Minor stable carbon isotope fractionation between respired carbon dioxide and bulk soil organic matter during laboratory incubation of topsoil

A common assumption in paleoenvironmental reconstructions using soils is that the carbon isotope composition of soil-respired CO₂ is equivalent to the carbon isotope composition of bulk soil organic matter (SOM). However, the occurrence of a non-zero per mil carbon isotope enrichment factor between CO₂ and SOM (\(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\)) during soil respiration is the most widely accepted explanation for the down-profile increase in SOM δ¹³C values commonly observed in well-drained soils. In order to shed light on this apparent discrepancy, we incubated soil samples collected from the top 2 cm of soils with pure C₃ vegetation and compared the δ¹³C values of soil-respired CO₂ to the δ¹³C values of bulk SOM. Our results show near-zero \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values (?0.3 to 0.4 ‰), supporting the use of paleosol organic matter as a proxy for paleo soil-respired CO₂. Significantly more negative \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values are required to explain the typical δ¹³C profiles of SOM in well-drained soils. Therefore our results also suggest that typical SOM δ¹³C profiles result from either (1) a process other than carbon isotope fractionation between CO₂ and SOM during soil respiration or (2) \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values that become increasingly negative as SOM matures.     

13CO2 pulse labelling of plants in tandem with stable isotope probing: methodological considerations for examining microbial function in the rhizosphere

Recently developed 13CO2 pulse labelling and stable isotope probing (SIP) methods offer the potential to track 13C-labelled plant photosynthate into phylogenetic groups of microbial taxa in the rhizosphere, permitting an examination of the link between soil microbial diversity and carbon flow in situ. We tested the feasibility of this approach to detect functional differences in microbial communities utilising recently fixed plant photosynthate in moisture perturbed grassland turfs. Specifically, we addressed two questions: (1) How does moisture perturbation (three treatments; continual wetting, drying, and drying followed by rewetting) affect the assimilation of 13C-labelled exudates carbon into the soil microbial community?; (2) Can 13C deposited in soil from pulse-labelled plants be used to identify microbes utilising plant exudates using SIP methodologies? Net CO2 fluxes showed that prior to 13CO2 pulse labelling, all treatments were photosynthetically active, but differences were observed in night time respiration, indicating moisture treatments had impacted on net CO2 efflux. Measurements of pulse-derived 13C incorporated into soil RNA over 2 months showed that there was only evidence of 13C enrichment in the continuously wetted treatments. However, isotopic values represented only a 0.1-0.2 13C at.% increase over natural abundance levels and were found to be insufficient for the application of RNA-SIP. These findings reveal that in this experimental system, the microbial uptake of labelled carbon from plant exudates is low, and further optimisation of methodologies may be required for application of SIP to natural plant-soil systems where 13C tracer dilution is a consideration.     

Respiration of the external mycelium in the arbuscular mycorrhizal symbiosis shows strong dependence on recent photosynthates and acclimation to temperature

* Although arbuscular mycorrhizal (AM) fungi are a major pathway in the global carbon cycle, their basic biology and, in particular, their respiratory response to temperature remain obscure. * A pulse label of the stable isotope (13)C was applied to Plantago lanceolata, either uninoculated or inoculated with the AM fungus Glomus mosseae. The extra-radical mycelium (ERM) of the fungus was allowed to grow into a separate hyphal compartment excluding roots. We determined the carbon costs of the ERM and tested for a direct temperature effect on its respiration by measuring total carbon and the (13)C:(12)C ratio of respired CO(2). With a second pulse we tested for acclimation of ERM respiration after 2 wk of soil warming. * Root colonization remained unchanged between the two pulses but warming the hyphal compartment increased ERM length. delta(13)C signals peaked within the first 10 h and were higher in mycorrhizal treatments. The concentration of CO(2) in the gas samples fluctuated diurnally and was highest in the mycorrhizal treatments but was unaffected by temperature. Heating increased ERM respiration only after the first pulse and reduced specific ERM respiration rates after the second pulse; however, both pulses strongly depended on radiation flux. * The results indicate a fast ERM acclimation to temperature, and that light is the key factor controlling carbon allocation to the fungus.     

