Carbon distribution in grass (Festuca pratensis L.) during regrowth after cutting—utilization of stored and newly assimilated carbon

Distribution of net assimilated C in meadow fescue (Fectuca pratensi L.) was followed before and after cutting of the shoots. Plants were continuously labelled in a growth chamber with ¹⁴C-labelled CO₂ in the atmosphere from seedling to cutting and with ¹³C-labelled CO₂ in the atmosphere during regrowth after the cutting. Labelled C, both ¹⁴C and ¹³C, was determined at the end of the two growth periods in shoots, crowns, roots, soil and rhizosphere respiration. Distribution of net assimilated C followed almost the same pattern at the end of the two growth periods, i.e. at the end of the ¹⁴C- and the ¹³C-labelling periods. Shoots retained 71–73% of net assimilated C while 9% was detected in the roots and 11–14% was released from the roots, determined as labelled C in soil and as rhizosphere respiration. At the end of the 2nd growth period, after cutting and regrowth, 21% of the residual plant ¹⁴C at cutting (¹⁴C in crowns and roots) was found in the new shoot biomass. A minor part of the residual plant ¹⁴C, 12%, was lost from the plants. The decreases in ¹⁴C in crowns and roots during the regrowth period suggest that ¹⁴C in both crowns and roots was translocated to new shoot tissue. Approximately half of the total root C at the end of the regrowth period after cutting was ¹³C-labelled C and thus represents new root growth. Root death after cutting could not be determined in this experiment, since the decline in root ¹⁴C during the regrowth period may also be assigned to root respiration, root exudation and translocation to the shoots. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

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.     

Carbon fluxes in the rhizosphere of sweet chestnut seedlings (Castanea sativa) grown under two atmospheric CO2 concentrations: 14C partitioning after pulse labelling

Partitioning of ¹⁴C was assessed in sweet chestnut seedlings (Castanea sativa Mill.) grown in ambient and elevated atmospheric [CO₂] environments during two vegetative cycles. The seedlings were exposed to ¹⁴CO₂ atmosphere in both high and low [CO₂] environments for a 6-day pulse period under controlled laboratory conditions. Six days after exposure to ¹⁴CO₂, the plants were harvested, their dry mass and the radioactivity were evaluated. ¹⁴C concentration in plant tissues, root-soil system respiratory outputs and soil residues (rhizodeposition) were measured. Root production and rhizodeposition were increased in plants growing in elevated atmospheric [CO₂]. When measuring total respiration, i.e. CO₂ released from the root/soil system, it is difficult to separate CO₂ originating from roots and that coming from the rhizospheric microflora. For this reason a model accounting for kinetics of exudate mineralization was used to estimate respiration of rhizospheric microflora and roots separately. Root activity (respiration and exudation) was increased at the higher atmospheric CO₂ concentration. The proportion attributed to root respiration accounted for 70 to 90% of the total respiration. Microbial respiration was related to the amount of organic carbon available in the rhizosphere and showed a seasonal variation dependent upon the balance of root exudation and respiration. The increased carbon assimilated by plants grown under elevated atmospheric [CO₂] stayed equally distributed between these increased root activities. ei]H Lambers     

Bi-directional transfer of nitrogen between alfalfa and bromegrass: Short and long term evidence

Transfer of N from legumes to associated non-legumes has been demonstrated under a wide range of conditions. Because legumes are able to derive their N requirements from N₂ fixation, legumes can serve, through the transfer of N, as a source of N for accompanying non-legumes. Studies, therefore, are often limited to the transfer of N from the legume to the non-legume. However, legumes preferentially rely on available soil N as their source of N. To determine whether N can be transferred from a non-legume to a legume, two greenhouse experiments were conducted. In the short-term N-transfer experiment, a portion of the foliage of meadow bromegrass (Bromus riparius Rhem.) or alfalfa (Medicago sativa L.) was immersed in a highly labelled ¹⁵N-solution and following a 64 h incubation, the roots and leaves of the associated alfalfa and bromegrass were analyzed for ¹⁵N. In the long-term N transfer experiment, alfalfa and bromegrass were grown in an ¹⁵N-labelled nutrient solution and transplanted in pots with unlabelled bromegrass and alfalfa plants. Plants were harvested at 50 and 79 d after transplanting and analyzed for ¹⁵N content. Whether alfalfa or bromegrass were the donor plants in the short-term experiment, roots and leaves of all neighbouring alfalfa and bromegrass plants were enriched with ¹⁵N. Similarly, when alfalfa or bromegrass was labelled in the long-term experiment, the roots and shoots of neighbouring alfalfa and bromegrass plants became enriched with ¹⁵N. These two studies conclusively show that within a short period of time, N is transferred from both the N₂-fixing legume to the associated non-legume and also from the non-legume to the N₂-fixing legume. The occurrence of a bi-directional N transfer between N₂-fixing and non-N₂-fixing plants should be taken into consideration when the intensity of N cycling and the directional flow of N in pastures and natural ecosystems are investigated.     

