectively, in HN and LN treatments; however, no significant effect was found at the jointing stage (early stage). Thus, the effect of increased pCO₂ on cumulative root respiration for the entire wheat growing season was not significant. Cumulative root respiration accounted for approximately 25–30% of cumulative soil respiration in the entire wheat growing season. Consequently, cumulative microbial respiration (soil respiration minus root respiration) increased by 22.5 and 21.1% due to elevated pCO₂ in HN and LN, respectively. High nitrogen application significantly increased root respiration at the late stage under both elevated pCO₂ and ambient pCO₂; however, no significant effects were found on cumulative soil respiration, root respiration, and microbial respiration. These findings suggest that heterotrophic respiration, which is influenced by increased substrate supplies from the plant to the soil, is the key process to determine C emission from agro-ecosystems with regard to future scenarios of enriched pCO₂.     

Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO2 efflux in two semiarid grasslands in southern Botswana

Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO₂ efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO₂ efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO₂ efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO₂ efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO₂ efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.     

Forest soil respiration across three climatically distinct chronosequences in Oregon

To assess the relative influence of edaphoclimatic gradients and stand replacing disturbance on the soil respiration of Oregon forests, we measured annual soil respiration at 36 independent forest plots arranged as three replicates of four age classes in each of three climatically distinct forest types. Annual soil respiration for the year 2001 was computed by combining periodic chamber measurements with continuous soil temperature measurements, which were used along with site-specific temperature response curves to interpolate daily soil respiration between dates of direct measurement. Results indicate significant forest type, age, and type × age interaction effects on annual soil respiration. Average annual soil respiration was 1100–1600, 1500–2100, and 500–900 g C m⁻² yr⁻¹ for mesic spruce, montane Douglas-fir, and semi-arid pine forests respectively. Age related trends in annual soil respiration varied between forest types. The variation in annual soil respiration attributable to the climatic differences between forest types was 48%(CV). Once weighted by the age class distribution for each forest type, the variation in annual soil respiration attributable to stand replacing disturbance was 15%(CV). Sensitivity analysis suggests that the regional variation in annual soil respiration is most dependent on summer base rates (i.e. soil respiration normalized to a common temperature) and much less dependent on the site-specific temperature response curves (to which annual rates are relatively insensitive) and soil degree-days (which vary only 10% among plots).     

Non-phototrophic CO 2 fixation by soil microorganisms

Although soils are generally known to be a net source of CO₂ due to microbial respiration, CO₂ fixation may also be an important process. The non-phototrophic fixation of CO₂ was investigated in a tracer experiment with ¹⁴CO₂ in order to obtain information about the extent and the mechanisms of this process. Soils were incubated for up to 91 days in the dark. In three independent incubation experiments, a significant transfer of radioactivity from ¹⁴CO₂ to soil organic matter was observed. The process was related to microbial activity and could be enhanced by the addition of readily available substrates such as acetate. CO₂ fixation exhibited biphasic kinetics and was linearly related to respiration during the first phase of incubation (about 20–40 days). The fixation amounted to 3–5% of the net respiration. After this phase, the CO₂ fixation decreased to 1–2% of the respiration. The amount of carbon fixed by an agricultural soil corresponded to 0.05% of the organic carbon present in the soil at the beginning of the experiment, and virtually all of the fixed CO₂ was converted to organic compounds. Many autotrophic and heterotrophic biochemical processes result in the fixation of CO₂. However, the enhancement of the fixation by addition of readily available substrates and the linear correlation with respiration suggested that the process is mainly driven by aerobic heterotrophic microorganisms. We conclude that heterotrophic CO₂ fixation represents a significant factor of microbial activity in soils.     

Soil carbon dioxide efflux determined from large undisturbed soil cores collected in different soil management systems

The aim of this study was to evaluate a measuring technique for determining soil CO₂ efflux from large soil samples having undisturbed structure under controlled laboratory conditions. Further objectives were to use the developed measuring method for comparing soil CO₂ efflux from samples, collected in three different soil management systems at various soil water content values. The experimental technique was tested and optimised for timing of sampling by taking air samples after 1, 3 and 6 hours of incubation. Based on the results, the incubation time was set to three hours. The CO₂ efflux measured for different soil management systems was the highest in the no-till and the lowest in the ploughing treatment, which was in accordance with measurements on accessible organic carbon for microbes. An increase in CO₂ efflux with increasing soil water content was found in the studied soil water content range. Our results indicate that soil respiration rates, measured directly after tillage operations, can highly differ from those measured long after.     