Fate of carbon in upland grassland subjected to liming using in situ 13CO2 pulse-labelling

Knowledge of the fate of plant assimilate is fundamental to our understanding of the terrestrial carbon cycle, particularly if we are to predict the effects of changes in climate and land management practices on agroecosystems. Pulse-labelling experiments have revealed that some of the carbon fixed by plants is rapidly allocated below-ground and released back into the atmosphere in respiration. However, little is known about the fate of plant assimilate, not accounted for in soil respiration, in the longer term and how current management practices such as liming may affect this. In southern Scotland, UK, limed and unlimed acid grassland plots were pulse-labelled with ¹³CO₂ and the turnover of ¹³C was studied one and two years after labelling. In this study the amount of labelled carbon remaining in shoot, root, and bulk soil pools, and how this differed between limed and unlimed plots was investigated. The results indicated that plant-root turnover was faster, and plants invested less nitrogen in the roots in the limed plots than in the unlimed plots. More ¹³C remained in the soil in the unlimed treatment compared to the limed treatment, but the main difference was found in the particulate organic matter, which turned over relatively quickly. The label was still above natural abundance one and two years after labelling in many cases. In addition, the results demonstrate that a ¹³CO₂ pulse-label administered for only a few hours can be a useful approach for investigating turnover of carbon several years later.     

Rhizosphere bacterial community composition responds to arbuscular mycorrhiza, but not to reductions in microbial activity induced by foliar cutting

Differences in bacterial community composition (BCC) between bulk and rhizosphere soil and between rhizospheres of different plant species are assumed to be strongly governed by quantitative and qualitative rhizodeposit differences. However, data on the relationship between rhizodeposit amounts and BCC are lacking. Other soil microorganisms, e.g. arbuscular mycorrhizal fungi (AMF), may also influence BCC. We simulated foliar herbivory (cutting) to reduce belowground carbon allocation and rhizodeposition of pea plants grown either with or without AMF. This reduced soil respiration, rhizosphere microbial biomass and bacteriovorous protozoan abundance, whereas none of these were affected by AMF. After labelling plants with (13)CO(2), root and rhizosphere soil (13)C enrichment of cut plants were reduced to a higher extent (24-46%) than shoot (13)C enrichment (10-24%). AMF did not affect (13)C enrichment. Despite these clear indications of reduced rhizosphere carbon-input, denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes PCR-amplified targeting DNA and RNA from rhizosphere soil did not reveal any effects of cutting on banding patterns. In contrast, AMF induced consistent differences in both DNA- and RNA-based DGGE profiles. These results show that a reduction in rhizosphere microbial activity is not necessarily accompanied by changes in BCC, whereas AMF presence inhibits proliferation of some bacterial taxa while stimulating others.     

Spatial variation of active microbiota in the rice rhizosphere revealed by in situ stable isotope probing of phospholipid fatty acids

This report is part of a serial study applying stable isotope labelling to rice microcosms to track the utilization of recently photosynthesized carbon by active microbiota in the rhizosphere. The objective of the present study was to apply phospholipid fatty acid-based stable isotope probing (PLFA-SIP) to detect the spatial variation of active microorganisms associated with rhizosphere carbon flow. In total, 49 pulses of 13CO2 were applied to rice plants in a microcosm over a period of 7 days. Rhizosphere soil was separated from bulk soil by a root bag. Soil samples were taken from rhizosphere and bulk soil, and the bulk soil samples were further partitioned both vertically (up layer and down layer) and horizontally with increasing distance to the root bag. Incorporation of 13C into PLFAs sharply decreased with distance to the roots. The labelling of 16:1omega9, 18:1omega7, 18:1omega9, 18:2omega6,9 and i14:0 PLFAs was relatively stronger in the rhizosphere while that of i15:0 and i17:0 increased in the bulk soil. The microorganisms associated with 16:1omega9 were active in both up- and down-layer soils. The microorganisms represented by i14:0, 18:1omega7 and 18:2omega6,9 exhibited a relatively higher activity in up-layer soil, whereas those represented by i15:0 and i17:0 were more active in down-layer soil. These results suggest that in the rhizosphere Gram-negative and eukaryotic microorganisms were most actively assimilating root-derived C, whereas Gram-positive microorganisms became relatively more important in the bulk soil. The active populations apparently differed between up- and down-layer soil and in particular changed with distance to the roots, demonstrating systematic changes in the activity of the soil microbiota surrounding roots.     

