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.     

Belowground nematode herbivores are resistant to elevated atmospheric CO2 concentrations in grassland ecosystems

Grasslands are considered to be one of the most sensitive ecosystems to rising atmospheric CO₂ concentrations, since, in addition to direct effects of elevated CO₂ on plant growth, indirect increases in water availability as an effect of elevated CO₂ may enhance primary production and alter plant community composition in these typically dry ecosystems. Moreover, grasslands support large populations of belowground herbivores that consume a major portion of plant biomass. The direct trophic link between herbivores and plants suggests that primary consumers may be particularly sensitive to rising CO₂ concentrations; however, the responses of belowground grassland herbivores have rarely been investigated. Here, we report the response of a range of herbivorous nematode populations to elevated CO₂ concentrations from three distinct grassland experiments. The three studies each involved 5–6 years of CO₂ fumigation, utilized natural or representative plant and soil communities, and were sampled at the end of the growing season. In the vast majority of cases, elevated CO₂ did not affect the abundance of nematode families; only two nematode families were significantly influenced by CO₂ enrichment (Anguinidae increased in one case and Hoplolaimidae decreased in another). Similarly, elevated CO₂ did not influence the total abundance, family richness, diversity or plant parasitic index of the herbivorous nematode community. These neutral responses to CO₂ enrichment occurred despite increased root production in all three experiments, suggesting a simultaneous antagonistic mechanism may have operated, possibly decreased root quality and/or changes in the soil environment. Whatever the mechanism, our findings suggest that herbivorous nematodes in grassland ecosystems are resistant to rising atmospheric CO₂ concentrations.     

Decomposition of C-labeled roots in a pasture soil exposed to 10 years of elevated CO2

The net flux of soil C is determined by the balance between soil C input and microbial decomposition, both of which might be altered under prolonged elevated atmospheric CO₂. In this study, we determined the effect of elevated CO₂ on decomposition of grass root material (Lolium perenne L.). ¹⁴C-labeled root material, produced under ambient (35 Pa pCO₂) or elevated CO₂ (70 Pa pCO₂) was incubated in soil for 64 days. The soils were taken from a pasture ecosystem which had been exposed to ambient (35 Pa pCO₂) or elevated CO₂ (60 Pa pCO₂) under FACE-conditions for 10 years and two fertilizer N rates: 140 and 560 kg N ha⁻¹ year⁻¹. In soil exposed to elevated CO₂, decomposition rates of root material grown at either ambient or elevated CO₂ were always lower than in the control soil exposed to ambient CO₂, demonstrating a change in microbial activity. In the soil that received the high rate of N fertilizer, decomposition of root material grown at elevated CO₂ decreased by approximately 17% after incubation for 64 days compared to root material grown at ambient CO₂. The amount of ¹⁴CO₂ respired per amount of ¹⁴C incorporated in the microbial biomass (q¹⁴CO₂) was significantly lower when roots were grown under high CO₂ compared to roots grown under low CO₂. We hypothesize that this decrease is the result of a shift in the microbial community, causing an increase in metabolic efficiency. Soils exposed to elevated CO₂ tended to respire more native SOC, both with and without the addition of the root material, probably resulting from a higher C supply to the soil during the 10 years of treatment with elevated CO₂. The results show the importance of using soils adapted to elevated CO₂ in studies of decomposition of roots grown under elevated CO₂. Our results further suggest that negative priming effects may obscure CO₂ data in incubation experiments with unlabeled substrates. From the results obtained, we conclude that a slower turnover of root material grown in an ‘elevated-CO₂ world’ may result in a limited net increase in C storage in ryegrass swards.     

Availability of soil nitrogen and phosphorus under elevated [CO2] and temperature in the Taihu Lake region,China

Global climate change affects the availability of soil nutrients, thereby influencing crop productivity. This study examined the effects of elevated [CO₂] and temperature on the availability of soil N and P in a paddy field in the Taihu Lake region, China. Winter wheat (Triticum aestivum L.) was planted at two levels of atmospheric [CO₂] (375 μmol L^–1 ambient; 575 μmol L^–1 elevated) and two temperature levels (ambient; ambient + 2°C). The results were as follows: Compared to ambient, the interaction effects of elevated [CO₂] and temperature significantly decreased soil NH$ _4^+ $ ‐N contents by 20.3%, 20.6%, and 18.7% in the jointing, heading, and ripening stages (p < 0.05), respectively, while soil NO₃^–‐N content had no clear variation trend under different [CO₂] and temperature conditions. Elevated [CO₂] significantly increased soil available P content by 14.3% in the jointing stage, and elevated temperature significantly decreased soil available P content by 18.8% in the jointing stage. Compared with ambient [CO₂], elevated [CO₂] significantly increased wheat biomass in jointing and heading stages (p < 0.05). The positive effect of elevated [CO₂] on wheat biomass was more significant at ambient temperature (AT) than at elevated temperature (ET) in the middle and late plant growth stages. These results explained that the availability of soil N and P varied under elevated [CO₂] and temperature conditions. The application of N and P should be adjusted to meet the need of wheat plants after the wintering stage.     

