Plant species influence microbial diversity and carbon allocation in the rhizosphere

Plant species effects on microbial communities are attributed to changes in microbial community composition and biomass, and may depend on plant species specific differences in the quality of resources (carbon) inputs. We examined the idea that plant-soil feedbacks can be explained by a chance effect, which is the probability of a highly productive or keystone plant species is present in the community and will influence the functions more than the number of species per se. A ¹³C pulse labelling technique was applied to three plant species and a species mixture in a greenhouse experiment to examine the carbon flow from plants to soil microbial communities. The ¹³C label was given as CO₂ to shoots of a legume (Lotus corniculatus), a forb (Plantago lanceolata), a grass (Holcus lanatus) and a mixture of the three species. Microbial phospholipid fatty acids (PLFA) was analysed in order to determine the biomass and composition of the soil microbial community. The incorporation of the stable isotope into soil microorganisms was determined through GC-IRMS analyses of the microbial PLFAs. Plant species identity did not influence the microbial biomass when determined as total carbon of microbial phospholipid fatty acids. However, the labelled carbon showed that the grass monoculture (H. lanatus) and the plant mixture allocated more ¹³C into bacteria and actinomycete biomass than the other plant species. H. lanatus monocultures had also the highest amounts of ¹³C allocated to AM-fungi and saprophytic fungi. The carbon allocation from plants to soil microorganisms in a plant species mixture can thus be explained by the presence of a highly productive species that influence soil functions.     

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

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

Effect of clipping and shading on C allocation and fluxes in soil under ryegrass and alfalfa estimated by 14C labelling

Photosynthesis of higher plants drives carbon (C) allocation below-ground and controls the supply of assimilates to roots and to rhizosphere microorganisms. To investigate the effect of limited photosynthesis on C allocation, redistribution and reutilization in plant and soil microorganisms, perennial grass Lolium perenne and legume Medicago sativa were clipped or shaded. Plants were labelled with three ¹⁴C pulses to trace allocation and reutilization of C assimilated before clipping or shading. Five days after the last ¹⁴C pulse, plants were clipped or shaded and the total CO₂ and ¹⁴CO₂ efflux from the soil was measured. ¹⁴C in above- and below-ground plant biomass and bulk soil, rhizosphere soil and microorganisms was determined 10 days after clipping or shading.After clipping, 2% of the total assimilated ¹⁴C originating mainly from root reserves were detected in the newly grown shoots. This corresponded to a translocation of 5 and 8% of total ¹⁴C from reserve organs to new shoots of L. perenne and M. sativa, respectively. The total CO₂ efflux from soil decreased after shading of both plant species, whereas after clipping, this was only true for L. perenne. The ¹⁴CO₂ efflux from soil did not change after clipping of both species. An increased ¹⁴CO₂ efflux from soil under shading for both plants indicated that lower assimilation was compensated by higher utilization of the reserve C for root and rhizomicrobial respiration.We conclude that C stored in roots is an important factor for plant recovery after limiting photosynthesis. This stored C is important for shoot regrowth after clipping, whereas after shading, it is utilized mainly for maintenance of root respiration. Based on these results as well as on a review of several studies on C reutilization for regrowth after clipping, we conclude that because of the high energy demand for nitrogen fixation, legumes use a higher portion (9–10%) of stored C for regrowth compared to grasses (5–7%). The effects of limited photosynthesis were of minor importance for the exudation of the reserve C and thus, have no effect on the uptake of this C by microorganisms.     

Photoassimilate allocation and dynamics of hotspots in roots visualized by 14C phosphor imaging

Understanding photoassimilate allocation into the roots and the release of organic substances from the roots into the rhizosphere is an important prerequisite for characterizing the belowground C input, the spatial and temporal distribution of C, and the interactions between plants and soil microorganisms. Based on ¹⁴C phosphor imaging, we visualized the allocation of assimilates into Lolium perenne roots and estimated the life time of hotspots at the root tips. Lolium shoots were labeled in a ¹⁴CO₂ atmosphere, and herbariums of roots and shoots were prepared 6 h, 2 d, and 11 d after the ¹⁴C pulse. The ¹⁴C distribution in roots and leaves revealed that pulse labeling does not yield homogeneously labeled plant material. The spatial distribution of assimilate allocation was evaluated based on the ¹⁴C specific activity expressed as digital light units (DLU mm^–2) of the imaging plates. Areas with high relative ¹⁴C activity were classified as hotspots. Strong ¹⁴C hotspots were detected mainly at the root tips already 6 h after the ¹⁴C assimilation, and they remained active for at least 2 d. Eleven days after the ¹⁴C assimilation, the hotspots at the root tips disappeared and the ¹⁴C distribution was much more even than after 6 h or after 2 d. ¹⁴C phosphor imaging proved to be a promising tool to visualize the allocation of photoassimilates into the roots and the rhizosphere and can be used to identify hotspots and their dynamics.     