CO2浓度倍增和土壤干旱对两种幼龄沙生灌木碳分配的影响

利用大型环境生长箱研究了两种幼龄沙地优势灌木柠条 (Caraganaintermedia) 和羊柴 (Hedysarummon golicum) 对CO2 浓度倍增和土壤干旱交互作用的响应。CO2 浓度倍增并没有改善两种沙生灌木叶片的水分状况, 而土壤干旱使叶片的相对含水量 (RWC) 显著降低。在土壤水分充足条件下, CO2 浓度倍增促进两种沙生灌木植株生长, 在干旱条件下则主要促进根的生长, 提高根冠比。土壤干旱显著减少了植株生物量, 但相对促进了根的生长, 特别是显著提高了羊柴的根冠比。CO2 倍增使稳定性碳同位素组分 (δ13 C) 降低, 但土壤干旱使之增加。两种沙生灌木叶片与根部的δ13 C值呈极显著线性关系, 羊柴的斜率大于柠条的, 表明前者叶片与根部在光合产物分配上具有较高的生态可塑性, 这和干旱条件下羊柴的根冠比增加相关联。羊柴的“源库”调节特性反映了对土壤水分胁迫具有较高的耐性。     

Effect of ecosystem warming on boreal black spruce bud burst and shoot growth

The boreal forest is predicted to experience the greatest warming of any forest biome during the next 50–100 years, but the effects of warming on vegetation phenology are not well known. The objectives of this study were to (1) examine the effects of whole ecosystem warming on bud burst and annual shoot growth of black spruce trees in northern Manitoba, Canada and (2) correlate bud burst to cumulative degree-days (CDD). The experimental design was a complete randomized block design that consisted of four replicated blocks. Each replicate block contained four treatments: soil warming only (heated outside, HO), soil and air warming (heated inside, HI), control outside (no chamber, no heating, CO), and inside a chamber maintained at ambient conditions (no soil or air warming, control inside, CI). Bud burst was measured during the first and second years of the experiment, starting in 2004, and annual shoot growth was measured for the first 3 years (2004–2006) of the study. On average, shoot bud burst occurred 11 and 9 days earlier in 2004 and 2005, respectively, for HI than for other treatments. However, mean CDD required for bud burst for HI was within the standard deviation of CO for both years. In year 1 of the treatments, shoot bud burst occurred earlier for HI than other treatments (CI, CO, HO), but final shoot length of HI trees was less than in CO trees. In the second year of warming, final shoot length was not different for HI than CO. By the third year of warming final shoot length was significantly greater for HI than all other treatments. Empirical results from this study suggest that soil and air warming causes an earlier bud burst for all years of observation and greater shoot lengths by the third season of warming. A longer growing season and greater annual shoot growth should increase carbon uptake by boreal black spruce trees in a warmer climate.     

A continuous labelling approach to recover photosynthetically fixed carbon in plant tissue and rhizosphere organisms of young beech trees (Fagus sylvatica L.) using 13C depleted CO2

A continuous labelling experiment using ¹³C-CO₂ was set up in open-top chambers in order to follow fluxes of assimilates from the plant into the rhizosphere. Labelling was performed for one growing season by adding low amounts of CO₂ depleted in ¹³C to the atmosphere of the open-top chambers, resulting in a difference of ? ¹³C 5‰ V-PDB compared to ambient conditions. The label was recovered in both plant parts and soil microbial communities, analysed via phospholipid fatty acid (PLFA) side chains. PLFA 18:2ω6,9 showed a significant incorporation of the ¹³C label in October, indicating that fungi utilized plant derived carbon. In bacterial PLFA no label incorporation was detected, probably due to a lower use of rhizodeposits or a preference to older carbon compounds as energy sources. This experimental setup represents a low-cost continuous labelling method for field experiments with only minor increase of CO₂ concentrations.     

Cluster Roots of Leucadendron laureolum (Proteaceae) and Lupinus albus (Fabaceae) Take Up Glycine Intact: An Adaptive Strategy to Low Mineral Nitrogen in Soils?