Can a mixed stand of N2-fixing and non-fixing plants restrict N2O emissions with increasing CO2 concentration?

Initial effects of elevated atmospheric CO₂ concentration on N₂O fluxes and biomass production of timothy/red clover were studied in the laboratory. The experimental design consisted of two levels of atmospheric CO₂ (ca. 360 and 720 μmol CO₂ mol⁻¹) and two N fertilisation levels (5 and 10 g N m⁻²). There was a total of 36 mesocosms comprising sandy loam soil, which were equally distributed in four thermo-controlled greenhouses. In two of the greenhouses, the CO₂ concentration was kept at ambient concentration and in the other two at doubled concentration. Forage was harvested and the plants fertilised three times during the basic experiment, followed by harvest, a fertilisation with the double amount of nitrogen and rise of water level. Under elevated CO₂, harvestable and total aboveground dry biomass production of a mixed Trifolium/Phleum stand was increased at both N treatments compared to ambient CO₂. The N₂O flux rates under ambient CO₂ were significantly higher at both N treatments during the early growth of mixed Phleum/Trifolium mesocosms compared to the N₂O flux rate under elevated CO₂. However, when the conditions were favourable for denitrification at the end of the experiment, i.e. N availability and soil moisture were high enough, the elevated CO₂ concentration enhanced the N₂O efflux.     

Natural N abundances in a tallgrass prairie ecosystem exposed to 8-y of elevated atmospheric CO2

After 8-y of elevated CO₂, we previously detected greater amounts of total soil nitrogen, suggesting that rates of ecosystem N flux into or out of tallgrass prairie had been altered. Denitrification and associative N fixation rates are the two primary biological processes that are known to control N loss and accumulation in tallgrass prairie soil. Therefore, our objective was to assess the natural abundance of plant and soil ¹⁵N isotopes as a cumulative index of potential change in efflux or influx of N into and out of the tallgrass prairie after 8-y of exposure to elevated CO₂. Aboveground plant delta ¹⁵N values of Andropogon gerardii were close to zero and more positive as a result of elevated CO₂, but whole-soil values at the 5-30 cm depth were significantly reduced (6.8 vs 7.3; P<0.05) under elevated CO₂-chamber (EC) relative to ambient CO₂- chamber (AC). Total, aboveground plant biomass, root-in-growth, extractable N, microbial biomass N, and soil pools collectively exhibited a range of delta ¹⁵N values from −2.8 to 7.3. Measurements of surface soil ¹⁵N indicate that a change in N inputs and outputs has occurred as a result of elevated atmospheric CO₂. In addition to possible changes in denitrification and N₂ fixation, other sources of N such as the re-translocation of N to the surface from deeper soil layers are needed to explain how soil N accrues in surface soils as a consequence of elevated CO₂. Our results support the notion that C accrual may promote N accrual, possibly driven by high plant and microbial N demand amplified by soil N limitation.     

Assessing the potential of using biochar in mine rehabilitation under elevated atmospheric CO<Subscript>2</Subscript> concentration

Purpose

Re-establishment of soil nitrogen (N) capital is a priority in mine rehabilitation. We aimed to evaluate the effects of biochar addition on improving mine spoil N pools and the influence of elevated CO₂ concentration on mine rehabilitation.

Materials and methods

We assessed the effects of pinewood biochar, produced at three temperatures (650, 750 and 850 °C, referred as B₆₅₀, B₇₅₀ and B₈₅₀, respectively), on mine spoil total N concentrations with five different plant species, including a tree species (Eucalyptus crebra), N-fixing shrubs (Acacia floribunda and Allocasuarina littoralis) and C₃ and C₄ grasses (Austrodanthonia tenuior and Themeda australis) incubated at ambient (400 μL L^?1) and elevated (700 μL L^?1) atmospheric CO₂ concentrations, as well as the effects of elevated CO₂ on mine rehabilitation.

