Effects of shoot excision on in situ soil and root respiration of wheat and soybean under drought stress

Little information is available about the variability of root-derived respiration rate in relation to biotic factors such as photosynthesis and substrate availability in roots. Here we examine the role of decreased carbohydrates availability on root-derived respiration through removal of above ground biomass. Spring wheat (Triticum aestivum L. cv. Longchun 8139) and soybean (Glycina max L. cv. Tianchan 2) were grown in the field under a moveable rain shelter, and subjected to three different water regimes: (1) well-watered control; (2) moderate drought stress, and (3) severe drought stress. Root-derived respiration before and after shoot clipping, and the concentration of total nonstructural carbohydrate, malic and citric acid were measured for spring wheat and soybean. Root-derived CO₂ flux and total nonstructural carbohydrate concentration of clipped wheat decreased by 38% and 31%, respectively. However, for soybean the root- derived CO₂ flux and total nonstructural carbohydrate concentrations were only 58% and 62% of control, respectively, indicating the root respiration rate was controlled by the availability of carbon in the root. A significant positive correlation between total nonstructural carbohydrate concentration of the root and soil water content was observed in unclipped plants. Total nonstructural carbohydrate contributed 93% of the variance in root-derived respiration. Our results clearly show, that in the field, the availability of carbon substrate in roots determines root-derived respiration and plays a key link between soil moisture and root-derived respiration. A period of time is needed for root respiration to return to “steady-state” after shoot removal and this period needed is strongly dependent on species and soil water content.     

Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas

Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO₂ efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO₂ efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the ‘autotrophic’ root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO₂ efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO₂ flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO₂ flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO₂ flux component represents an overflow ‘CO₂ tap’ through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO₂.     

Carbon use in root respiration as affected by elevated atmospheric O2

The use of fossil fuel is predicted to cause an increase of the atmospheric CO₂ concentration, which will affect the global pattern of temperature and precipitation. It is therefore essential to incorporate effects of temperature and water supply on the carbon requirement for root respiration of plants to predict effects of elevated [CO₂] on the carbon budget of natural and managed systems.There is insufficient information to support the contentention that an increase in the concentration of CO₂ in the atmosphere will enhance the CO₂ concentration in the soil to an extent that is likely to affect root respiration. Moreover, there is no convincing evidence for a direct effect of elevated atmospheric [CO₂] on the rate of root respiration per unit root mass or the fraction of carbon required for root respiration. However, there are likely to be indirect effects of elevated [CO₂] on the carbon requirement of plants in natural systems.Firstly, it is very likely that the carbon requirement of root respiration relative to that fixed in photosynthesis will increase when elevated [CO₂] induces a decrease in nutrient status of the plants. Although earlier papers have emphasized that elevated [CO₂] favours investment of biomass in roots relative to that in leaves, these are in fact indirect effects. The increase in root weight ratio is due to the more rapid depletion of nutrients in the root environment as a consequence of enhanced growth. This will decrease the specific rate of root respiration, but increase the carbon requirement as a fraction of the carbon fixed in photosynthesis. It is likely that these effects will be minor in systems where the nutrient supply is very high, e.g. in many managed arable systems, and increase with decreasing soil fertility, i.e. in many natural systems.Secondly, a decrease in rainfall in some parts of the world may cause a shortage in water supply which favours the carbon partitioning to roots. Water stress is likely to reduce rates of root respiration per unit root mass, but enhance the fraction of total assimilates required for root respiration, due to greater allocation of biomass to roots.Increased temperatures are unlikely to affect the specific rate of root respiration in all species. Broadly generalized, the effect of temperature on biomass allocation is that the relative investment of biomass in roots is lowest at a certain optimum temperature and increases at both higher and lower temperatures. The root respiration of some species acclimates to growth temperature, so that the effect of global temperature rise is entirely accounted for by the effect of temperature on biomass allocation. The specific rate of root respiration of other species will increase with global warming. In response to global warming the carbon requirement of roots is likely to decrease in temperate regions, when temperatures are suboptimal for the roots' capacity to acquire water. Here global warming will induce a smaller biomass allocation to the roots. Conversely, the carbon requirements are more likely to increase in mediterranean environments, where temperatures are often supraoptimal and a rise in temperature will induce greater allocation of biomass to the roots.     