Root and microbial involvement in the kinetics of 14C-partitioning to rhizosphere respiration after a pulse labelling of maize assimilates

In a previous study, we examined the kinetics of radioactivity evolution from rhizosphere respiration after the pulse labelling of maize shoots with ¹⁴CO₂ (Nguyen et al., 1999). The specific activity of rhizosphere respiration demonstrated two peaks of ¹⁴CO₂ production. The first one occurred a few hours after the pulse of ¹⁴CO₂ and was followed by a second peak, which took place during the night following the labelling. In the present work, we demonstrate that the second phase of activity occurred in both sterile and non sterile plant–soil systems. This was inconsistent with the results obtained for wheat by Warembourg and Billès (1979) who observed the second peak solely in the case of non-sterile cultures. These authors suggested that this second phase of ¹⁴CO₂ production was related to microbial mineralisation of labelled complex compounds. Their synthesis and breakdown into smaller molecules delayed their utilisation by micro-organisms. However, in the present work, we also demonstrate that the second phase of activity was closely related to photoperiod. When plants were transferred from a 16 h to 20 h photoperiod, the second mineralisation of labelled rhizosphere compounds occurred sooner after the initiation of the dark period and it was strongly attenuated. Therefore, we suggest that the second phase of activity resulted from the utilisation by roots and by micro-organisms of stored ¹⁴C-compounds, which accumulated during the previous light period.     

Compartmentation and Fluxes of Sugars in Roots of Phaseolus Vulgaris Under Phosphate Deficiency

The influence of phosphate deficiency on the sugar accumulation and sugar partitioning in the root cells of bean (Phaseolus vulgaris L.) was studied. Bean plants were cultured 17 - 19 d on a phosphate-sufficient and phosphate-deficient nutrient medium. Phosphate deficit in the growth medium resulted in increased sugar concentration for about 30 % in the apoplastic and cytoplasmic compartments as well as in the vacuoles of root cells. However, the distribution of sugars between apoplast and cytoplasm compartment and vacuole was not affected by decreased phosphate concentration. About 20 % of sugars were found in the apoplast and cytoplasm, about 80 % in the vacuole. Low phosphate concentration enhanced influx of exogenous ¹⁴C-sucrose into meristematic and elongation zones of root. The ¹⁴C-labelled sugar content in the root tips increased for about 60 % as compared to control plants. Phosphate deficiency increased also ¹⁴C-glucose uptake and content in the root tips. However, the amount of ¹⁴CO₂ liberated during respiration of P-deficient roots (after feeding with uniformly labelled ¹⁴C-glucose) was lower than ¹⁴CO₂ respired by control plants, thus a large part of accumulated sugars seems to be metabolically inactive. This revised version was published online in July 2006 with corrections to the Cover Date.     

Model for rhizodeposition and CO2 efflux from planted soil and its validation by 14C pulse labelling of ryegrass

A model for rhizodeposition and root respiration was developed and parameterised based on ¹⁴C pulse labelling of Lolium perenne. The plants were grown in a two-compartment chamber on a loamy Haplic Luvisol under controlled laboratory conditions. The dynamics of ¹⁴CO₂ efflux from the soil and ¹⁴C content in shoots, roots, micro-organisms, dissolved organic carbon (DOC) and soil were measured during the first 11 days after labelling. Modelled parameters were estimated by fitting on measured ¹⁴C dynamics in the different pools. The model and the measured ¹⁴C dynamics in all pools corresponded well (r ²=0.977). The model describes well ¹⁴CO₂ efflux from the soil and ¹⁴C dynamics in shoots, roots and soil, but predicts unsatisfactorily the ¹⁴C content in micro-organisms and DOC. The model also allows for division of the total ¹⁴CO₂ efflux from the soil in ¹⁴CO₂ derived from root respiration and ¹⁴CO₂ derived from rhizomicrobial respiration by use of exudates and root residues. Root respiration and rhizomicrobial respiration amounted for 7.6% and 6.0% of total assimilated C, respectively, which accounts for 56% and 44% of root-derived ¹⁴CO₂ efflux from the soil planted with 43-day-old Lolium perenne, respectively. The sensitivity analysis has shown that root respiration rate affected the curve of ¹⁴CO₂ efflux from the soil mainly during the first day after labelling. The changes in the exudation rate influenced the ¹⁴CO₂ efflux later than first 24 h after labelling.     