Results and discussion

Soil total N significantly improved following biochar incorporation under all plant species (P < 0.05) except for T. Australis. E. crebra had the highest soil total N (0.197%, 0.198% and 0.212% for B₆₅₀, B₇₅₀ and B₈₅₀, respectively). Different from the negligible influence of elevated CO₂ on soil properties under the grasses and the N-fixing shrubs, elevated CO₂ significantly increased soil water and hot water extractable organic C (WEOC and HWEOC, respectively) and decreased total C under E. crebra, indicating that the nutrient demands were not met.

Conclusions

Biochar addition showed the potential in mine rehabilitation in terms of improving soil N pool, especially with E. crebra. However, it would be more difficulty to rehabilitate mine spoils in future with the rising atmospheric CO₂ concentration.

     

Effect of elevated carbon dioxide and water stress on gas exchange and water use efficiency in corn

CO₂ has been predicted to increase in the future, and thus leading to possible changes in precipitation patterns. The objectives of this study were to investigate water use and canopy level photosynthesis of corn plants, and to quantify water use efficiency in corn plants under two different CO₂ levels combined with four different water stress levels. Corn plants were planted in sunlit plant growth chambers and a day/night temperature of (28/18 °C) was applied. From 21 days after emergence (DAE), the eight treatments including two levels of carbon dioxide concentrations (400 and 800 μmol mol⁻¹) and four levels of water stress (well-watered control, “mild”, “moderate”, and “severe” water stress) treatments at each CO₂ level were imposed. Height, number of leaves, leaf lengths, and growth stages of corn plants were monitored from nine plants twice a week. Corn plants were separately collected, dried, and analyzed for the biomass accumulation at 21 and 60 DAE. Soil water contents were monitored by a time domain reflectometry (TDR) system (15 probes per chamber). The “breaking points” (changes from high to low rates of soil water uptake) were observed in the bottom of soil depth for the water stressed conditions, and the “breaking points” under ambient CO₂ appeared 6-9 days earlier than under elevated CO₂. Although approximately 20-49% less water was applied for the elevated CO₂ treatments than for ambient CO₂ from 21 DAE, higher soil water contents were recorded under elevated CO₂ than under ambient CO₂. However, corn growth variables such as height, leaf area, and biomass accumulation were not significantly different in CO₂ or water stressed treatments. This result may be explained by considering that significant differences in canopy level gross photosynthesis among the water stress treatments was observed only toward the end of the experiment. The higher soil water contents observed under elevated CO₂ resulted mainly from less water use than under ambient CO_2. WUE (above ground biomass per water use since 21 DAE) at the final harvest was consistently higher and varied with a smaller range under elevated CO₂ than under ambient CO₂. This study suggests that less water will be required for corn under high-CO₂ environment in the future than at present.     

Residue incorporation and N fertilization affect the response of soil nematodes to the elevated CO2 in a Chinese wheat field

The interplay between the carbon and other nutrient cycles is the key to understand the responses of soil ecosystems to climatic change. Using the free-air CO₂ enrichment (FACE) techniques, we carried out a multifactorial experiment in a Chinese rice-wheat rotation system, to investigate the response of soil nematodes to elevated CO₂ under different application rates of N fertilizer (225.0 kg N ha⁻¹ (HN) and 112.5 kg N ha⁻¹(LN), respectively) and residue incorporation (0 kg C ha⁻¹ (ZR), 1000 kg C ha⁻¹ (MR) and 2000 kg C ha⁻¹ (HR), respectively). This study was conducted during the wheat growing season of 2007 after expose to the elevated CO₂ for three years. The results in our study indicated that seasonality is an important factor in determining changes in the nematode abundance and diversity. The residue addition effects were more obvious than the elevated CO₂, which significantly influenced the abundance of total nematodes and plant-parasites, and some ecological indices. The interactions between residue addition and CO₂ significantly influenced nematode dominance and structure indices. High level of N fertilization was found to decrease the nematode diversity, generic richness and maturity indices at wheat jointing stage. There are significant interactions between N fertilization and elevated CO₂ for abundance of total nematodes and different trophic groups.     