Nitrogen rhizodeposition of young wheat plants under elevated CO<Subscript>2</Subscript> and drought stress

The objective of this study was to determine the effect of drought stress and elevated CO₂ concentrations around the shoots on N rhizodeposition of young wheat plants. In a pot experiment, the plant N pool was labeled through ¹⁵NH₃ application to shoots at nontoxic NH₃ concentrations, and the impact of low water supply (40% field capacity), elevated CO₂ (720 μmol mol⁻¹ CO₂), and the combination of both factors on the ¹⁵N distribution was studied. Total ¹⁵N rhizodeposition ranged from 5 to 11% of the total ¹⁵N recovered in the plant/soil system. Elevated CO₂ concentration as well as drought stress increased the belowground transport of N and increased the relative portion of N rhizodeposition on total ¹⁵N in the plant/soil system. However, while the increased N rhizodeposition with elevated CO₂ was the result of increased total belowground N transport, drought stress additionally increased the portion of ¹⁵N found in rhizodeposition vs roots. Elevated CO₂ intensified the effect of drought stress. The percentage of water soluble ¹⁵N in the ¹⁵N rhizodeposition was very low under all treatments, and it was significantly decreased by the drought-stressed treatments.     

Diurnal and seasonal carbon dioxide exchange and its components in temperate grasslands in the Netherlands — an outline of the methodology

A methodological outline is presented of a study into the diurnal and seasonal cycle of carbon fluxes within grassland ecosystems in the Netherlands in relation to their environment. At experimental sites Lelystad and Zegveld ?redominantlyLolium perenne L. at a clay and peat soil, respectively — measurements will be made on (1) net CO₂ assimilation of the grassland vegetation using infrared gas analysis; (2) carbon distribution within the plant using¹⁴C pulse labeling; and (3) carbon and CO₂ fluxes associated with root respiration and soil organic matter decomposition using¹⁴C pulse labeling. At both sites and at experimental site Cabauw additional measurements will be made on total CO₂ fluxes between the grassland vegetation and the lower part of the atmospheric boundary layer. For the analysis of the experimental results and generalisation of the relationships between carbon fluxes and environmental and plant factors use will be made of dynamic simulation models of grass growth and soil organic matter dynamics.     

Short and long-term effects of elevated CO2 on Lolium perenne rhizodeposition and its consequences on soil organic matter turnover and plant N yield

It is still unclear whether elevated CO₂ increases plant root exudation and consequently affects the soil microbial biomass. The effects of elevated CO₂ on the fate of the C and nitrogen (N) contained in old soil organic matter pools is also unclear. In this study the short and long-term effects of elevated CO₂ on C and N pools and fluxes were assessed by growing isolated plants of ryegrass (Lolium perenne) in glasshouses at elevated and ambient atmospheric CO₂ and using soil from the New Zealand FACE site that had >4 years exposure to CO₂ enrichment. Using ¹⁴CO₂ pulse labelling, the effects of elevated CO₂ on C allocation within the plant-soil system were studied. Under elevated CO₂ more root derived C was found in the soil and in the microbial biomass 48 h after labelling. The increased availability of substrate significantly stimulated soil microbial growth and acted as priming effect, enhancing native soil organic matter decomposition regardless of the mineral N supply. Despite indications of faster N cycling in soil under elevated CO₂, N availability to plants stayed unchanged. Soil previously exposed to elevated CO₂ exhibited a higher N cycling rate but again there was no effect on plant N uptake. With respect to the difficulties of extrapolating glasshouse experiment results to the field, we concluded that the accumulation of coarse organic matter observed in the field under elevated CO₂ was probably not created by an imbalance between C and N but was likely to be due to more complex phenomena involving soil mesofauna and/or other nutrients limitations.     

