Climate Change Affects Carbon Allocation to the Soil in Shrublands

Climate change may affect ecosystem functioning through increased temperatures or changes in precipitation patterns. Temperature and water availability are important drivers for ecosystem processes such as photosynthesis, carbon translocation, and organic matter decomposition. These climate changes may affect the supply of carbon and energy to the soil microbial population and subsequently alter decomposition and mineralization, important ecosystem processes in carbon and nutrient cycling. In this study, carried out within the cross-European research project CLIMOOR, the effect of climate change, resulting from imposed manipulations, on carbon dynamics in shrubland ecosystems was examined. We performed a ¹⁴C-labeling experiment to probe changes in net carbon uptake and allocation to the roots and soil compartments as affected by a higher temperature during the year and a drought period in the growing season. Differences in climate, soil, and plant characteristics resulted in a gradient in the severity of the drought effects on net carbon uptake by plants with the impact being most severe in Spain, followed by Denmark, with the UK showing few negative effects at significance levels of p 0.10. Drought clearly reduced carbon flow from the roots to the soil compartments. The fraction of the ¹⁴C fixed by the plants and allocated into the soluble carbon fraction in the soil and to soil microbial biomass in Denmark and the UK decreased by more than 60%. The effects of warming were not significant, but, as with the drought treatment, a negative effect on carbon allocation to soil microbial biomass was found. The changes in carbon allocation to soil microbial biomass at the northern sites in this study indicate that soil microbial biomass is a sensitive, early indicator of drought- or temperature-initiated changes in these shrubland ecosystems. The reduced supply of substrate to the soil and the response of the soil microbial biomass may help to explain the observed acclimation of CO₂ exchange in other ecosystems.     

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.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Do plants modulate biomass allocation in response to petroleum pollution?

Biomass allocation is an important plant trait that responds plastically to environmental heterogeneities. However, the effects on this trait of pollutants owing to human activities remain largely unknown. In this study, we investigated the response of biomass allocation of Phragmites australis to petroleum pollution by a ¹³CO₂ pulse-labelling technique. Our data show that plant biomass significantly decreased under petroleum pollution, but the root–shoot ratio for both plant biomass and ¹³C increased with increasing petroleum concentration, suggesting that plants could increase biomass allocation to roots in petroleum-polluted soil. Furthermore, assimilated ¹³C was found to be significantly higher in soil, microbial biomass and soil respiration after soils were polluted by petroleum. These results suggested that the carbon released from roots is rapidly turned over by soil microbes under petroleum pollution. This study found that plants can modulate biomass allocation in response to petroleum pollution.     

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}     

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.     

The effect of soil pH on rhizosphere carbon flow of Lolium perenne

Perennial rye-grass plants were grown at 15°C in microcosms containing soil sampled from field plots that had been maintained at constant pH for the last 30 years. Six soil pH values were tested in the experiment, with pH ranging from 4.3–6.5. After 3 weeks growth in the microcosms, plant shoots were exposed to a pulse of ¹⁴C-CO₂. The fate of this label was determined by monitoring ¹⁴C-CO₂ respired by the plant roots/soil and by the shoots. The ¹⁴C remaining in plant roots and shoots was determined when the plants were harvested 7 days after receiving the pulse label. The amount of ¹⁴C (expressed as a percentage of the total ¹⁴C fixed by the plant) lost from the plant roots increased from 12.3 to 30.6% with increasing soil pH from 4.3 to 6. Although a greater percentage of the fixed ¹⁴C was respired by the root/soil as soil pH increased, plant biomass was greater with increasing soil pH. Possible reasons for observed changes in the pattern of ¹⁴C distribution are discussed and, it is suggested that changes in the soil microbial biomass and in plant nitrogen nutrition may, in particular be key factors which led to increased loss of carbon from plant roots with increasing soil pH.     

