Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles

We tested a conceptual model describing the influence of elevated atmospheric CO₂ on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C₃) grown in an elevated CO₂ atmosphere often increase production and allocation to belowground structures. We predicted that greater belowground C inputs at elevated CO₂ should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO₂ concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO₂ with no open top chamber, ii) at ambient CO₂ in an open top chamber, and iii) at twice-ambient CO₂ in an open top chamber. Plants were fertilized with 4.5 g N m⁻² over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO₂ effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO₂ treatments. Rates of photosynthesis in plants grown at elevated CO₂ were significantly greater than those measured under ambient conditions. The number of roots, root length, and root length increment were also substantially greater at elevated CO₂. Total and belowground biomass were significantly greater at elevated CO₂. Under N-limited conditions, plants allocated 50–70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient treatments; there were no significant differences between labile C pools in the bulk soil of ambient and elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of plants grown at elevated CO₂ compared to that in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO₂ concentrations can have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

Defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter under a sunflower species: Helianthus annuus

In the present study, `natural <sup>13</sup>C tracer method' was used to partition the belowground respiration into rhizosphere respiration and soil microbial respiration to test the hypothesis that defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter (SOM). A C<sub>3</sub> plant species, Helianthus annuus (sunflower), was grown in `C₄' soil in microcosms so that the CO₂ evolved from plant-soil system can be partitioned. Four levels of defoliation intensities were established by manual clipping. CO₂ evolved from plant-soil system was trapped during 0–4 h after defoliation (HAD), 5–22 HAD and 23–46 HAD using a closed circulating system, respectively. We found that both rhizosphere respiration and soil microbial respiration of the clipped plants were either unchanged or significantly enhanced compared to unclipped plants at 45% defoliation level during all sampling intervals. Soil microbial respiration increased significantly at all defoliation levels during 0–4 HAD, however, both rhizosphere respiration and soil microbial respiration decreased significantly during 5–22 HAD or 23–46 HAD when 20% or 74 clearly demonstrated that the defoliation treatments modified the rhizosphere priming effect on SOM decomposition. The total cumulative rhizosphere priming effects on SOM decomposition during 0–46 HAD were 146%, 241%, 204% and 205% when 0%, 20%, 45% and <74%.     

Elevated atmospheric CO2 increases microbial growth rates in soil: results of three CO2 enrichment experiments

Increasing the belowground translocation of assimilated carbon by plants grown under elevated CO₂ can cause a shift in the structure and activity of the microbial community responsible for the turnover of organic matter in soil. We investigated the long‐term effect of elevated CO₂ in the atmosphere on microbial biomass and specific growth rates in root‐free and rhizosphere soil. The experiments were conducted under two free air carbon dioxide enrichment (FACE) systems: in Hohenheim and Braunschweig, as well as in the intensively managed forest mesocosm of the Biosphere 2 Laboratory (B2L) in Oracle, AZ. Specific microbial growth rates (μ) were determined using the substrate‐induced respiration response after glucose and/or yeast extract addition to the soil. For B2L and both FACE systems, up to 58% higher μ were observed under elevated vs. ambient CO₂, depending on site, plant species and N fertilization. The μ‐values increased linearly with atmospheric CO₂ concentration at all three sites. The effect of elevated CO₂ on rhizosphere microorganisms was plant dependent and increased for: Brassica napus=Triticum aestivum<Beta vulgaris<Populus deltoides. N deficiency affected microbial growth rates directly (N limitation) and indirectly (changing the quantity of fine roots). So, 50% decrease in N fertilization caused the overall increase or decrease of microbial growth rates depending on plant species. The μ‐value increase was lower for microorganisms growing on yeast extract then for those growing on glucose, i.e. the effect of elevated CO₂ was smoothed on rich vs. simple substrate. So, the r/K strategies ratio can be better revealed by studying growth on simple (glucose) than on rich substrate mixtures (yeast extract). Our results clearly showed that the functional characteristics of the soil microbial community (i.e. specific growth rates) rather than total microbial biomass amount are sensitive to increased atmospheric CO₂. We conclude that the more abundant available organics released by roots at elevated CO₂ altered the ecological strategy of the soil microbial community specifically a shift to a higher contribution of fast‐growing r‐selected species was observed. These changes in functional structure of the soil microbial community may counterbalance higher C input into the soil under elevated atmospheric CO₂ concentration.     