BACKGROUND AND AIMS: South African soils are not only low in phosphorus (P) but most nitrogen (N) is in organic form, and soil amino acid concentrations can reach 2.6 g kg(-1) soil. The Proteaceae (a main component of the South African Fynbos vegetation) and some Fabaceae produce cluster roots in response to low soil phosphorus. The ability of these roots to acquire the amino acid glycine (Gly) was assessed. METHODS: Uptake of organic N as 13C-15N-Gly was determined in cluster roots and non-cluster roots of Leucadendron laureolum (Proteaceae) and Lupinus albus (Fabaceae) in hydroponic culture, taking account of respiratory loss of 13CO2. KEY RESULTS: Both plant species acquired doubly labelled (intact) Gly, and respiratory losses of 13CO2 were small. Lupin (but not leucadendron) acquired more intact Gly when cluster roots were supplied with 13C-15N-Gly than when non-cluster roots were supplied. After treatment with labelled Gly (13C : 15N ratio = 1), lupin cluster roots had a 13C : 15N ratio of about 0.85 compared with 0.59 in labelled non-cluster roots. Rates of uptake of label from Gly did not differ between cluster and non-cluster roots of either species. The ratio of C : N and 13C : 15N in the plant increased in the order: labelled roots < rest of the root < shoot in both species, owing to an increasing proportion of 13C translocation. CONCLUSIONS: Cluster roots of lupin specifically acquired more intact Gly than non-cluster roots, whereas Gly uptake by the cluster and non-cluster roots of leucadendron was comparable. The uptake capacities of cluster roots are discussed in relation to spatial and morphological characteristics in the natural environment.     

Temporal dynamics of the carbon isotope composition in a Pinus sylvestris stand: from newly assimilated organic carbon to respired carbon dioxide

The (13)C isotopic signature (C stable isotope ratio; delta(13)C) of CO(2) respired from forest ecosystems and their particular compartments are known to be influenced by temporal changes in environmental conditions affecting C isotope fractionation during photosynthesis. Whereas most studies have assessed temporal variation in delta(13)C of ecosystem-respired CO(2) on a day-to-day scale, not much information is available on its diel dynamics. We investigated environmental and physiological controls over potential temporal changes in delta(13)C of respired CO(2) by following the short-term dynamics of the (13)C signature from newly assimilated organic matter pools in the needles, via phloem-transported organic matter in twigs and trunks, to trunk-, soil- and ecosystem-respired CO(2). We found a strong 24-h periodicity in delta(13)C of organic matter in leaf and twig phloem sap, which was strongly dampened as carbohydrates were transported down the trunk. Periodicity reappeared in the delta(13)C of trunk-respired CO(2), which seemed to originate from apparent respiratory fractionation rather than from changes in delta(13)C of the organic substrate. The diel patterns of delta(13)C in soil-respired CO(2) are partly explained by soil temperature and moisture and are probably due to changes in the relative contribution of heterotrophic and autotrophic CO(2) fluxes to total soil efflux in response to environmental conditions. Our study shows that direct relations between delta(13)C of recent assimilates and respired CO(2) may not be present on a diel time scale, and other factors lead to short-term variations in delta(13)C of ecosystem-emitted CO(2). On the one hand, these variations complicate ecosystem CO(2) flux partitioning, but on the other hand they provide new insights into metabolic processes underlying respiratory CO(2) emission.     

Carbon (13C) uptake and allocation in pasture plants following field pulse-labelling