Time-dependent responses of soil CO2 efflux components to elevated atmospheric [CO2] and temperature in experimental forest mesocosms

We previously used dual stable isotope techniques to partition soil CO₂ efflux into three source components (rhizosphere respiration, litter decomposition, and soil organic matter (SOM) oxidation) using experimental chambers planted with Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings. The components responded differently to elevated CO₂ (ambient + 200 mol mol^–1) and elevated temperature (ambient + 4 °C) treatments during the first year. Rhizosphere respiration increased most under elevated CO₂, and SOM oxidation increased most under elevated temperature. However, many studies show that plants and soil processes can respond to altered climates in a transient way. Herein, we extend our analysis to 2 years to evaluate the stability of the responses of the source components. Total soil CO₂ efflux increased significantly under elevated CO₂ and elevated temperature in both years (1994 and 1995), but the enhancement was much less in 1995. Rhizosphere respiration increased less under elevated temperature in 1995 compared with 1994. Litter decomposition also tended to increase comparatively less in 1995 under elevated CO₂, but was unresponsive to elevated temperature between years. In contrast, SOM oxidation was similar under elevated CO₂ in the 2 years. Less SOM oxidation occurred under elevated temperature in 1995 compared with 1994. Our results indicate that temporal variations can occur in CO₂ production by the sources. The variations likely involve responses to antecedent physical disruption of the soil and physiological processes.     

On the assessment of root and soil respiration for soils of different textures: interactions with soil moisture contents and soil CO2 concentrations

Estimates of root and soil respiration are becoming increasingly important in agricultural and ecological research, but there is little understanding how soil texture and water content may affect these estimates. We examined the effects of soil texture on (i) estimated rates of root and soil respiration and (ii) soil CO₂ concentrations, during cycles of soil wetting and drying in the citrus rootstock, Volkamer lemon (Citrus volkameriana Tan. and Pasq.). Plants were grown in soil columns filled with three different soil mixtures varying in their sand, silt and clay content. Root and soil respiration rates, soil water content, plant water uptake and soil CO₂ concentrations were measured and dynamic relationships among these variables were developed for each soil texture treatment. We found that although the different soil textures differed in their plant-soil water relations characteristics, plant growth was only slightly affected. Root and soil respiration rates were similar under most soil moisture conditions for soils varying widely in percentages of sand, silt and clay. Only following irrigation did CO₂ efflux from the soil surface vary among soils. That is, efflux of CO₂ from the soil surface was much more restricted after watering (therefore rendering any respiration measurements inaccurate) in finer textured soils than in sandy soils because of reduced porosity in the finer textured soils. Accordingly, CO₂ reached and maintained the highest concentrations in finer textured soils (> 40 mmol CO₂ mol⁻¹). This study revealed that changes in soil moisture can affect interpretations of root and soil measurements based on CO₂ efflux, particularly in fine textured soils. The implications of the present findings for field soil CO₂ flux measurements are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Separating root and soil microbial contributions to soil respiration: A review of methods and observations

Forest soil respiration is the sum of heterotrophic (microbes, soil fauna) and autotrophic (root) respiration. The contribution of each group needs to be understood to evaluate implications of environmental change on soil carbon cycling and sequestration. Three primary methods have been used to distinguish hetero- versus autotrophic soil respiration including: integration of components contributing to in situ forest soil CO₂ efflux (i.e., litter, roots, soil), comparison of soils with and without root exclusion, and application of stable or radioactive isotope methods. Each approach has advantages and disadvantages, but isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots. Published data from all methods indicate that root/rhizosphere respiration can account for as little as 10 percent to greater than 90 percent of total in situ soil respiration depending on vegetation type and season of the year. Studies which have integrated percent root contribution to total soil respiration throughout an entire year or growing season show mean values of 45.8 and 60.4 percent for forest and nonforest vegetation, respectively. Such average annual values must be extrapolated with caution, however, because the root contribution to total soil respiration is commonly higher during the growing season and lower during the dormant periods of the year.     