Differences in rhizosphere carbon-partitioning among plant species of different families

Interspecific variations in carbon (C) allocation and partitioning in the rhizosphere were investigated on 12 Mediterranean species belonging to different family groups (grasses, legumes, non-legume forbs) and having different life cycles. Plants grown individually in artificial soil, in a greenhouse and inoculated with rhizosphere microflora were labelled with ¹⁴CO₂ for 3 h at the vegetative stage. Rhizosphere respiration was measured during 6 days after which labelled C partitioning between shoots, roots, soil, root washing solution and respiration was estimated. The percentage of assimilated ¹⁴C allocated below ground differed significantly between species (41 – 76%) but no significant difference was found between grasses, legumes and non-legume forbs. When expressed as percentage of below-ground ¹⁴C, rhizosphere respiration was significantly smaller for non-legume forbs (42%) than for grasses (46%) and legumes (51%). Consequently more ¹⁴C was incorporated into root biomass in the former. Half-life of ¹⁴CO₂ evolution through respiration ranged from 23 h in legumes to 27 h for non-legume forbs and 37 h for grasses. This suggested differences in microbial activities due to quantities and quality of root exuded C. Rhizosphere respiration was positively correlated with the amount of ¹⁴C in the solution used to wash the roots on one hand, and root N concentration on the other hand. This led to a functional hierarchy between plant family groups of the overall rhizosphere activity. It went from non-legume forbs being the less active (except Crepis sancta)in terms of respiration and exudation, to grasses and then legumes, the most active but also the richest in nitrogen.     

Carbon distribution and nitrogen partitioning in a soil-plant system with barley (Hordeum vulgare L.), ryegrass (Lolium perenne) and rape (Brassica napus L.) grown in a14CO2-atmosphere

To examine the influence of plant-microorganism interactions on soil-N transformations (e.g. net mineralization, net immobilization) a pot experiment was conducted in a¹⁴C-labelled atmosphere by using different (two annuals, one perennial) plants species. It was assumed that variation in below-ground, microorganism-available C would influence N transformations in soil. Plant species were fertilized (low rate) with¹⁵N-labelled nitrogen and grown, during days 13 and 62 after germination, in a growth chamber with a¹⁴C-labelled atmosphere. Nitrification was inhibited by using nitrapyrin (N-Serve). During the chamber period, shoots were harvested, and associated roots and soil were collected on two sampling occasionm, e.g. after 4 and 7 weeks in the growth chamber.The distribution of net (%) assimilated¹⁴C was significantly affected by both plant and time factors, and there was a significant plant × time interaction. There were significant differences between plants in all plant-soil compartments examined as well as in the degree of the plant × time interaction.Differences in the¹⁴C distribution between plants were due to both interspecific and developmental variation. In general, when comparing¹⁵N and¹⁴C quantities between species, many of the differences found between plants can be explained by the differences determined in the weight of shoot or root parts. Despite the fact that amounts of C released were greater in ryegrass than in the other plant-treatments no unequivocal evidence was found to show that the effects of plant-microorganism interactions on soil-N mineralization were greater under ryegrass. Possible mechanisms accounting for the partitioning of N found among plant biomass, soil biomass and soil residues are discussed.     

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.     

Transformations and movement of P in the rhizosphere

Summary Wheat plants labelled with³³P were grown in thin layers of soil amended with³²P-labelled fertiliser. Roots were separated from the soil during plant growth by a porous membrane to overcome difficulties in measuring microbial P in rhizosphere soil. Over the 22 day growth period, net movement of³³P out of healthy growing roots varied from 0.9–4.9% of the total³³P translocated to the root. Over the same period the plants took up 12.0% and the microbial biomass 14.1% of the fertiliser³²P. On drying and rewetting of the soil after the plants were harvested, a large proportion of root P moved into soil fractions while³²P appeared to accumulate in the biomass and stable P forms.     

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).     

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.     