Root-derived respiration and non-structural carbon of rice seedlings

Various methods have been suggested to separate root and microbial contributions to soil respiration. However, to date there is no ideal approach available to partition below-ground CO₂ fluxes in its components although the combination of traditional methods with approaches based on isotopes seems especially promising for the future improvement of estimates. Here we provide evidence for the applicability of a new approach based on the hypothesis that root-derived (rhizomicrobial) respiration, including root respiration and CO₂ derived from microbial activity in the immediate vicinity of the root, is proportional to non-structural carbon contents (sugars and organic acids) of plant tissues. We examined relationships between root-derived CO₂ and non-structural carbon of rice (Oryza sativa) seedlings using ¹⁴C pulse labelling techniques, which partitioned the ¹⁴C fixed by photosynthesis into root-derived ¹⁴CO₂, and ¹⁴C in sugars and organic acids of roots and shoots. After the ¹⁴C pulse ¹⁴C in both sugars and organic acids of plant tissues decreased steeply during the first 12 h, and then decreased at a lower rate during the remaining 60 h. Soil ¹⁴CO₂ efflux and soil CO₂ efflux strongly depended on ¹⁴C pools in non-structural carbon of the plant tissues. Based on the linear regression between root-derived respiration and total non-structural carbon (sugars and organic acids) of roots, non-rhizomicrobial respiration (SOM-derived) was estimated to be 0.25 mg C g⁻¹ root d.w. h⁻¹. Assuming the value was constant, root-derived respiration contributed 85–92% to bulk soil respiration.     

Carbon allocation in a sweet sorghum‐soil system using 14C as a tracer

Carbon (C) distribution in a sweet sorghum‐soil system was studied by ¹⁴CO₂ pulse‐labeling of shoots at three dates during the growth cycle in order to assess the contribution of the crop to carbon storage in the soil. Soil and plant samplings were performed 24 h after the ¹⁴C‐labeling and at final harvest (October) to determine the assimilate allocation and estimate the amount of plant‐derived soil carbon. Approximately 4‐16% of the ¹⁴C present in the sorghum‐soil system was located in the soil fine fraction (< 2 mm) over a 24 h period. At final harvest, the proportion of ¹⁴C in the soil accounted for 7‐9% of the ¹⁴C present in the sorghum‐soil system. The plant‐derived soil carbon was estimated at 0.10‐0.12 g C plant^‐1 day^‐1. The total amount of carbon captured by sweet sorghum was estimated at 1.44 kg C m^‐2 over the whole growth cycle: 0.82 kg C m^‐2 in the above‐ground biomass, 0.52 kg C m^‐2 in the below‐ground biomass and 0.10 kg C m^‐2 in the soil carbon pool. No significant increase in soil ¹⁴C was detected over the next 14 months.     

二氧化碳升高与干旱对植物生理、土壤碳及土壤酶活的影响

Global climate models have indicated high probability of drought occurrences in the coming future decades due to the impacts of climate change caused by a mass release of CO2.Thus,climate change regarding elevated ambient CO2 and drought may consequently affect the growth of crops.In this study,plant physiology,soil carbon,and soil enzyme activities were measured to investigate the impacts of elevated CO2 and drought stress on a Stagnic Anthrosol planted with soybean (Glycine max).Treatments of two CO2 levels,three soil moisture levels,and two soil cover types were established.The results indicated that elevated CO2 and drought stress significantly affected plant physiology.The inhibition of plant physiology by drought stress was mediated via prompted photosynthesis and water use efficiency under elevated CO2 conditions.Elevated CO2 resulted in a longer retention time of dissolved organic carbon (DOC) in soil,probably by improving the soil water effectiveness for organic decomposition and mineralization.Drought stress significantly decreased C:N ratio and microbial biomass carbon (MBC),but the interactive effects of drought stress and CO2 on them were not significant.Elevated CO2 induced an increase in invertase and catalase activities through stimulated plant root exudation.These results suggested that drought stress had significant negative impacts on plant physiology,soil carbon,and soil enzyme activities,whereas elevated CO2 and plant physiological feedbacks indirectly ameliorated these impacts.     

Responses of soil respiration and its components to drought stress

Purpose

Climate change is likely to increase both intensity and frequency of drought stress. The responses of soil respiration (R _s) and its components (root respiration, R _r; mycorrhizal respiration, R _m; and heterotrophic respiration, R _h) to drought stress can be different. This work aims to review the recent and current literature about the variations in R _s during the period of drought stress, to explore potential coupling processes and mechanisms between R _s and driving factors in the context of global climate change.