Assimilate partitioning affects 13C fractionation of recently assimilated carbon in maize

Coupling ¹³C natural abundance and ¹⁴C pulse labelling enabled us to investigate the dependence of ¹³C fractionation on assimilate partitioning between shoots, roots, exudates, and CO₂ respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling of maize shoots in a ¹⁴CO₂ atmosphere, ¹⁴C was traced to determine the amounts of recently assimilated C in the four pools and the δ¹³C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered ¹⁴C in NS and DNS treatments) led to a 0.3‰ ¹³C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered ¹⁴C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings support the hypothesis that ¹³C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ¹³C of CO₂ evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced (NS treatment), the respired CO₂ became 0.7‰ ¹³C depleted compared to roots. Increasing amounts of recently assimilated C in the CO₂ from NS via DNS to DW treatments resulted in a 1.6‰ δ¹³C increase of root respired CO₂ from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool led to a δ¹³C increase. In DW and DNS plants there was no ¹³C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ ¹³C enrichment in exudates compared to roots. We conclude that ¹³C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools. Section Editor: H. Lambers     

Below-ground carbon distribution in barley (Hordeum vulgare L.) with and without nitrogen fertilization

The distribution of net assimilated C in barley (Hordeum vulgare L.) grown at two N-levels was determined in a growth chamber. The N-fertilization involved 0 and 3.61 mol N g^-1 dry soil. After growth for seven weeks in an atmosphere with continuously ¹⁴C-labelled CO₂, ¹⁴C was determined in shoots, roots, rhizosphere respiration and soil. At the low N-level, 32% of the net assimilated ¹⁴C was translocated below ground, whereas at the high N-level 27% was translocated below ground. The release of C from roots (root respiration, microbial respiration originating from decomposition of ¹⁴C-labelled root material and ¹⁴C remaining in soil) was greater with no N-supply (19% of net assimilated ¹⁴C) than in the treatment with N-supply (15%). Thus, the effect of N-supply on both translocation of assimilated ¹⁴C below ground and the release of ¹⁴C from growing roots was relatively small.     

Response of root respiration and root exudation to alterations in root C supply and demand in wheat

Rising atmospheric CO₂ concentrations have highlighted the importance of being able to understand and predict C fluxes in plant-soil systems. We investigated the responses of the two fluxes contributing to below-ground efflux of plant root-dependent CO₂, root respiration and rhizomicrobial respiration of root exudates. Wheat (Triticum aestivum L., var. Consort) plants were grown in hydroponics at 20°C, pulse-labelled with ¹⁴CO₂ and subjected to two regimes of temperature and light (12 h photoperiod or darkness at either 15°C or 25°C), to alter plant C supply and demand. Root respiration was increased by temperature with a Q ₁₀ of 1.6. Root exudation was, in itself, unaltered by temperature, however, it was reduced when C supply to the roots was reduced and demand for C for respiration was increased by elevated temperature. The rate of exudation responded much more rapidly to the restriction of C input than did respiration and was approximately four times more sensitive to the decline in C supply than respiration. Although temporal responses of exudation and respiration were treatment dependent, at the end of the experimental period (2 days) the relative proportion of C lost by the two processes was conserved despite differences in the magnitude of total root C loss. Approximately 77% of total C and 67% of ¹⁴C lost from roots was accounted for by root respiration. The ratio of exudate specific activity to CO₂ specific activity converged to a common value for all treatments of 2, suggesting that exudates and respired CO₂were not composed of C of the same age. The results suggest that the contributions of root and rhizomicrobial respiration to root-dependent below-ground respiration are conserved and highlight the dangers in estimating short-term respiration and exudation only from measurements of labelled C. The differences in responses over time and in the age of C lost may ultimately prove useful in improving estimates of root and rhizomicrobial respiration.     

Validation of the design of feeding experiments involving [(14)C]substrates used to monitor metabolic flux in higher plants