Root systems and root:mass ratio-carbon allocation under current and projected atmospheric conditions in arable crops

Roots of annual crop plants are a major sink for carbon particularly during early, vegetative growth when up to one-half of all assimilated carbon may be translocated belowground. Flowering marks a particularly important change in resource allocation, especially in determinate species, with considerably less allocation to roots and, depending on environmental conditions, there may be insufficient for maintenance. Studies with ¹⁴C indicate the rapid transfer belowground of assimilates with typically 50% translocated in young cereal plants of which 50% is respired; exudation/rhizodeposition is generally <5% of the fixed carbon. Root: total plant mass decreases through the season and is affected by soil and atmospheric conditions. Limited water availability increased the allocation of ¹³C to roots of wheat grown in columns so that at booting 0.38 of shoot C (ignoring shoot respiration) was belowground compared to 0.31 in well-watered plants. Elevated CO₂ (700 mol CO₂ mol^–1 air) increased the proportion of root:total mass by 55% compared with normal concentration, while increasing the air temperature by a mean of 3 °C decreased the proportion from 0.093 in the cool treatment to 0.055 in the warm treatment.     

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     

Inhibition of growth,and effects on nutrient uptake of arctic graminoids by leaf extracts — allelopathy or resource competition between plants and microbes?

Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.     

Estimating seasonal and annual carbon inputs,and root decomposition rates in a temperate pasture following field 14C pulse-labelling

Using a ¹⁴C pulse-labelling technique, we studied the seasonal changes in assimilation and partitioning of photoassimilated C in the plant–root–soil components of a temperate pasture. Pasture and soil samples were taken after 4-h, and 35-day chase periods, to examine these seasonal ¹⁴C fluxes. Total C and ¹⁴C were determined in the shoot, root and soil system. The amounts of C translocated annually to roots and soil were also estimated from the seasonal ¹⁴C distribution and pasture growth. The in situ field decomposition of newly formed roots during different seasons, also using ¹⁴C-labelling, was studied for one year in undisturbed rhizosphere soil. The ¹⁴C-labelled roots were sampled five times and decomposition rates were calculated assuming first-order decomposition.Annual pasture production at the site was 16 020 kg DM ha^–1, and pasture growth varied with season being highest (75–79 kg ha^–1 d^–1) in spring and lowest (18–20 kg ha^–1 d^–1) in winter. The above- and below-ground partitioning of ¹⁴C also varied with the season. The respiratory ¹⁴C–CO₂ losses, calculated as the difference between the total amounts of ¹⁴C recovered in the soil-plant system at 4 h and 35 days, were high (66–70%) during the summer, autumn and winter season, and low (37–39%) during the spring and late-spring season. Pasture plants partitioned more C below-ground during spring compared with summer, autumn and winter seasons. Overall, at this high fertility dairy pasture site, 18 220 kg C/ha was respired, 6490 kg remained above-ground in the shoot, and 6820 kg was translocated to roots and 1320 kg to soil. Root decomposition rate constant (k) differed widely with the season and were the highest for the autumn roots. The half-life was highest (111 days) for autumn roots and lowest (64 days) for spring roots. About one-third of the root label measured in the spring season disappeared in the first 5 weeks after the initial 35 Day of allocation period. The late spring, summer, late summer and winter roots had intermediate half-lives (88–94 days). These results indicate that seasonal changes in root growth and decomposition should be accounted for to give a better quantification of root turnover.     

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     

Loss of exudates from the roots of perennial ryegrass inoculated with a range of micro-organisms

To determine the effect of microbial metabolites on the release of root exudates from perennial ryegrass, seedlings were pulse labelled with [¹⁴C]-CO₂ in the presence of a range of soil micro-organisms. Microbial inoculants were spatially separated from roots by Millipore membranes so that root infection did not occur. Using this technique, only microbial metabolites affected root exudation. The effect of microbial metabolites on carbon assimilation and distribution and root exudation was determined for 15 microbial species. Assimilation of a pulse label varied by over 3.5 fold, dependent on inoculant. Distribution of the label between roots and shoots also varied with inoculant, but the carbon pool that was most sensitive to inoculation was root exudation. In the absence of a microbial inoculant only 1% of assimilated label was exuded. Inoculation of the microcosms always caused an increase in exudation but the percentage exuded varied greatly, within the range of 3–34%.