Results and discussion

The sensitivity of soil respiration and its components to drought stress depended on the ecosystems and seasonality. Drought stress depressed R _s in mesic and xeric ecosystems, while it stimulated R _s in hydric ecosystems. The reductions in supply and availability of substrate decreased both auto- and heterotrophic respirations, leading to the temporal decoupling of root and mycorrhizal respiration from canopy photosynthesis as well as C allocation. Drought stress also reduced the diffusion of soluble C substrate, and activities of extracellular enzymes, consequently, limited microbial activity and reduced soil organic matter decomposition. Drought stress altered Q ₁₀ values and broke the coupling between temperature and soil respiration. Under drought stress conditions, R _m is generally less sensitive to temperature than R _r and R _h. Elevated CO₂ concentration alleviated the negative effect of drought stress on soil respiration, principally due to the promotion of plant C assimilation subsequently, which increased substrate supply for respiration in both roots and soil microorganisms. Additionally, rewetting stimulated soil respiration dramatically in most cases, except for soil that experienced extreme drought stress periods. The legacy of drought stress can also regulate the response of soil respiration rate to rewetting events in terrestrial ecosystems through changing abiotic drivers and microbial community structure.

Conclusions and perspectives

There is increasing evidence that drought stress can result in the decoupling of the above- and belowground processes, which are associated with soil respiration. However, studies on the variation in rates of soil respiration and its components under different intensities and frequencies of drought stress over the ecosystems should be reinforced. Meanwhile, molecular phylogenetics and functional genomics should be applied to link microbial ecology to the process of R _s. In addition, we should quantify the relationship between soil respiration and global change parameters (such as warming and elevated [CO₂]) under drought stress. Models simulating the rates of soil respiration and its components under global climate change and drought stress should also be developed.     

Root-derived carbon in soil respiration and microbial biomass determined by 14C and 13C

Two approaches to quantitatively estimating root-derived carbon in soil CO₂ efflux and in microbial biomass were compared under controlled conditions. In the ¹⁴C labelling approach, maize (Zea mays) was pulse labelled and the tracer was chased in plant and soil compartments. Root-derived carbon in CO₂ efflux and in microbial biomass was estimated based on a linear relationship between the plant shoots and the below-ground compartment. Since the maize plants were grown on C₃ soil, in a second approach the differences in ¹³C natural abundance between C₃ and C₄ plants were used to calculate root-derived carbon in the CO₂ efflux and in the microbial biomass. The root-derived carbon in the total CO₂ efflux was between 69% and 94% using the ¹⁴C labelling approach and between 86% and 94% in the natural ¹³C labelling approach. At a ¹³C fractionation measured to be 5.2‰ between soil organic matter (SOM) and CO₂, the root-derived contribution to CO₂ ranged from 70% to 88% and was much closer to the results of the ¹⁴C labelling approach. Root-derived contributions to the microbial biomass carbon ranged from 2% to 9% using ¹⁴C labelling and from 16% to 36% using natural ¹³C labelling. At a 3.2‰ ¹³C fractionation between SOM and microbial biomass, both labelling approaches yielded an equal contribution of root-derived C in the microbial biomass. Both approaches may therefore be used to partition CO₂ efflux and to quantify the C sources of microbial biomass. However, the assumed ¹³C fractionation strongly affects the contributions of individual C sources.     

Selenium in lemon balm plants: Productivity,phytotoxicity and drought alleviation

Since studies on the effects of selenium (Se) supplementation in water-stressed plants have mainly focused on cereal crops, the specific reports regarding Se-mediated adaptation to drought stress in medicinal vegetables are scant. Thus, we investigated the responses of Melissa officinalis to Se supplementation. Selenium contents were increased in leaves and grains by supplemental Se. Selenium foliar application at 1 mg l^?1 could be useful to increase the vegetative and reproductive growth of Se-enriched plants under well-watered conditions but at 20 mg l^?1 led to toxicity and caused damage to shoots. Drought stress significantly inhibited plant growth by chlorophyll degradation and reduced net carbon dioxide (CO₂) assimilation rate. Although Se at 1 mg l^?1 could increase biomass production under well-watered conditions in addition to the stimulation of antioxidant system under water stress, it could not ameliorate the negative effect of drought on productivity.     