Soil microbiota in two annual grasslands: responses to elevated atmospheric CO2

We measured soil bacteria, fungi, protozoa, nematodes, and biological activity in serpentine and sandstone annual grasslands after 4 years of exposure to elevated atmospheric CO₂. Measurements were made during the early part of the season, when plants were in vegetative growth, and later in the season, when plants were approaching their maximum biomass. In general, under ambient CO₂, bacterial biomass, total protozoan numbers, and numbers of bactivorous nematodes were similar in the two grasslands. Active and total fungal biomasses were higher on the more productive sandstone grassland compared to the serpentine. However, serpentine soils contained nearly twice the number of fungivorous nematodes compared to the sandstone, perhaps explaining the lower standing crop of fungal biomass in the serpentine and suggesting higher rates of energy flow through the fungal-based soil food web. Furthermore, root biomass in the surface soils of these grasslands is comparable, but the serpentine contains 6 times more phytophagous nematodes compared to the sandstone, indicating greater below-ground grazing pressure on plants in stressful serpentine soils. Elevated CO₂ increased the biomass of active fungi and the numbers of flagellates in both grasslands during the early part of the season and increased the number of phytophagous nematodes in the serpentine. Elevated CO₂ had no effect on the total numbers of bactivorous or fungivorous nematodes, but decreased the diversity of the nematode assemblage in the serpentine at both sampling dates. Excepting this reduction in nematode diversity, the effects of elevated CO₂ disappeared later in the season as plants approached their maximum biomass. Elevated CO₂ had no effect on total and active bacterial biomass, total fungal biomass, or the total numbers of amoebae and ciliates in either grassland during either sampling period. However, soil metabolic activity was higher in the sandstone grassland in the early season under elevated CO₂, and elevated CO₂ altered the patterns of use of individual carbon substrates in both grasslands at this time. Rates of substrate use were also significantly higher in the sandstone, indicating increased bacterial metabolic activity. These changes in soil microbiota are likely due to an increase in the flux of carbon from roots to soil in elevated CO₂, as has been previously reported for these grasslands. Results presented here suggest that some of the carbon distributed below ground in response to elevated CO₂ affects the soil microbial food web, but that these effects may be more pronounced during the early part of the growing season.     

Soil microbial response in tallgrass prairie to elevated CO2

Terrestrial responses to increasing atmospheric CO₂ are important to the global carbon budget. Increased plant production under elevated CO₂ is expected to increase soil C which may induce N limitations. The objectives of this study were to determine the effects of increased CO₂ on 1) the amount of carbon and nitrogen stored in soil organic matter and microbial biomass and 2) soil microbial activity. A tallgrass prairie ecosystem was exposed to ambient and twice-ambient CO₂ concentrations in open-top chambers in the field from 1989 to 1992 and compared to unchambered ambient CO₂ during the entire growing season. During 1990 and 1991, N fertilizer was included as a treatment. The soil microbial response to CO₂ was measured during 1991 and 1992. Soil organic C and N were not significantly affected by enriched atmospheric CO₂. The response of microbial biomass to CO₂ enrichment was dependent upon soil water conditions. In 1991, a dry year, CO₂ enrichment significantly increased microbial biomass C and N. In 1992, a wet year, microbial biomass C and N were unaffected by the CO₂ treatments. Added N increased microbial C and N under CO₂ enrichment. Microbial activity was consistently greater under CO₂ enrichment because of better soil water conditions. Added N stimulated microbial activity under CO₂ enrichment. Increased microbial N with CO₂ enrichment may indicate plant production could be limited by N availability. The soil system also could compensate for the limited N by increasing the labile pool to support increased plant production with elevated atmospheric CO₂. Longer-term studies are needed to determine how tallgrass prairie will respond to increased C input.     