Trace gas emissions from soil of the central highlands of Mexico as affected by natural vegetation: a laboratory study

In the central highlands of Mexico, mesquite (Prosopis laevigata) and huisache (Acacia schaffneri), N₂-fixing trees or shrubs, dominate the vegetation and are currently used in a reforestation program to prevent erosion. We investigated how natural vegetation or cultivation of soil affected oxidation of CH₄, and production of N₂O. Soil was sampled under the canopy of mesquite (MES treatment) and huisache trees (HUI treatment), outside their canopy (OUT treatment) and from fields cultivated with maize (ARA treatment) at three different sites while production of CO₂, and dynamics of CH₄, N₂O and inorganic N (NH₄⁺, and NO₃^–) were monitored in an aerobic incubation. The production of CO₂ was 2.3 times higher and significantly greater in the OUT treatment, 3.0 times higher in the MES treatment and 4.0 times higher in the HUI treatment compared to the ARA treatment. There was no significant difference in oxidation of CH₄ between the treatments, which ranged from 0.019 g CH₄–C kg^–1 day^–1 for the HUI treatment to 0.033 CH₄–C kg^–1 day^–1 for the MES treatment. The production of N₂O was 30 g N₂O–N kg^–1 day^–1 in the MES treatment and >8 times higher compared to the other treatments. The average concentration of NO₃^– was 2 times higher and significantly greater in the MES treatment than in the HUI treatment, 3 times greater than in the OUT treatment and 10 times greater than in the ARA treatment. It was found that cultivation of soil decreased soil organic matter content, C and N mineralization, but not oxidation of CH₄ or production of N₂O.     

Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2

Elevated CO₂ may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO₂-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO₂. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to ¹³C labeling at ambient (392 μmol mol⁻¹) and elevated (792 μmol mol⁻¹) CO₂ concentrations, in soil containing ¹⁵N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-¹³C was enhanced by 50% under elevated compared to ambient CO₂. Concurrently, plant ¹⁵N uptake increased (+7%) under elevated CO₂ while ¹⁵N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO₂. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO₂ can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO₂.     

Long-term changes in topsoil nematode populations in grazed pasture under elevated atmospheric carbon dioxide

Increasing atmospheric CO₂ concentrations are expected to affect ecosystems processes, and while a New Zealand study reported a response in soil biological activity after 4 years of CO₂ enrichment, apparently reflecting increased populations of Longidorus elongatus, similar findings have not been reported from other sites. Soil microfauna in 0–10 cm soil under a sheep-grazed pasture on a sand was assessed quarterly in FACE rings that were either at ambient CO₂ or had been exposed to 475 μl l⁻¹ CO₂ for some 9 years. Although the area had been subject to a severe drought and microfaunal populations were lower than previously found, the effects of elevated CO₂ on microfaunal populations were broadly similar to those at 4 years. Average populations of the root-feeding L. elongatus increased from 67,000 to 233,000 m⁻² (3.48×) compared with a 4.26× increase after 4 years; microbial-feeding nematodes increased slightly, while predacious nematodes showed a 2.0× increase. A pot experiment showed an additive effect of elevated CO₂ and L. elongatus abundance in reducing specific root length. That similar effects have been found 4 and 9 years after CO₂ enrichment commenced suggests they are real, and emphasises the difference to other sites around the world where much lower responses to elevated CO₂ have been found. This, in part, reflects the unique combination of soil, plant and soil biological conditions at each site and confirms the strong effect of soil type and vegetation on soil biological processes. Just as the effects of global climate change on a given region are idiosyncratic, so it seems are the effects of elevated CO₂ on soil and ecosystem processes. In part, this reflects our limited understanding of below-ground processes.     

Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands

Several recent studies have indicated that an enriched atmosphere of carbon dioxide (CO₂) could exacerbate the intensity of plant invasions within natural ecosystems, but little is known of how rising CO₂ impacts the belowground characteristics of these invaded systems. In this study, we examined the effects of elevated CO₂ and nitrogen (N) inputs on plant and soil microbial community characteristics of plant communities invaded by reed canary grass, Phalaris arundinacea L. We grew the invasive grass under two levels of invasion: the invader was either dominant (high invasion) at >90% plant cover or sub-dominant (low invasion) at <50% plant cover. Experimental wetland communities were grown for four months in greenhouses that received either 600 or 365 μl l⁻¹ (ambient) CO₂. Within each of three replicate rooms per CO₂ treatment, the plant communities were grown under high (30 mg l⁻¹) or low (5 mg l⁻¹) N. In contrast to what is often predicted under N limitation, we found that elevated CO₂ increased native graminoid biomass at low N, but not at high N. The aboveground biomass of reed canary grass did not respond to elevated CO₂, despite it being a fast-growing C3 species. Although elevated CO₂ had no impact on the plant biomass of heavily invaded communities, the relative abundance of several soil microbial indicators increased. In contrast, the moderately invaded plant communities displayed increased total root biomass under elevated CO_2, while little impact occurred on the relative abundance of soil microbial indicators. Principal components analysis indicated that overall soil microbial community structure was distinct by CO₂ level for the varying N and invasion treatments. This study demonstrates that even when elevated CO₂ does not have visible effects on aboveground plant biomass, it can have large impacts belowground.     