Repeated CO2 pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment (FACE)

Rising levels of atmospheric CO₂ have often been found to increase above and belowground biomass production of C3 plants. The additional translocation of organic matter into soils by increased root mass and exudates are supposed to possibly increase C pools in terrestrial ecosystems. Corresponding investigations were mostly conducted under more or less artificial indoor conditions with disturbed soils. To overcome these limitations, we conducted a ¹⁴CO₂ pulse-labelling experiment within the German FACE project to elucidate the role of an arable crop system in carbon sequestration under elevated CO₂. We cultivated spring wheat cv. “Minaret” with usual fertilisation and ample water supply in stainless steel cylinders forced into the soil of a control and a FACE plot. Between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with ¹⁴CO₂ in the field. Soil born total CO₂ and ¹⁴CO₂ was monitored daily till harvest. Thereafter, the distribution of ¹⁴C was analysed in all plant parts, soil, soil mineral fractions and soil microbial biomass. Due to the small number of grown wheat plants (40) in each ring and the inherent low statistical power, no significant above and belowground growth effect of elevated CO₂ was detected at harvest. But in comparison to ambient conditions, 28% more ¹⁴CO₂ and 12% more total CO₂ was evolved from soil under elevated CO₂ (550 μmol CO₂ mol⁻¹). In the root-free soil 27% more residual ¹⁴C was found in the FACE soil than in the soil from the ambient ring. In soil samples from both treatments about 80% of residual ¹⁴C was found in the clay fraction and 7% in the silt fraction. Very low ¹⁴C contents in the CFE extracts of microbial biomass in the soil from both CO₂ treatments did not allow assessing their influence on this parameter. Since the calculated specific radioactivity of soil born ¹⁴CO₂ gave no indication of an accelerated priming effect in the FACE soil, we conclude that wheat plants grown under elevated CO₂ can contribute to an additional net carbon gain in soils.     

Drying-rewetting events reduce C and N losses from a Norway spruce forest floor

Periods of prolonged summer drought are likely to be expected for this century, with possibly strong effects on carbon (C) and nitrogen (N) mineralization in soils. Drought generally reduces mineralization rates, but the possibility of excess mineralization pulses during rewetting raises the question about the net effect of drying-rewetting events. In this experiment, we measured C and N mineralization in undisturbed soil columns that were either kept under continuously moist conditions (control) or that were subjected to drying-rewetting. We had three treatments (D1-D3) with different drying intensity (increasing from D1 to D3) but uniform rewetting intensity (4 mm d⁻¹). Soil columns were taken from a Norway spruce forest in Bavaria, Germany. The CO₂ fluxes from control and treatment groups were identical before drying. Over the 80 d drought period, total CO₂ emissions from D1, D2, and D3 were only 72, 52 and 43% of that from the control, respectively. Rewetting resulted in a fast increase of CO₂ fluxes to approx. the same level as in the control. Rewetting could not restore soil moisture of the dry soil to the level of the control, presumably because of preferential flow and water repellency of soil organic matter. No significant excess C mineralization during the 40 d rewetting period was observed. Adding up total CO₂ fluxes during drought and rewetting period, the treatments D1, D2, and D3 emitted only 88, 71 and 67% of the CO₂ emitted by the control. Measurements of dissolved organic carbon (DOC) did only show minor differences between control and treatment columns, indicating that no significant accumulation of DOC took place during the drought period. Radiocarbon signature of emitted CO₂ indicated that C mineralization was reduced with decreasing water availability and no new substrate became bioavailable. Net N mineralization over the course of the whole experiment was reduced by drought to 77, 65 or 52% of the control. Net nitrification was virtually zero during drought whereas net ammonification continued at reduced levels. In summary, we found that drying-rewetting generally reduced C and N mineralization in this soil and that the total reduction increased with drought intensity.     

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.     

Characterisation of the dynamics of C-partitioning within Lolium perenne and to the rhizosphere microbial biomass using 14C pulse chase