Plant rhizosphere influence on microbial C metabolism: the role of elevated CO2, N availability and root stoichiometry

Microbial decomposer C metabolism is considered a factor controlling soil C stability, a key regulator of global climate. The plant rhizosphere is now recognized as a crucial driver of soil C dynamics but specific mechanisms by which it can affect C processing are unclear. Climate change could affect microbial C metabolism via impacts on the plant rhizosphere. Using continuous ¹³C labelling under controlled conditions that allowed us to quantify SOM derived-C in all pools and fluxes, we evaluated the microbial metabolism of soil C in the rhizosphere of a C4 native grass exposed to elevated CO₂ and under variation in N concentrations in soil and in plant root C:N stoichiometry. Our results demonstrated that this plant can influence soil C metabolism and further, that elevated CO₂ conditions can alter this role by increasing microbial C efficiency as indicated by a reduction in soil-derived C respiration per unit of soil C-derived microbial biomass. Moreover, under elevated CO₂ increases in soil N, and notably, root tissue N concentration increased C efficiency, suggesting elevated CO₂ shifted the stoichiometric balance so N availability was a more critical factor regulating efficiency than under ambient conditions. The root C:N stoichiometry effect indicates that plant chemical traits such as root N concentration are able to influence the metabolism of soil C and that elevated CO₂ conditions can modulate this role. Increased efficiency in soil C use was associated with negative rhizosphere priming and we hypothesize that the widely observed phenomenon of rhizosphere priming may result, at least in part, from changes in the metabolic efficiency of microbial populations. Observed changes in the microbial community support that shifting microbial populations were a contributing factor to the observed metabolic responses. Our case study points at greater efficiency of the SOM-degrading populations in a high CO₂, high N world, potentially leading to greater C storage of microbially assimilated C in soil.     

Stimulation of microbial extracellular enzyme activities by elevated CO2 depends on soil aggregate size

Increased belowground carbon (C) transfer by plant roots at elevated CO₂ may change properties of the microbial community in the rhizosphere. Previous investigations that focused on total soil organic C or total microbial C showed contrasting results: small increase, small decrease or no changes. We evaluated the effect of 5 years of elevated CO₂ (550 ppm) on four extracellular enzymes: β‐glucosidase, chitinase, phosphatase, and sulfatase. We expected microorganisms to be differently localized in aggregates of various sizes and, therefore analyzed microbial biomass (C_mic by SIR) and enzyme activities in three aggregate‐size classes: large macro‐ (> 2 mm), small macro‐ (0.25–2 mm), and microaggregates (< 0.25 mm). To estimate the potential enzyme production, we activated microorganisms by substrate (glucose and nutrients) amendment. Although C_total and C_mic as well as the activities of β‐glucosidase, phosphatase, and sulfatase were unaffected in bulk soil and in aggregate‐size classes by elevated CO₂, significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO₂ were 1.2–1.9‐fold higher than under ambient CO₂. This indicates the increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO₂. Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO₂ revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungal hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO₂ affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools.     

Rhizosphere interactions, carbon allocation, and nitrogen acquisition of two perennial North American grasses in response to defoliation and elevated atmospheric CO2

Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere. Plant C allocation patterns and rhizosphere interactions can also be affected by rising atmospheric CO(2) concentrations, which in turn could influence plant and microbial responses to defoliation. We studied two widespread perennial grasses native to rangelands of western North America to test whether (1) defoliation-induced enhancement of rhizodeposition would stimulate rhizosphere N availability and plant N uptake, and (2) defoliation-induced enhancement of rhizodeposition, and associated effects on soil N availability, would increase under elevated CO(2). Both species were grown at ambient (400 μL L(-1)) and elevated (780 μL L(-1)) atmospheric [CO(2)] under water-limiting conditions. Plant, soil and microbial responses were measured 1 and 8 days after a defoliation treatment. Contrary to our hypotheses, we found that defoliation and elevated CO(2) both reduced carbon inputs to the rhizosphere of Bouteloua gracilis (C(4)) and Pascopyrum smithii (C(3)). However, both species also increased N allocation to shoots of defoliated versus non-defoliated plants 8 days after treatment. This response was greatest for P. smithii, and was associated with negative defoliation effects on root biomass and N content and reduced allocation of post-defoliation assimilate to roots. In contrast, B. gracilis increased allocation of post-defoliation assimilate to roots, and did not exhibit defoliation-induced reductions in root biomass or N content. Our findings highlight key differences between these species in how post-defoliation C allocation to roots versus shoots is linked to shoot N yield, but indicate that defoliation-induced enhancement of shoot N concentration and N yield is not mediated by increased C allocation to the rhizosphere.     

Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2 for 8 years

Alterations in microbial mineralization and nutrient cycling may control the long-term response of ecosystems to elevated CO₂. Because micro-organisms constitute a labile fraction of potentially available N and are regulators of decomposition, an understanding of microbial activity and microbial biomass is crucial. Tallgrass prairie was exposed to twice ambient CO₂ for 8 years beginning in 1989. Starting in 1991 and ending in 1996, soil samples from 0 to 5 and 5 to 15 cm depths were taken for measurement of microbial biomass C and N, total C and N, microbial activity, inorganic N and soil water content. Because of increased water-use-efficiency by plants, soil water content was consistently and significantly greater in elevated CO₂ compared to ambient treatments. Soil microbial biomass C and N tended to be greater under elevated CO₂ than ambient CO₂ in the 5–15 cm depth during most years, and in the month of October, when analyzed over the entire study period. Microbial activity was significantly greater at both depths in elevated CO₂ than ambient conditions for most years. During dry periods, the greater water content of the surface 5 cm soil in the elevated CO₂ treatments increased microbial activity relative to the ambient CO₂ conditions. The increase in microbial activity under elevated CO₂ in the 5–15 cm layer was not correlated with differences in soil water contents, but may have been related to increases in soil C inputs from enhanced root growth and possibly greater root exudation. Total soil C and N in the surface 15 cm were, after 8 years, significantly greater under elevated CO₂ than ambient CO₂. Our results suggest that decomposition is enhanced under elevated CO₂ compared with ambient CO₂, but that inputs of C are greater than the decomposition rates. Soil C sequestration in tallgrass prairie and other drought-prone grassland systems is, therefore, considered plausible as atmospheric CO₂ increases. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impact of elevated atmospheric CO2 concentration on soil microbial biomass and activity in a complex, weedy field model ecosystem

Although soil organisms play an essential role in the cycling of elements in terrestrial ecosystems, little is known of the impact of increasing atmospheric CO₂ concentrations on soil microbial processes. We determined microbial biomass and activity in the soil of multitrophic model ecosystems housed in the Ecotron (NERC Centre for Population Biology, Ascot, UK) under two atmospheric CO₂ concentrations (ambient vs. ambient + 200 ppm). The model communities consist of four annual plant species which naturally co-occur in weedy fields and disturbed ground throughout southern England, together with their herbivores, parasitoids and soil biota. At the end of two experimental runs lasting 9 and 4.5 months, respectively, root dry weight and quality showed contradictory responses to elevated CO₂ concentrations, probably as a consequence of the different time-periods (and hence number of plant generations) in the two experiments. Despite significant root responses no differences in microbial biomass could be detected. Effects of CO₂ concentration on microbial activity were also negligible. Specific enzymes (protease and xylanase) showed a significant decrease in activity in one of the experimental runs. This could be related to the higher C:N ratio of root tissue. We compare the results with data from the literature and conclude that the response of complex communities cannot be predicted on the basis of oversimplified experimental set-ups.     