Tree-girdling to separate root and heterotrophic respiration in two Eucalyptus stands in Brazil

The release of carbon as CO₂ from belowground processes accounts for about 70% of total ecosystem respiration. Insights about factors controlling soil CO₂ efflux are constrained by the challenge of apportioning sources of CO₂ between autotrophic tree roots (and mycorrhizal fungi) and heterotrophic microorganisms. In some temperate conifer forests, the reduction in soil CO₂ efflux after girdling (phloem removal) has been used to separate these sources. Girdling stops the flow of carbohydrates to the belowground portion of the ecosystem, which should slow respiration by roots and mycorrhizae while heterotrophic respiration should remain constant or be enhanced by the decomposition of newly dead roots. Therefore, the reduction in CO₂ efflux after girdling should be a conservative estimate of the belowground flux of C from trees. We tested this approach in two tropical Eucalyptus plantations. Tree canopies remained intact for more than 3 months after girdling, showing no reduction in light interception. The reduction in soil CO₂ efflux averaged 16–24% for the 3-month period after girdling. The reduction in CO₂ efflux was similar for plots with one half of the trees girdled and those with all of the trees girdled. Girdling did not reduce live fine root biomass for at least 5 months after treatment, indicating that large reserves of carbohydrates in the root systems of Eucalyptus trees maintained the roots and root respiration. Our results suggest that the girdling approach is unlikely to provide useful insights into the contribution of tree roots and heterotrophs to soil CO₂ efflux in this type of forest ecosystem.     

Root xylem CO<Subscript>2</Subscript> flux: an important but unaccounted-for component of root respiration

Key message

In tree roots, a large fraction of root-respired CO ₂ remains within the root system rather than diffusing into the soil. This CO ₂ is transported in xylem sap into the shoot, and because respiration is almost always measured as the flux of CO ₂ into the atmosphere from plant tissues, it represents an unaccounted-for component of tree root metabolism.

Abstract

Root respiration has been considered a large component of forest soil CO₂ efflux, but recent findings indicate that it may be even more important than previous measurements have shown because a substantial fraction of root-respired CO₂ remains within the tree root system and moves internally with the transpiration stream. The high concentration of CO₂ in roots appears to originate mainly within the root. It has been suggested that plants can take up dissolved inorganic carbon (DIC) from soil, but under most conditions uptake from soil is minimal due to the root-to-soil diffusion gradient, which suggests that most of the CO₂ in root xylem is derived from root respiration. Estimates of the internal flux of CO₂ through root xylem are based on combined measurements of sap flow and internal [CO₂]. Results quantifying root xylem CO₂ flux, obtained for a limited number of species, have raised important concerns regarding our understanding of tree respiration. Taken together, the results of these studies call into question the partitioning of ecosystem respiration into its above- and belowground components, and redefine the energetic costs of tree root metabolism and hence estimates of belowground carbon allocation. Expanding our observations of root xylem CO₂ flux to more species and at longer time scales, as well as improving the techniques used to study this process, could be fruitful avenues for future research, with the potential to substantially revise our understanding of root respiration and forest carbon cycles.

     

Influence of rhizodeposition under elevated CO2 on plant nutrition and soil organic matter

Atmospheric CO₂ concentrations can influence ecosystem carbon storage through net primary production (NPP), soil carbon storage, or both. In assessing the potential for carbon storage in terrestrial ecosystems under elevated CO₂, both NPP and processing of soil organic matter (SOM), as well as the multiple links between them, must be examined. Within this context, both the quantity and quality of carbon flux from roots to soil are important, since roots produce specialized compounds that enhance nutrient acquisition (affecting NPP), and since the flux of organic compounds from roots to soil fuels soil microbial activity (affecting processing of SOM).From the perspective of root physiology, a technique is described which uses genetically engineered bacteria to detect the distribution and amount of flux of particular compounds from single roots to non-sterile soils. Other experiments from several labs are noted which explore effects of elevated CO₂ on root acid phosphatase, phosphomonoesterase, and citrate production, all associated with phosphorus nutrition. From a soil perspective, effects of elevated CO₂ on the processing of SOM developed under a C4 grassland but planted with C3 California grassland species were examined under low (unamended) and high (amended with 20 g m^–2 NPK) nutrients; measurements of soil atmosphere 13C combined with soil respiration rates show that during vegetative growth in February, elevated CO₂ decreased respiration of carbon from C4 SOM in high nutrient soils but not in unamended soils.This emphasis on the impacts of carbon loss from roots on both NPP and SOM processing will be essential to understanding terrestrial ecosystem carbon storage under changing atmospheric CO₂ concentrations.Abbreviations SOM soil organic matter - NPP net primary productivity - NEP net ecosystem productivity - PNPP p-nitrophenyl phosphate     