Effect of increasing oxygen concentration on total denitrification and nitrous oxide release from soil by different bacteria

Summary The effect of increasing oxygen concentrations (0, 5, 10 and 20 Vol% O₂) on total denitrification and N₂0 release was studied in model experiments using a neutral pH loamy soil relatively rich in easily decomposable organic matter and supplied with nitrate (300 g nitrate N/g dry soil). The sterilized soil was inoculated with three different denitrifying bacteria (Bacillus licheniformis,Aeromonas denitrificans andAzospirillum lipoferum) and incubated (80% WHC, 30°C). The gas volume was analysed for O₂, CO₂, N₂O, NO and N₂ by gas chromatography and the soil investigated for changes in ammonium, nitrite, nitrate, pH, total N and C as well as water-extractable C. WithB. licheniformis andAeromonas denitrificans total denitrification increased remarkably with increasing pO₂ as the result of intensified mineralization.Azospirillum lipoferum, however, showed the highest activity at 5 vol% O₂. WithB. licheniformis N₂O was released only in anaerobic conditions and at 5 Vol% O₂ (maximum) or 10 Vol% 02, but not at 20 Vol%, whereasAeromonas denitrificans produced N₂O only in the presence of He gas (maximum) or at 5 Vol% O₂. In contrast to these bacteria, N₂O production withAzospirillum lipoferum was restricted to 10 Vol% O₂ (maximum) and to 20 Vol% 02, with some traces at 5 vol% O₂. With a certain set of conditions, total denitrification and N₂O formation seem to be governed by the mineralization rate of the organisms in question. The increased demand for electron acceptors by a high turnover rate rather than the presence of anaerobic conditions seems to have determined the rate of denitrification.     

Quantitative analysis of soluble exudates produced by ectomycorrhizal roots as a response to ambient and elevated CO2

Despite its potential impact on soil carbon flow, few studies have attempted to quantify the effects of elevated carbon dioxide (CO₂) on production of exudates by mycorrhizal plants. In this study we quantified low molecular weight (LMW) organic compounds exuded by non-mycorrhizal (NM) and ectomycorrhizal (ECM) plants in relation to exposure to elevated CO₂. Scots pine seedlings, either colonized by one of eight different ECM fungi or non-mycorrhizal (NM), were exposed to either ambient (350 ppm) or elevated (700 ppm) concentrations of CO₂. Exudation of LMW organic acids (LMWOAs), amino acids, dissolved monosaccharides and total dissolved organic carbon (DOC) was determined and exudation rates were calculated per g root and fungal dry mass. CO₂ had a significant impact on exudation. Under elevated CO₂, exudation of total LMWOAs increased by 120-160%, amino acids by 250%, dissolved monosaccharides by 130-270% and DOC by 180-220% compared to ambient CO₂ treatment. Net CO₂ assimilation rates increased significantly by 41-47% for seedlings exposed to elevated CO₂. Exuded C calculated as a percentage of assimilated CO₂ increased by 41-88% in the elevated CO₂ treatment compared to ambient CO₂ treatment.     

Nitrogen Transformation as Influenced by Soil Microbial Nitrogen under Carbon Dioxide Enrichment Induced in Created Riparian Wetlands

Limited research has been conducted on how atmospheric carbon dioxide (CO₂) affects water and soil nitrogen (N) transformation in wetland ecosystems. A stable isotope technique is suitable for conducting a detailed investigation of mechanistic nutrient transformations. Nutrient ammonium sulfate (NH₄)₂SO₄ input in culture water under elevated CO₂ (700 μL L^?1) and ambient CO₂ (380 μL L^?1) was studied to analyze N transformations with N blanks for both water and soil. It was measured by ¹⁵N pool dilution using analytical equations in a riparian wetland during a 3-month period. Soil gross ammonium (NH₄ ⁺) mineralization and consumption rates increased significantly by 22% and 404%, Whereas those of water decreased greatly by??57% and??57% respectively in enriched CO₂. In contrast, gross nitrate (NO₃ ^?) consumption and nitrification rates of soil decreased by??11% and??14% and those of water increased by 29% and 27% respectively in enrichment CO₂. These may be due to the extremely high soil microbial biomass nitrogen (MBN), which increased by 94% in elevated soil. The results can show when CO₂ concentrations are going to rise in the future. Consequently soil microbial activity initiates the decreased N concentration in sediment and increased N concentration in overlying water in riparian wetland ecosystems.     