The allocation of carbon (C) to plant roots and conversion to soil organic matter is a major determinant of the size of the terrestrial C pool in pastoral ecosystems. The aim was to quantify C allocation to roots in contrasting pastoral ecosystems. Pastures on long-term research sites in Canterbury, New Zealand were pulse-labelled using ¹³CO₂ within portable gas-tight enclosures. Sites included Winchmore (with or without superphosphate fertiliser, and with or without irrigation) and Tara Hills (low, medium or high grazing intensity with continuous or alternating grazing). Separate micro-plots were labelled in late spring, summer and autumn at Winchmore and in spring at Tara Hills. Herbage label ¹³C recoveries were greatest one hour after pulse labelling and declined by 21 days, whereas in roots they were initially lower but generally continued to increase until 21 days. The greatest recoveries of ¹³C in roots, one hour and 21 days after labelling, were in summer and autumn respectively. The proportion of label ¹³C allocated to roots by 21 days was 0.50 in the absence of superphosphate and 0.41 in the superphosphate treatment, and was 0.39, 0.43 and 0.51 respectively in spring, summer and autumn. Irrigation had no significant effect on root allocation. The low stocking rate at Tara Hills, which had the greatest herbage biomass, also had greater total ¹³C, tussock herbage ¹³C and root ¹³C recoveries than the higher stocking rate treatments. Inter-tussock root recovery and allocation of ¹³C to roots increased with increasing stocking rate, whereas tussock root allocation was greatest in the high and least in the medium stocking rate treatment. By 21 days there was a greater inter-tussock and tussock root recovery and lower inter-tussock herbage recovery in the continuous than in the alternating grazing management treatment. The root allocation was generally greater in the continuous than in alternating grazed treatments, except for tussocks one hour after labelling where the reverse was the case. In conclusion the ¹³C pulse labelling showed pasture plants allocate more C to roots with low soil fertility, high grazing intensity, continuous grazing, and in autumn.     

Short-term dynamics of the carbon isotope composition of CO2 emitted from a wheat agroecosystem--physiological and environmental controls

Understanding environmental and physiological controls of the variations in δ(13) C of CO(2) respired (δ(13) C(R)) from different compartments of an ecosystem is important for separation of CO(2) fluxes and to assess coupling between assimilation and respiration. In a wheat field, over 3 days we characterised the temporal dynamics of δ(13) C(R) from shoots and roots, from the soil and from the whole agroecosystem. To evaluate the basis of potential variations in δ(13) C(R), we also measured δ(13) C in different organic matter pools, as well as meteorological and gas exchange parameters. We observed strong diel variations up to ca. 6% in shoot, root and soil δ(13) C(R), but not in δ(13) C of the putative organic substrates for respiration, which varied by not more than ca. 1% within 24 h. Whole ecosystem-respired CO(2) was least depleted in (13) C in the afternoon and most negative in the early morning. We assume that temporally variable respiratory carbon isotope fractionation and changes in fluxes through metabolic pathways, rather than photosynthetic carbon isotope fractionation, governs the δ(13) C of respired CO(2) at the diel scale, and thus provides insights into the metabolic processes related to respiration under field conditions.     

Pulse-labelling a cover crop with 13C to follow its decomposition in soil under field conditions

A preliminary study was conducted using the stable isotope ¹³C to pulse label the cover crop phacelia (Phacelia tanacetifolia) to examine its decomposition in soil, under field conditions. Plants were grown, in pots, in the greenhouse and after four weeks of growth were labelled with ¹³CO₂ six times, at 1–2 week intervals. A single chamber was placed over the pots, and ¹³CO₂ was generated, inside the chamber, by injecting lactic acid into sodium carbonate (99 atom % ¹³C). For calculating the quantity of Na₂CO₃ required, a target enrichment of 5 atom% ¹³C within the shoots of plants, assuming no respiration losses, was used. When harvested, at flowering, the mean enrichment of the shoot material was 3.0466 atom% ¹³C, or 1.9654 atom% excess ¹³C. To assess uniformity of labelling within plants, the shoot of a single plant was divided into leaves and stem from three sections of equal length. Ninety-three percent of this plant's dry matter had a ¹³C enrichment within 20 % of the weighted mean. At a field site with sandy soil, ¹³C labelled shoot and root material were combined and mixed with soil (0–15 cm). The soil was sampled 16 and 179 days later to determine the recovery of the added excess ¹³C in soil total C. The recoveries in soil (0–30 cm) were, respectively, 78 and 40 % at 16 and 179 days; there was appreciable variation associated with the recovery data from day 16, much less so at day 179. Methodological procedures for (i) enhancing the uniformity of labelling with ¹³C within plants, and (ii) minimising variability in the recovery of ¹³C from soil are suggested. ei]R Merckx     