Response of soil biota to elevated atmospheric CO2 in poplar model systems

We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO₂ would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO₂ in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO₂. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO₂. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO₂. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO₂ treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO₂, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO₂ was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose. Received: 18 March 1997 / Accepted: 5 August 1997     

Changes in microbial activity and composition in a pasture ecosystem exposed to elevated atmospheric carbon dioxide

Elevated atmospheric CO₂ increases aboveground plant growth and productivity. However, carbon dioxide-induced alterations in plant growth are also likely to affect belowground processes, including the composition of soil biota. We investigated the influence of increased atmospheric CO₂on bacterial numbers and activity, and on soil microbial community composition in a pasture ecosystem under Free-Air Carbon Dioxide Enrichment (FACE). Composition of the soil microbial communities, in rhizosphere and bulk soil, under two atmospheric CO₂ levels was evaluated by using phospholipid fatty acid analysis (PLFA), and total and respiring bacteria counts were determined by epifluorescence microscopy. While populations increased with elevated atmospheric CO₂ in bulk soil of white clover (Trifolium repens L.), a higher atmospheric CO₂ concentration did not affect total or metabolically active bacteria in bulk soil of perennial ryegrass (Lolium perenne L.). There was no effect of atmospheric CO₂ on total bacteria populations per gram of rhizosphere soil. The combined effect of elevated CO₂ on total root length of each species and the bacterial population in these rhizospheres, however, resulted in an 85% increase in total rhizosphere bacteria and a 170% increase in respiring rhizosphere bacteria for the two plant species, when assessed on a per unit land area basis. Differences in microbial community composition between rhizosphere and bulk soil were evident in samples from white clover, and these communities changed in response to CO₂ enrichment. Results of this study indicate that changes in soil microbial activity, numbers, and community composition are likely to occur under elevated atmospheric CO₂, but the extent of those changes depend on plant species and the distance that microbes are from the immediate vicinity of the plant root surface.     

Global change belowground: impacts of elevated CO2, nitrogen,and summer drought on soil food webs and biodiversity

The world's ecosystems are subjected to various anthropogenic global change agents, such as enrichment of atmospheric CO₂ concentrations, nitrogen (N) deposition, and changes in precipitation regimes. Despite the increasing appreciation that the consequences of impending global change can be better understood if varying agents are studied in concert, there is a paucity of multi‐factor long‐term studies, particularly on belowground processes. Herein, we address this gap by examining the responses of soil food webs and biodiversity to enrichment of CO₂, elevated N, and summer drought in a long‐term grassland study at Cedar Creek, Minnesota, USA (BioCON experiment). We use structural equation modeling (SEM), various abiotic and biotic explanatory variables, and data on soil microorganisms, protozoa, nematodes, and soil microarthropods to identify the impacts of multiple global change effects on drivers belowground. We found that long‐term (13‐year) changes in CO₂ and N availability resulted in modest alterations of soil biotic food webs and biodiversity via several mechanisms, encompassing soil water availability, plant productivity, and – most importantly – changes in rhizodeposition. Four years of manipulation of summer drought exerted surprisingly minor effects, only detrimentally affecting belowground herbivores and ciliate protists at elevated N. Elevated CO₂ increased microbial biomass and the density of ciliates, microarthropod detritivores, and gamasid mites, most likely by fueling soil food webs with labile C. Moreover, beneficial bottom‐up effects of elevated CO₂ compensated for detrimental elevated N effects on soil microarthropod taxa richness. In contrast, nematode taxa richness was lowest at elevated CO₂ and elevated N. Thus, enrichment of atmospheric CO₂ concentrations and N deposition may result in taxonomically and functionally altered, potentially simplified, soil communities. Detrimental effects of N deposition on soil biodiversity underscore recent reports on plant community simplification. This is of particular concern, as soils house a considerable fraction of global biodiversity and ecosystem functions.     