Partitioning the components of soil respiration: a research challenge

Little is known about the respiratory components of CO₂ emitted from soils and attaining a reliable quantification of the contribution of root respiration remains one of the major challenges facing ecosystem research. Resolving this would provide major advances in our ability to predict ecosystem responses to climate change. The merits and technical and theoretical difficulties associated with different approaches adopted for partitioning respiration components are discussed here. The way forward is suggested to be the development of non-invasive regression analysis validated by stable isotope approaches to increase the sensitivity of model functions to include components of rhizosphere microbial activity, changing root biomass and the dynamics of a wide range of soil C pools. Section Editor: A. Hodge     

Photosynthesis controls of CO2 efflux from maize rhizosphere

The effects of different shading periods of maize plants on rhizosphere respiration and soil organic matter decomposition were investigated by using a ¹³C natural abundance and ¹⁴C pulse labeling simultaneously. ¹³C was a tracer for total C assimilated by maize during the whole growth period, and ¹⁴C was a tracer for recently assimilated C. CO₂ efflux from bare soil was 4 times less than the total CO₂ efflux from planted soil under normal lighting. Comparing to the normal lighting control (12/12 h day/night), eight days with reduced photosynthesis (12/36 h day/night period) and strongly reduced photosynthesis (12/84 h day/night period) resulted in 39% and 68% decrease of the total CO₂ efflux from soil, respectively. The analysis of ¹³C natural abundance showed that root-derived CO₂ efflux accounted for 82%, 68% and 56% of total CO₂ efflux from the planted soil with normal, prolonged and strongly prolonged night periods, respectively. Clear diurnal dynamics of the total CO₂ efflux from soil with normal day-night period as well as its strong reduction by prolonged night period indicated tight coupling with plant photosynthetic activity. The light-on events after prolonged dark periods led to increases of root-derived and therefore of total CO₂ efflux from soil. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO₂ efflux, and thereby, total CO₂ efflux from soils. ¹⁴C labeling of plants before the first light treatment did not show any significant differences in the ¹⁴CO₂ respired in the rhizosphere between different dark periods because the assimilate level in the plants was high. Second labeling, conducted after prolonged night phases, showed higher contribution of recently assimilated C (¹⁴C) to the root-derived CO₂ efflux by shaded plants. Results from ¹³C natural abundance showed that the cultivation of maize on Chromic Luvisol decreased soil organic matter (SOM) mineralization compared to unplanted soil (negative priming effect). A more important finding is the observed tight coupling of the negative rhizosphere effect on SOM decomposition with photosynthesis.     

Long-term Effects of Free Air CO2 Enrichment (FACE) on Soil Respiration

Emissions of CO₂ from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO₂ to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO₂ Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO₂ concentration affects soil CO₂ dynamics over 7 years of continuous enrichment. Soil respiration, soil CO₂ concentrations, and the isotopic signature of soil CO₂ were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO₂ efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO₂ enrichment effect on soil CO₂ efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO₂ concentrations and depletion in the δ¹³C of soil CO₂ at all but the shallowest soil depths.     

Diurnal cycle of carbon isotope ratio in soil CO2 in various ecosystems

Our investigations of diurnal variations of the ¹³C/¹²C ratio and CO₂ content in soil air were carried out in three environments during periods of high biosphere activity. It has been observed that diurnal variation of CO₂ concentration is negatively correlated ¹³. Particularly great variations occurred at shallow soil depths (10–30 cm) when the plant cover activity was high while the soil temperature was rather low. Under such conditions the ¹³ variations had the magnitude of 4, while the CO₂ concentration varied more than doubly. The maximum of the ¹³C/¹²C ratlo and the minimum of the CO₂ concentration in a cultivated field with winter wheat took place in the afternoon, whereas in deciduous forest similar patterns were observed at dawn. In these cases soil temperatures at 10 cm depths varied less than 2°C. Hence, under wheat the variation in root respiration rate seem to be the main reason of the recorded varations. In an uncultivated grass-field during the hottest period in summer we did not measure any distinct variations of CO₂ properties in spite of the fact that soil temperature varied up to 5°C. This might be due to dominant microbial respiration at the high soil temperature, which exceeded 20°C.     

The effect of plant material grown under elevated CO2 on soil respiratory activity

The biodegradability of aerial material from a C4 plant, sorghum grown under ambient (345 µmol mol^–1) and elevated (700 µmol mol^–1) atmospheric CO₂ concentrations were compared by measuring soil respiratory activity. Initial daily respiratory activity (measured over 10 h per day) increased four fold from 110 to 440 cm³ CO₂ 100g dry weight soil^–1 in soils amended with sorghum grown under either elevated or ambient CO₂. Although soil respiratory activity decreased over the following 30 days, respiration remained significantly higher (t-test;p>0.05) in soils amended with sorghum grown under elevated CO₂ concentrations. Analysis of the plant material revealed no significant differences in C:N ratios between sorghum grown under elevated or ambient CO₂. The reason for the differences in soil respiratory activity have yet to be elucidated. However if this trend is repeated in natural ecosystems, this may have important implications for C and N cycling.