Rapid response of soil protozoa to elevated CO2

Short-term changes in bacterial and protozoan populations from the soil of plants grown under elevated atmospheric CO₂ were quantified. We grew Brassica nigra at either ambient or twice-ambient CO₂ levels within open-top chambers in the field for 4 weeks. Plant biomass, above- and belowground, was unaffected by elevated CO₂. Direct count bacterial density was unchanged under elevated CO₂. Flagellate density tended to increase, whereas amoebal density significantly declined under elevated CO₂. This change in protozoan community structure suggests trophic transfer of the elevated CO₂ fertilization effect through the soil food chain. Received: 20 August 1996     

Composition and function of microbial communities during the early decomposition stages of foliar litter exposed to elevated CO2 concentrations

Foliar litter derived from trees exposed to elevated CO₂ concentration decreases in quality relative to foliar litter exposed to ambient CO₂. This change in quality could in turn affect the composition and function of microbial communities associated with litter transformation and mineralization. We evaluated the effects of decomposing litter obtained from three tree species, Pinus koraiensis, P. sylvestriformis and Quercus mongolicus, on the structure and function of microbial communities by burying litter‐bags and sampling the litters at intervals over a 429‐day period. Foliar litter collected from trees exposed to elevated CO₂ had greater C and lignin contents, and lost the mass at a slower rate than that collected from trees growing in ambient CO₂, except for P. koraiensis litter. The CO₂ responses of enzyme activity in litter were species‐dependent. Molecular fingerprinting by means of denaturing gradient gel electrophoresis (DGGE) demonstrated that elevated CO₂ more strongly affected the fungal community than the bacterial community in decomposing litter. In samples of P. koraiensis litter that had been exposed to elevated CO₂, sequences belonging to Basidiomycota showed increases in relative intensity, which were accompanied by significant increases in enzyme activities. Our results showed that enzyme activities and micro‐decomposer composition have different, litter‐specific responses to changes in litter biochemistry, which indicate that exposure of trees to elevated CO₂ has varying effects on litter decomposition.     

Increased moisture and methanogenesis contribute to reduced methane oxidation in elevated CO2 soils

Awareness of global warming has stimulated research on environmental controls of soil methane (CH₄) consumption and the effects of increasing atmospheric carbon dioxide (CO₂) on the terrestrial CH₄ sink. In this study, factors impacting soil CH₄ consumption were investigated using laboratory incubations of soils collected at the Free Air Carbon Transfer and Storage I site in the Duke Forest, NC, where plots have been exposed to ambient (370 μL L⁻¹) or elevated (ambient + 200 μL L⁻¹) CO₂ since August 1996. Over 1 year, nearly 90% of the 360 incubations showed net CH₄ consumption, confirming that CH₄-oxidizing (methanotrophic) bacteria were active. Soil moisture was significantly (p < 0.01) higher in the 25–30 cm layer of elevated CO₂ soils over the length of the study, but soil moisture was equal between CO₂ treatments in shallower soils. The increased soil moisture corresponded to decreased net CH₄ oxidation, as elevated CO₂ soils also oxidized 70% less CH₄ at the 25–30 cm depth compared to ambient CO₂ soils, while CH₄ consumption was equal between treatments in shallower soils. Soil moisture content predicted (p < 0.05) CH₄ consumption in upper layers of ambient CO₂ soils, but this relationship was not significant in elevated CO₂ soils at any depth, suggesting that environmental factors in addition to moisture were influencing net CH₄ oxidation under elevated CO₂. More than 6% of the activity assays showed net CH₄ production, and of these, 80% contained soils from elevated CO₂ plots. In addition, more than 50% of the CH₄-producing flasks from elevated CO₂ sites contained deeper (25–30 cm) soils. These results indicate that subsurface (25 cm+) CH₄ production contributes to decreased net CH₄ consumption under elevated CO₂ in otherwise aerobic soils.