黄土高原半干旱区典型草地生态系统CO2交换对刈割的响应

草地生态系统碳循环在全球气候变化中扮演着重要的角色。刈割是实现草地资源合理利用的主要管理措施,但其对草地生态系统碳通量的影响过程和机理尚不清楚。以黄土高原半干旱区典型草地为研究对象,设计刈割和对照两种处理,于2013—2015年对生态系统碳通量各组分,土壤温度和水分进行了测定。结果表明:生态系统碳通量季节性变化均呈明显的单峰模式,以夏季最高,春秋季节最低,但不同组分峰值出现的时间有所不同。刈割使整个试验期土壤呼吸增加了17%,但对总初级生产力(GEP)、生态系统呼吸(Re)和净初级生产力(NEE)的影响不显著。GEP和NEE在正常降水年(2014)高于干旱年(2015),而且其对刈割的响应与降水年型有关。在正常降水年,刈割后GEP和NEE显著减低,而在干旱年显著增加。这些结果表明,降雨年际变化是造成半干旱草地生态系统碳通量变化的主导因素,而刈割增大了这一生态系统的土壤碳排放。     

Diel patterns of autotrophic and heterotrophic respiration among phenological stages

Improved understanding of the links between aboveground production and allocation of photosynthate to belowground processes and the temporal variation in those links is needed to interpret observations of belowground carbon cycling processes. Here, we show that combining a trenching manipulation with high‐frequency soil respiration measurements in a temperate hardwood forest permitted identification of the temporally variable influence of roots on diel and seasonal patterns of soil respiration. The presence of roots in an untrenched plot caused larger daily amplitude and a 2–3 h delay in peak soil CO₂ efflux relative to a root‐free trenched plot. These effects cannot be explained by differences in soil temperature, and they were significant only when a canopy was present during the growing season. This experiment demonstrated that canopy processes affect soil CO₂ efflux rates and patterns at hourly and seasonal time scales, and it provides evidence that root and microbial processes respond differently to environmental factors.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Estimation of carbon allocation to the roots from soil respiration measurements of oil palm

CO₂ flux from the soil was measured in situ under oil palms in southern Benin. The experimental design took into account the spatial variability of the root density, the organic matter in the soil-palm agrosystem and the effect of factors such as the soil temperature and moisture.Measurements of CO₂ release in situ, and a comparison with the results obtained in the laboratory from the same soil free of roots, provided an estimation of the roots contribution to the total CO₂ flux. The instantaneous values for total release in situ were between 3.2 and 10.0 mol CO₂ m^-2 s^-1. For frond pile zones rich in organic matter, and around oil palm trunks, root respiration accounted for 30% of the efflux when the soil was at field capacity and 80% when the soil was dry with a pF close to 4.2. This proportion remained constant in interrow zones at around 75%, irrespective of soil moisture.Subsequently carbon allocation to the roots was determined. Total CO₂ release over a year was 57 Mg of CO₂ ha^-1 yr^-1 (around 1610 g of C per m² per year), and carbon allocation to the roots was approximately 53 Mg of CO₂ ha^-1 yr^-1 of which approximately 13 Mg CO₂ ha^-1 yr^-1 (25%) was devoted to turn-over and 40 Mg CO₂ ha^-1 yr^-1 (75%) to respiration.     

Root respiration in temperate mountain grasslands differing in land use

In grasslands the proportionally largest emission of CO₂ comes from the soil. This study aimed to assess how root respiration, a major flux component, is affected by land management and changes in land use. Respiration of roots, separated to classes of different diameter, was measured in 11 temperate mountain grasslands, including meadows, pastures and abandoned sites at three geographic locations. Specific root respiration was affected by nitrogen (N) concentration, root class and land use. The relationship between root N concentration and respiration differed between locations. With increasing root diameter there was a decrease in root respiration, N concentration, respiration per unit N and Q₁₀. In grasslands abandoned for several years specific root respiration was lower than in meadows, pastures and a recently abandoned site. This was due to lower root N concentrations and/or lower respiration rates per unit N within each root class. Since root biomass was higher on abandoned grasslands, total ecosystem root respiration did not differ consistently between sites. Ecosystem root respiration showed distinct seasonal changes due to changes in root biomass, which were less pronounced on abandoned grasslands. Fine roots generally made up the largest portion of ecosystem root respiration, their contribution varying between 35% and 96%. On meadows, clipping increased soil and root respiration by increasing soil temperature. When corrected for temperature effects soil respiration was reduced by 20–50%, whilst root respiration was little affected, suggesting that carbohydrate reserves sustained root metabolism for several days and that microbial respiration strongly responded to short‐term changes in assimilate supply.     