Plant diversity drives soil microbial biomass carbon in grasslands irrespective of global environmental change factors

Soil microbial biomass is a key determinant of carbon dynamics in the soil. Several studies have shown that soil microbial biomass significantly increases with plant species diversity, but it remains unclear whether plant species diversity can also stabilize soil microbial biomass in a changing environment. This question is particularly relevant as many global environmental change (GEC) factors, such as drought and nutrient enrichment, have been shown to reduce soil microbial biomass. Experiments with orthogonal manipulations of plant diversity and GEC factors can provide insights whether plant diversity can attenuate such detrimental effects on soil microbial biomass. Here, we present the analysis of 12 different studies with 14 unique orthogonal plant diversity × GEC manipulations in grasslands, where plant diversity and at least one GEC factor (elevated CO₂, nutrient enrichment, drought, earthworm presence, or warming) were manipulated. Our results show that higher plant diversity significantly enhances soil microbial biomass with the strongest effects in long‐term field experiments. In contrast, GEC factors had inconsistent effects with only drought having a significant negative effect. Importantly, we report consistent non‐significant effects for all 14 interactions between plant diversity and GEC factors, which indicates a limited potential of plant diversity to attenuate the effects of GEC factors on soil microbial biomass. We highlight that plant diversity is a major determinant of soil microbial biomass in experimental grasslands that can influence soil carbon dynamics irrespective of GEC.     

Interactive effects of elevated carbon dioxide and environmental stresses on root mass fraction in plants: a meta-analytical synthesis using pairwise techniques

Rising atmospheric CO₂ greatly enhances plant production, but its effect on biomass allocation, particularly in the presence of environmental stresses, is not well understood. Here, we used meta-analysis combined with pairwise techniques to examine root mass fraction (RMF; i.e., the fraction of root to total biomass) as affected by elevated CO₂ and environmental stresses. Our results showed that lower soil fertility increased RMF and the magnitude was similar for ambient and elevated CO₂-grown plants. Lower soil water also increased RMF, but to a greater extent at elevated than at ambient CO₂. While CO₂ enrichment had little effect on the magnitude of O₃-caused reduction in RMF in herbaceous species, it alleviated the adverse effect of higher O₃ on root production in woody species. These results demonstrate that CO₂ has less pronounced effects on RMF than other environmental factors. Under abiotic stresses, e.g., drought and higher O₃, elevated CO₂-grown plants will likely increase biomass allocation below-ground. Because of the non-uniform changes in drought and O₃ projected for different parts of the world, we conclude that elevated CO₂ will have regional, but not global, effects on biomass allocation under various global change scenarios.     

Effects of Elevated CO2 and Defoliation on Compensatory Growth and Photosynthesis of Seedlings in a Tropical Tree,Copaifera aromatica1

After defoliation by herbivores, some plants exhibit enhanced rates of photosynthesis and growth that enable them to compensate for lost tissue, thus maintaining their fitness relative to competing, undefoliated plants. Our aim was to determine whether compensatory photosynthesis and growth would be altered by increasing concentrations of atmospheric CO₂. Defoliation of developing leaflets on seedlings of a tropical tree, Copaifera aromatica, caused increases in photosynthesis under ambient CO₂, but not under elevated CO₂. An enhancement in the development of buds in the leaf axils followed defoliation at ambient levels of CO₂. In contrast, under elevated CO₂, enhanced development of buds occurred in undefoliated plants with no further enhancement in bud development due to exposure to elevated CO₂. Growth of leaf area after defoliation was increased, particularly under elevated CO₂. Despite this increase, defoliated plants grown under elevated CO₂ were further from compensating for tissue lost during defoliation after 5¹/₂ weeks than those grown under ambient CO₂ concentrations.     