The aim of this study was to examine whether flux through the pathways of carbohydrate oxidation is accurately reflected in the pattern of ¹⁴CO₂ release from positionally labelled [¹⁴C]substrates in conventional radiolabel feeding studies. Heterotrophic cell suspension cultures of Arabidopsis thaliana were used for this work. The presence of an alkaline trap to capture metabolically generated ¹⁴CO₂ had no significant effect on the ratio of ¹⁴CO₂ release from specifically labelled [¹⁴C]substrates, or on the metabolism of [U-¹⁴C]glucose by the cells. Although the amount of ¹⁴CO₂ captured in a conventional time-course study was only about half of that released from a sample acidified at an equivalent time point, the ratios of ¹⁴CO₂ released from different positionally labelled [¹⁴C]glucose and [1-¹⁴C]gluconate were the same in untreated and acidified samples. Less than 5% of radioactivity supplied to the growth medium as [¹⁴C]bicarbonate was incorporated into acid-stable compounds, and there was no evidence for appreciable reassimilation of ¹⁴CO₂ generated intracellularly during oxidation of [1-¹⁴C]gluconate by the cells. It is concluded that the ratio of label captured from specifically labelled [¹⁴C]glucose is a valid and convenient measure of the relative rates of oxidation of the different positional carbon atoms within the supplied respiratory substrate. However, it is argued that failure to compensate for the incomplete absorption of ¹⁴CO₂ by an alkaline trap may distort estimates of respiration that rely on an absolute measure of the amount of ¹⁴CO₂ generated by metabolism.     

Photosynthesis controls of CO2 efflux from maize rhizosphere

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

15N2 incorporation by rhizosphere soil influence of rice variety,organic matter and combined nitrogen

Summary Heterotrophic nitrogen fixation by rhizosphere soil samples from 20 rice cultivars grown under uniform field conditions was estimated employing¹⁵N-tracer technique. Rhizosphere soil samples from different rice cultivars showed striking differences with regard to their ability to incorporate¹⁵N₂. Rhizosphere samples from rice straw-amended (3 and 6 tons/ha) soil exhibited more pronounced nitrogen-fixing activity than the samples from unamended soil; while the activity of the rhizosphere samples from soils receiving combined nitrogen (40 and 80 kg N/ha) was relatively low. However, the inhibitory effect of combined nitrogen was not expressed in the presence of rice straw at 6 tons/ha. Results suggest that plant variety, application of combined nitrogen and organic matter influence the rhizosphere nitrogen fixation.     

Distribution and utilization of assimilated carbon in red clover during the first year of vegetation

Summary The pattern of distribution of¹⁴C labelled assimilates and translocation with time was measured in red clover during one reproductive cycle. Measurements were made on whole plants grown outdoors in pots by exposing the aerial parts to¹⁴CO₂ during one photoperiod. Simultaneously, root respiration and N₂ fixation were recorded.At the beginning of the vegetative period, 2/3 of the assimilates remained in the leaves (basal leaves), and 1/3 were directed to the root system. Then the development of branches required as much as 40% of the C and the root allocation decreased. Reproductive structures diverted 17% of the current photosynthates. Nitrogen fixation was optimal during the maximum extension of the basal leaves and decreased during the development of branches. During this period, C allocation to the nodulated roots was high with an estimated amount of 3.2 mg of C per mg of N fixed.With time, translocation occured within the foliage, from basal leaves to the leaves of the branches and to the new basal leaves developed after senescence of the branches. Remobilization to the reproductive structures remained minimal indicating that flower and seed growth was supported by current photosynthesis.     

Does carbon partitioning in ectomycorrhizal pine seedlings under elevated CO2 vary with fungal species?

Enhanced soil respiration in response to elevated atmospheric CO₂ has been demonstrated, and ectomycorrhizal (ECM) fungi are of particular interest since they partition host-derived photoassimilates belowground. Although a strong response of ECM fungi to elevated CO₂ has been shown, little is still known about the functional diversity among species. We studied carbon (C) partitioning in mycorrhizal Scots pine seedlings in response to short-term CO₂ enrichment, using seven ECM species with different ecological strategies. Mycorrhizal associations were synthesised and seedlings grown in large Petri dishes containing peat:vermiculite and nutrient solution for 10–15 weeks, after which half of the microcosms were exposed to elevated CO₂ treatment (710 ppm) for 15 days and the other half were kept in ambient CO₂ treatment. Partitioning of C was quantified by pulse labelling the seedlings with ¹⁴CO₂ and examining the distribution of labelled assimilates in shoot, root and extraradical mycelial compartments by destructive harvest and liquid scintillation counting. Fungal biomass was determined with PLFA analysis. The respiratory loss of ¹⁴CO₂ was on average greater in the elevated CO₂ treatment for most species compared to the ambient CO₂ treatment. More label was retrieved in the shoots in the ambient CO₂ treatment compared to elevated CO₂ (significant for P. involutus and P. fallax). Greater amounts of label were found in the extraradical mycelial compartment in all species (except P. involutus) in elevated CO₂ compared to ambient CO₂ (significant for L. bicolor, P. byssinum, P. fallax and R. roseolus). Fungal biomass production increased significantly with elevated CO₂ for two species (H. velutipes and A. muscaria); three species (P. fallax, P. involutus and R. roseolus) showed a similar but non-significant trend, whereas L. bicolor and P. byssinum produced less biomass in elevated CO₂ compared to ambient CO₂. When ¹⁴C in the mycelial compartment and respiration was expressed per unit fungal PLFA the difference between CO₂ treatments disappeared. We demonstrated that different ECM fungal isolates respond differently in C partitioning in response to CO₂ enrichment. These results suggest that under certain growth conditions, when nutrients are not limiting, ECM fungi respond rapidly to increasing C-availability through changed biomass production and respiration.     