Soil CO2 concentration does not affect growth or root respiration in bean or citrus

Contrasting effects of soil CO₂ concentration on root respiration rates during short-term CO₂ exposure, and on plant growth during long-term CO₂ exposure, have been reported. Here we examine the effects of both short- and long-term exposure to soil CO₂ on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. & Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO₂. Respiration rates of citrus roots were unaffected by the CO₂ concentration used during the respiration measurements (200 and 2000 μmol mol⁻¹), regardless of the soil CO₂, concentration during the previous month (600 and 20 000 μmol mol⁻¹). Bean plants were grown with their roots exposed to either a natural CO₂ diffusion gradient, or to an artificially maintained CO₂ concentration of 600 or 20 000 μmol mol⁻¹. These treatments had no effect on shoot and root growth. Growth respiration and maintenance respiration of bean roots were also unaffected by CO₂ pretreatment and the CO₂ concentration used during the respiration measurements (200–2000 μmol mol⁻¹). We conclude that soil CO₂ concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean.     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

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.     

Spatial and temporal patterns of soil respiration over the Japanese Archipelago: a model intercomparison study

We used terrestrial ecosystem models to estimate spatial and temporal variability in and uncertainty of estimated soil carbon dioxide (CO₂) efflux, or soil respiration, over the Japanese Archipelago. We compared five carbon-cycle models to assess inter-model variability: Biome-BGC, CASA, LPJ, SEIB, and VISIT. These models differ in approaches to soil carbon dynamics, root respiration estimation, and relationships between decomposition and environmental factors. We simulated the carbon budget of natural ecosystems over the archipelago for 2001–2006 at 1-day time steps and 2-min (latitude and longitude) spatial resolution. The models were calibrated using measured flux data to accurately represent net ecosystem CO₂ exchange. Each model successfully reproduced seasonal changes and latitudinal gradients in soil respiration. The five-model average of estimated total soil respiration of Japanese ecosystems was 295 Tg C year⁻¹, with individual model estimates ranging from 210 to 396 Tg C year⁻¹ (1 Tg = 10¹² g). The differences between modeled estimates were more evident in summer and in warmer years, implying that they were mainly attributable to differences in modeling the temperature dependence of soil respiration. There was a large discrepancy between models in the estimated contribution of roots to total soil respiration, ranging from 3.9 to 48.4%. Although model calibration reduced the uncertainty of flux estimates, substantial uncertainties still remained in estimates of underground processes from these terrestrial carbon-cycle models.     

Components of forest soil CO2 efflux estimated from Δ14C values of soil organic matter

Aims

The partitioning of the total soil CO₂ efflux into its two main components: respiration from roots (and root-associated organisms) and microbial respiration (by means of soil organic matter (SOM) and litter decomposition), is a major need in soil carbon dynamics studies in order to understand if a soil is a net sink or source of carbon.

Methods

The heterotrophic component of the CO₂ efflux was estimated for 11 forest sites as the ratio between the carbon stocks of different SOM pools and previously published (Δ¹⁴C derived) turnover times. The autotrophic component, including root and root-associated respiration, was calculated by subtracting the heterotrophic component from total soil chamber measured CO₂ efflux.

Results

Results suggested that, on average, 50.4 % of total soil CO₂ efflux was derived from the respiration of the living roots, 42.4 % from decomposition of the litter layers and less than 10 % from decomposition of belowground SOM.

Conclusions

The Δ¹⁴C method proved to be an efficient tool by which to partition soil CO₂ efflux and quantify the contribution of the different components of soil respiration. However the average calculated heterotrophic respiration was statistically lower compared with two previous studies dealing with soil CO₂ efflux partitioning (one performed in the same study area; the other a meta-analysis of soil respiration partitioning). These differences were probably due to the heterogeneity of the SOM fraction and to a sub-optimal choice of the litter sampling period.     