Post-uptake metabolism affects quantification of amino acid uptake

? The quantitative significance of amino acids to plant nutrition remains controversial. This experiment determined whether post-uptake metabolism and root to shoot export differ between glycine and glutamine, and examined implications for estimation of amino acid uptake. ? Field soil containing a Eucalyptus pauciflora seedling was injected with uniformly (13)C- and (15)N-labelled glycine or glutamine. I quantified (15)N and (13)C excess in leaves and roots and intact labelled amino acids in leaves, roots and stem xylem sap. A tunable diode laser quantified fluxes of (12)CO(2) and (13)CO(2) from leaves and soil. ? 60-360 min after addition of amino acid, intact molecules of U-(13)C,(15)N glutamine were < 5% of (15)N excess in roots, whereas U-(13)C,(15)N glycine was 30-100% of (15)N excess in roots. Intact molecules of glutamine, but not glycine, were exported from roots to shoots. ? Post-uptake metabolism and transport complicate interpretation of isotope labelling such that root and shoot contents of intact amino acid, (13)C and (15)N may not reflect rates of uptake. Future experiments should focus on reconciling discrepancies between intact amino acid, (13)C and (15)N by determining the turnover of amino acids within roots. Alternatively, post-uptake metabolism and transport could be minimized by harvesting plants within minutes of isotope addition.     

Theoretical Proof and Empirical Confirmation of a Continuous Labeling Method Using Naturally ^13C-Depleted Carbon Dioxide

Continuous Isotope labeling and tracing is often needed to study the transformation, movement, and allocation of carbon in plant-soil systems. However, existing labeling methods have numerous limitations. The present study Introduces a new continuous labeling method using naturally ^13C-depleted CO2. We theoretically proved that a stable level of ^13C-CO2 abundance in a labeling chamber can be maintained by controlling the rate of CO2-free air Injection and the rate of ambient airflow with coupling of automatic control of CO2 concentration using a CO2 analyzer. The theoretical results were tested and confirmed in a 54 day experiment in a plant growth chamber. This new continuous labeling method avoids the use of radioactive ^14C or expensive ^13C-enrlched CO2 required by existing methods and therefore eliminates issues of radiation safety or unaffordable isotope cost, as well as creating new opportunities for short- or long-term labeling experiments under a controlled environment.     

The dynamics of neutral sugars in the rhizosphere of wheat. An approach by13C pulse-labelling and GC/C/IRMS

Rhizodeposition, i.e. the release of carbon into the soil by growing roots, is an important part of the terrestrial carbon cycle. However thein situ nature and dynamics of root-derived carbon in the soil are still poorly understood. Here we made an investigation of the latter in laboratory experiments using¹³CO₂ pulse chase labelling of wheat (Triticum aestivum L.). We analyzed the kinetics of¹³C-labelled carbon and more specially¹³C carbohydrates in the rhizosphere. Wheat seedlings-soil mesocosms were exposed to¹³CO₂ for 5 hours in controlled chambers and sampled repeatedly during two weeks for¹³C/C analysis of organic carbon. After a two-step separation of the soil from the roots, the amount of total organic¹³C was determined by isotope ratio mass spectrometry as well as the amounts of¹³C in arabinose, fructose, fucose, glucose, galactose, mannose, rhamnose and xylose. The amount and isotopic ratio of monosaccharides were obtained by capillary gas chromatography coupled with isotope ratio mass spectrometry (GC/C/IRMS) after trimethyl-silyl derivatization. Two fractions were analyzed : total (hydrolysable) and soluble monomeric (water extractable) soil sugars. The amount of organic¹³C found in the soil, expressed as a percentage of the total photosynthetically fixed¹³C at the end of the labelling period, reached 16% in the day following labelling and stabilised at 9% after one week. We concluded that glucose under the form of polymers was the dominant moietie of rhizodeposits. Soluble glucose and fructose were also present. But after 2 days, these soluble sugars had disappeared. Forty percent of the root-derived carbon was in the form of neutral sugars, and exhibited a time-increasing signature of microbial sugars. The composition of rhizospheric sugars rapidly tended towards that of bulk soil organic matter.     