Allocation of carbon to shoots,roots, soil and rhizosphere respiration by barrel medic (Medicago truncatula) before and after defoliation

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a ¹⁴CO₂-labelled atmosphere for 30 days, defoliated and then grown in a ¹³CO₂-labelled atmosphere for 19 days. Allocation of ¹⁴C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of ¹⁴C and ¹³C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (¹³C) exceeded that of C assimilated before defoliation (¹⁴C). This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrate influx kinetic parameters of five potato cultivars during vegetative growth

This study examines the effect of elevated CO₂ on short-term partitioning of inorganic N between a grass and soil micro-organisms. ¹⁵N-labelled NH₄⁺ was injected in the soil of mesocosms of Holcus lanatus (L.) that had been grown for more than 15 months at ambient or elevated CO₂ in reconstituted grassland soil. After 48 h, the percentage recovery of added ¹⁵N was increased in soil microbial biomass N at elevated CO₂, was unchanged in total plant N and was decreased in soil extractable N. However, plant N content and microbial biomass N were not significantly affected by elevated CO₂. These results and literature data from plant–microbial ¹⁵N partitioning experiments at elevated CO₂ suggest that the mechanisms controlling the effects of CO₂ on short- vs. long-term N uptake and turnover differ. In particular, short-term immobilisation of added N by soil micro-organisms at elevated CO₂ does not appear to lead to long-term increases in N in soil microbial biomass. In addition, the increased soil microbial C:N ratios that we observed at elevated CO₂ suggest that long-term exposure to CO₂ alters either the functioning or structure of these microbial communities.     

Elevated temperature shifts soil N cycling from microbial immobilization to enhanced mineralization,nitrification and denitrification across global terrestrial ecosystems

We assessed the response of soil microbial nitrogen (N) cycling and associated functional genes to elevated temperature at the global scale. A meta‐analysis of 1,270 observations from 134 publications indicated that elevated temperature decreased soil microbial biomass N and increased N mineralization rates, both in the presence and absence of plants. These findings infer that elevated temperature drives microbially mediated N cycling processes from dominance by anabolic to catabolic reaction processes. Elevated temperature increased soil nitrification and denitrification rates, leading to an increase in N₂O emissions of up to 227%, whether plants were present or not. Rates of N mineralization, denitrification and N₂O emission demonstrated significant positive relationships with rates of CO₂ emissions under elevated temperatures, suggesting that microbial N cycling processes were associated with enhanced microbial carbon (C) metabolism due to soil warming. The response in the abundance of relevant genes to elevated temperature was not always consistent with changes in N cycling processes. While elevated temperature increased the abundances of the nirS gene with plants and nosZ genes without plants, there was no effect on the abundances of the ammonia‐oxidizing archaea amoA gene, ammonia‐oxidizing bacteria amoA and nirK genes. This study provides the first global‐scale assessment demonstrating that elevated temperature shifts N cycling from microbial immobilization to enhanced mineralization, nitrification and denitrification in terrestrial ecosystems. These findings infer that elevated temperatures have a profound impact on global N cycling processes with implications of a positive feedback to global climate and emphasize the close linkage between soil microbial C and N cycling.     

Are there links between responses of soil microbes and ecosystem functioning to elevated CO2, N deposition and warming? A global perspective

In recent years, there has been an increase in research to understand how global changes’ impacts on soil biota translate into altered ecosystem functioning. However, results vary between global change effects, soil taxa, and ecosystem processes studied, and a synthesis of relationships is lacking. Therefore, here we initiate such a synthesis to assess whether the effect size of global change drivers (elevated CO₂, N deposition, and warming) on soil microbial abundance is related with the effect size of these drivers on ecosystem functioning (plant biomass, soil C cycle, and soil N cycle) using meta‐analysis and structural equation modeling. For N deposition and warming, the global change effect size on soil microbes was positively associated with the global change effect size on ecosystem functioning, and these relationships were consistent across taxa and ecosystem processes. However, for elevated CO₂, such links were more taxon and ecosystem process specific. For example, fungal abundance responses to elevated CO₂ were positively correlated with those of plant biomass but negatively with those of the N cycle. Our results go beyond previous assessments of the sensitivity of soil microbes and ecosystem processes to global change, and demonstrate the existence of general links between the responses of soil microbial abundance and ecosystem functioning. Further we identify critical areas for future research, specifically altered precipitation, soil fauna, soil community composition, and litter decomposition, that are need to better quantify the ecosystem consequences of global change impacts on soil biodiversity.