How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth?

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of ¹⁴C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of ¹⁴C to roots. In parallel, N fertilisation increased the root soluble ¹⁴C and the ¹⁴C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).     

Measurement of rhizosphere respiration and organic matter decomposition using natural 13C

Due to the limitations in methodology it has been a difficult task to measure rhizosphere respiration and original soil carbon decomposition under the influence of living roots. ¹⁴C-labeling has been widely used for this purpose in spite of numerous problems associated with the labeling method. In this paper, a natural ¹³C method was used to measure rhizosphere respiration and original soil carbon decomposition in a short-term growth chamber experiment. The main objective of the experiment was to validate a key assumption of this method: the ¹³C value of the roots represents the ¹³C value of the rhizosphere respired CO₂. Results from plants grown in inoculated carbon-free medium indicated that this assumption was valid. This natural ¹³C method was demonstrated to be advantageous for studying rhizosphere respiration and the effects of living roots on original soil carbon decomposition.     

Carbon translocation to the rhizosphere of maize and wheat and influence on the turnover of native soil organic matter at different soil nitrogen levels

Wheat and maize were grown in a growth chamber with the atmospheric CO₂ continuously labelled with ¹⁴C to study the translocation of assimilated carbon to the rhizosphere. Two different N levels in soil were applied. In maize 26–34% of the net assimilated ¹⁴C was translocated below ground, while in wheat higher values (40–58%) were found. However, due to the much higher shoot production in maize the total amount of carbon translocated below ground was similar to that of wheat. At high N relatively more of the C that was translocated to the root, was released into the soil due to increased root respiration and/or root exudation and subsequent microbial utilization and respiration. The evolution rate of unlabelled CO₂ from the native soil organic matter decreased after about 25 days when wheat was grown at high N as compared to low N. This negative effect of high N in soil was not observed with maize.     

Contribution of Lolium perenne rhizodeposition to carbon turnover of pasture soil

Carbon rhizodeposition and root respiration during eight development stages of Lolium perenne were studied on a loamy Gleyic Cambisol by ¹⁴CO₂ pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total ¹⁴CO₂ efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first 8 days after ¹⁴C pulse labelling decreased during plant development from 14 to 6.5% of the total ¹⁴C input. Root respiration accounted for was between 1.5 and 6.5% while microbial respiration of easily available rhizodeposits and dead root remains were between 2 and 8% of the ¹⁴C input. Both respiration processes were found to decline during plant development, but only the decrease in root respiration was significant. The average contribution of root respiration to total ¹⁴CO₂ efflux from the soil was approximately 41%. Close correlation was found between cumulative ¹⁴CO₂ efflux from the soil and the time when maximum ¹⁴CO₂ efflux occurred (r=0.97). The average total of CO₂ Defflux from the soil with Lolium perenne was approximately 21 μg C-CO₂ d⁻¹ g⁻¹. It increased slightly during plant development. The contribution of plant roots to total CO₂ efflux from the soil, calculated as the remainder from respiration of bare soil, was about 51%. The total ¹⁴C content after 8 days in the soil with roots ranged from 8.2 to 27.7% of assimilated carbon. This corresponds to an underground carbon transfer by Lolium perenne of 6–10 g C m⁻² at the beginning of the growth period and 50–65 g C m⁻² towards the end of the growth period. The conventional root washing procedure was found to be inadequate for the determination of total carbon input in the soil because 90% of the young fine roots can be lost. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon balance and allocation of assimilated CO2 in Scots pine, Norway spruce, and Silver birch seedlings determined with gas exchange measurements and 14C pulse labelling

Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO₂ efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with ¹⁴C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P _max) was 2.21 μg CO₂ s⁻¹ g⁻¹ in Scots pine, 1.22 μg CO₂ s⁻¹ g⁻¹ in Norway spruce and 3.01 μg CO₂ s⁻¹ g⁻¹ in Silver birch seedlings. According to the pulse labelling experiments, 43–75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9–26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13–21% of the total assimilated CO₂. The ¹⁴CO₂ pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.