Temporal variation in δ<Superscript>13</Superscript>C of ecosystem respiration in the Pacific Northwest: links to moisture stress

We measured seasonal and interannual variations in delta(13)C values within the carbon reservoirs (leaves and soil) and CO(2) fluxes (soil and ecosystem respired CO(2)) of an old growth coniferous forest in the Pacific Northwest USA with relation to local meteorological conditions. There were significant intra-annual and interannual differences in the carbon isotope ratios of CO(2) respired at both the ecosystem (delta(13)C(R)) and the soil levels (delta(13)C(R-soil)), but only limited variations in the carbon isotope ratios of carbon stocks. The delta(13)C(R) values varied by as much as 4.4 per thousand over a growing season, while delta(13)C(R-soil )values changed as much as 6.2 per thousand. The delta(13)C of soil organic carbon (delta(13)C(SOC)) and needle organic carbon (delta(13)C(P)) exhibited little or no significant changes over the course of this study. Carbon isotope discrimination within leaves (Delta(p)) showed systematic decreases with increased canopy height, but remained fairly constant throughout the year (Delta(p)=17.9 per thousand -19.2 per thousand at the top of the canopy, Delta(p)=19.6 per thousand -20.9 per thousand at mid-canopy, Delta(p)=23.3 per thousand -25.1 per thousand at the canopy base). The temporal variation in the delta(13)C of soil and ecosystem respired CO(2) was correlated ( r=0.93, P<0.001) with soil moisture levels, with dry summer months having the most (13)C-enriched values. The dynamic seasonal changes in delta(13)C of respired CO(2) are hypothesized to be the result of fast cycling of recently fixed carbon back to the atmosphere. One scaling consequence of the seasonal and interannual variations in delta(13)C(R) is that inversion-based carbon-cycle models dependent on observed atmospheric CO(2) concentration and isotope values may be improved by incorporating dynamic delta(13)C(R) values to interpret regional carbon sink strength.     

Influence of elevated CO(2) on the fungal community in a coastal scrub oak forest soil investigated with terminal-restriction fragment length polymorphism analysis

Sixteen open-top chambers (diameter, 3.66 m) were established in a scrub oak habitat in central Florida where vegetation was removed in a planned burn prior to chamber installation. Eight control chambers have been continuously exposed to ambient air and eight have been continuously exposed to elevated CO(2) at twice-ambient concentration (approximately 700 ppm) for 5 years. Soil cores were collected from each chamber to examine the influence of elevated atmospheric CO(2) on the fungal community in different soil fractions. Each soil sample was physically fractionated into bulk soil, rhizosphere soil, and roots for separate analyses. Changes in relative fungal biomass were estimated by the ergosterol technique. In the bulk soil and root fractions, a significantly increased level of ergosterol was detected in the elevated CO(2) treatments relative to ambient controls. Fungal community composition was determined by terminal-restriction fragment length polymorphism (T-RFLP) analysis of the internal transcribed spacer (ITS) region. The specificities of different ITS primer sets were evaluated against plant and fungal species isolated from the experimental site. Changes in community composition were assessed by principal component analyses of T-RFLP profiles resolved by capillary electrophoresis. Fungal species richness, defined by the total number of terminal restriction fragments, was not significantly affected by either CO(2) treatment or soil fraction.     

Canopy CO2 enrichment permits tracing the fate of recently assimilated carbon in a mature deciduous forest

How rapidly newly assimilated carbon (C) is invested into recalcitrant structures of forests, and how closely C pools and fluxes are tied to photosynthesis, is largely unknown. A crane and a purpose-built free-air CO2 enrichment (FACE) system permitted us to label the canopy of a mature deciduous forest with 13C-depleted CO2 for 4 yr and continuously trace the flow of recent C through the forest without disturbance. Potted C4 grasses in the canopy ('isometers') served as a reference for the C-isotope input signal. After four growing seasons, leaves were completely labelled, while newly formed wood (tree rings) still contained 9% old C. Distinct labels were found in fine roots (38%) and sporocarps of mycorrhizal fungi (62%). Soil particles attached to fine roots contained 9% new C, whereas no measurable signal was detected in bulk soil. Soil-air CO2 consisted of 35% new C, indicating that considerable amounts of assimilates were rapidly returned back to the atmosphere. These data illustrate a relatively slow dilution of old mobile C pools in trees, but a pronounced allocation of very recent assimilates to C pools of short residence times.