Estimating respiration of roots in soil: Interactions with soil CO2, soil temperature and soil water content

Little information is available on the variability of the dynamics of the actual and observed root respiration rate in relation to abiotic factors. In this study, we describe I) interactions between soil CO₂ concentration, temperature, soil water content and root respiration, and II) the effect of short-term fluctuations of these three environmental factors on the relation between actual and observed root respiration rates. We designed an automated, open, gas-exchange system that allows continuous measurements on 12 chambers with intact roots in soil. By using three distinct chamber designs with each a different path for the air flow, we were able to measure root respiration over a 50-fold range of soil CO₂ concentrations (400 to 25000 ppm) and to separate the effect of irrigation on observed vs. actual root respiration rate. All respiration measurements were made on one-year-old citrus seedlings in sterilized sandy soil with minimal organic material.Root respiration was strongly affected by diurnal fluctuations in temperature (Q₁₀ = 2), which agrees well with the literature. In contrast to earlier findings for Douglas-fir (Qi et al., 1994), root respiration rates of citrus were not affected by soil CO₂ concentrations (400 to 25000 ppm CO₂; pH around 6). Soil CO₂ was strongly affected by soil water content but not by respiration measurements, unless the air flow for root respiration measurements was directed through the soil. The latter method of measuring root respiration reduced soil CO₂ concentration to that of incoming air. Irrigation caused a temporary reduction in CO₂ diffusion, decreasing the observed respiration rates obtained by techniques that depended on diffusion. This apparent drop in respiration rate did not occur if the air flow was directed through the soil. Our dynamic data are used to indicate the optimal method of measuring root respiration in soil, in relation to the objectives and limitations of the experimental conditions.     

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     

Process-based isotope partitioning of winter soil respiration in a subalpine ecosystem reveals importance of rhizospheric respiration

Deep snow in sub-alpine ecosystems may reduce or eliminate soil freezing, thus contributing to the potential for winter soil respiration to account for a significant fraction of annual CO₂ efflux to the atmosphere. Quantification of carbon loss from soils requires separation of respiration produced by roots and rhizosphere organisms from that produced by heterotrophic, decomposer organisms because the former does not result in a net loss of stored carbon. Our objective was to quantify winter soil respiration rates in a sub-alpine forest and meadow, and to partition that flux into its rhizosphere and heterotrophic components. We were particularly interested in comparing early winter soil respiration to late winter/early spring soil respiration of each component because previous work has shown a consistent increase in soil respiration of subalpine systems from early winter to late winter/spring. Field data on the total soil CO₂ flux and its carbon isotope composition were coupled with data from laboratory incubations using a novel process-based stable isotope mixing model implemented in a hierarchical Bayesian framework. We found that soil respiration generally increased from early to later winter and was greatest mid-summer. After correcting for the effect of wind on snowpack δ¹³C–CO₂, the δ¹³C of soil-respired CO₂ varied little over winter, and the contributions of rhizospheric (~35 %) and heterotrophic (~65 %) respiration were relatively constant. The significance of winter respiration from the rhizosphere and apparent coupling of increases in rhizospheric and heterotrophic respiration in late winter are likely to be important for predicting changes in soil carbon in sub-alpine ecosystems.     

The use of C14O2 canopy techniques for measuring carbon transfer through the plant-soil system

Summary Methods for labelling growing plants by exposing them to C¹⁴O₂ under a cellulose acetate-butyrate canopy have been developed for laboratory and field use. The length of labelling ranged from 2 to 33 days and the C¹⁴O₂ content of the atmosphere was automatically controlled. This made it possible to measure carbon assimilation by the plants, transfer of photosynthates beneath ground and respiration of the roots.In the laboratory, root respiration of wheat plants was measured by separating the above and beneath ground plant parts using a RTV rubber partition. Half to two thirds of the assimilated carbon was found above ground, 15 to 25 per cent in the roots and shoot bases below the partition and 17 to 25 per cent was lost by underground respiration. The variability of these proportions was related to the stage of maturity of the plants.On native grassland, the relative above and beneath ground productivity was 50 per cent. The time required for the photosynthates to reach the roots at various depths ranged from 1 to 5 days and the amount of material deposited in the roots changed with time and soil moisture content. The use of tubes inserted at various depths beneath the canopy permitted sampling of soil air for C¹⁴ and CO₂ measurements. The soil C¹⁴O₂ flux indicated that root respiration during 8 days accounted for 24 per cent of the labelled carbon translocated to the roots after a two days labelling period.