The dynamics of C partitioning with Lolium perenne and its associated rhizosphere was investigated in plant-soil microcosms using ¹⁴C pulse-chase labelling. The ¹⁴CO₂ pulse was introduced into the shoot chamber and the plants allowed to assimilate the label for a fixed period. The microcosm design facilitated independent monitoring of shoot and root/soil respiration during the chase period. Partitioning between above- and below-ground pools was determined between 30 min and 168 h after the pulse, and the distribution was found to vary with the length of the chase period. Initially (30 min after the pulse), the ¹⁴C was predominantly (99%) in the shoot biomass and declined thereafter. The results indicate that translocation of recent photoassimilate is rapid, with ¹⁴C detected below ground within 30 min of pulse application. The translocation rate of ¹⁴C below ground was maximal (6.2% h^-1) between 30 min and 3 h after the pulse, with greatest incorporation into the microbial biomass detected at 3 h. After 3 h, the microbial biomass ¹⁴C pool accounted for 74% of the total ¹⁴C rhizosphere pool. By 24 h, approximately 30% of ¹⁴C assimilate had been translocated below ground; thereafter ¹⁴C translocation was greatly reduced. Partitioning of recent assimilate changed with increasing CO₂ concentration. The proportion of ¹⁴C translocated below ground almost doubled from 17.76% at the ambient atmospheric CO₂ concentration (450 ppm) to 33.73% at 750 ppm CO₂ concentration. More specifically, these changes occurred in the root biomass and the total rhizosphere pools, with two- and threefold ¹⁴C increases at an elevated CO₂ concentration compared to ambient, respectively. The pulselabelling strategy developed in this study provided sufficient sensitivity to determine perturbations in C dynamics in L. perenne, in particular rhizosphere C pools, in response to an elevated atmospheric CO₂ concentration.     

A simple chamber technique for the in situ labelling of pasture sward with carbon (14C)

Abstract

Information on carbon (C) flows and transformations in the rhizosphere provides a basis for understanding the functioning of the system. However, the sophisticated growth cabinet facilities required for collecting quantitative data, with ¹⁴C labelling, generally limit their application under field conditions. We determined the feasibility of ‘pulse‐labelling’ pasture swards with ¹⁴C [exposing the plants to a single large ¹⁴C‐carbon dioxide (CO₂) pulse] to monitor C transformations under field conditions using a simple chamber modified to form a sealed hemisphere over an area of pasture. The ¹⁴C‐CO₂ was introduced into the hemisphere to ¹⁴C label the plant material. Assimilation of ¹⁴C‐CO₂ was checked by taking samples of the chamber atmosphere. Any leakage of ¹⁴C‐CO₂ from the chamber was also checked by taking air samples from around and outside the chamber during the assimilation period. The chamber was subsequently removed, and the pasture was opened to natural conditions. Cores were taken periodically from the treated area. Herbage, roots and soils were separated and analyzed for ¹⁴C. Incorporation of ¹⁴C‐CO₂ into the pasture sward was rapid and the variability was non‐limiting. Up to 78% of the calculated ¹⁴C‐CO₂ produced in the syringe and injected into the chamber could be accounted for in the plant/soil system four hours after labelling. The fate of the ¹⁴C label was monitored after an allocation period of 4 hour to 35 days in the plant/soil system using well established methods of analysis. This simple chamber technique appears to be useful for studying C transfers through the pasture plant/soil system and for understanding C dynamics in the field.     

Carbon partitioning in ectomycorrhizal Scots pine seedlings

The complete carbon budget and the turnover rate of assimilated carbon of ectomycorrhizal Scots pine seedlings growing on natural humus were determined in microcosm conditions. The main aim was to improve understanding of the partitioning of the assimilated carbohydrates within seedlings associated with multiple ectomycorrhizal fungi, and to discover carbon dynamics of the mycorrhizosphere.Plant photosynthesis and below-ground respiration were measured in order to obtain the actual carbon assimilation and respiration rates at the time of measurements. Soon after the photosynthesis and respiration rate measurements the seedlings were pulse-labeled with ¹⁴CO₂ to follow carbon allocation to different plant, fungal and soil compartments and rhizosphere respiration. Long-term carbon allocation during the entire life span of the seedlings was estimated by measuring plant and mycorrhizal root-tip biomass. The ectomycorrhizal community was analyzed using morphotyping and ITS-sequencing.The ¹⁴C label was detected in rhizosphere respiration after 12 h and it peaked between 36 and 60 h after labeling. More than half of the assimilated carbon was allocated below-ground as biomass or respiration and higher mycorrhizal biomass increased the below-ground carbon turnover. The presence of Suillus variegatus affected the plant carbon balance in several ways. When S. variegatus was present, the below-ground respiration increased and this carbon loss was compensated by higher photosynthetic activity. Other fungal species did not differ between each other in their effects on carbon balance. Our findings indicate that some root-associated mycorrhizal fungal symbionts can significantly alter plant CO₂ exchange, biomass distribution, and the allocation of recently photosynthesized plant-derived carbon.     

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.