Effect of elevated atmospheric CO2 concentration on soil and root respiration in winter wheat by using a respiration partitioning chamber

Soil respiration in a cropland is the sum of heterotrophic (mainly microorganisms) and autotrophic (root) respiration. The contribution of both these types to soil respiration needs to be understood to evaluate the effects of environmental change on soil carbon cycling and sequestration. In this paper, the effects of free-air CO₂ enrichment (FACE) on hetero- and autotrophic respiration in a wheat field were differentiated and evaluated by a novel split-root growth and gas collection system. Elevated atmospheric pCO₂ of approximately 200 μmol mol⁻¹ above the ambient pCO₂ significantly increased soil respiration by 15.1 and 14.8% at high nitrogen (HN) and low nitrogen (LN) application rates, respectively. The effect of elevated atmospheric pCO₂ on root respiration was not consistent across the wheat growth stages. Elevated pCO₂ significantly increased and decreased root respiration at the booting-heading stage (middle stage) and the late-filling stage (late stage), respectively, in HN and LN treatments; however, no significant effect was found at the jointing stage (early stage). Thus, the effect of increased pCO₂ on cumulative root respiration for the entire wheat growing season was not significant. Cumulative root respiration accounted for approximately 25–30% of cumulative soil respiration in the entire wheat growing season. Consequently, cumulative microbial respiration (soil respiration minus root respiration) increased by 22.5 and 21.1% due to elevated pCO₂ in HN and LN, respectively. High nitrogen application significantly increased root respiration at the late stage under both elevated pCO₂ and ambient pCO₂; however, no significant effects were found on cumulative soil respiration, root respiration, and microbial respiration. These findings suggest that heterotrophic respiration, which is influenced by increased substrate supplies from the plant to the soil, is the key process to determine C emission from agro-ecosystems with regard to future scenarios of enriched pCO₂.     

Quantitative estimation of root exudation of maize plants

Summary The rate of root exudation of maize plants was estimated by measuring the rate of denitrification in a hermetically sealed root system. While CO₂ production measured in the rhizosphere results both from root respiration and microbial respiration N₂O production during nitrate respiration is solely related to the amount of root exudates available for bacterial degradation. With 4 week old plants growing in quartz sand or soil root exudation amounted to 7% of the net photosynthates. Calculations revealed that about 25% of the organic matter flowing into the root system was excreted into the rhizosphere.     

Contribution of root respiration to soil respiration in a C3/C4 mixed grassland

The spatial and temporal variations of soil respiration were studied from May 2004 to June 2005 in a C₃/C₄ mixed grassland of Japan. The linear regression relationship between soil respiration and root biomass was used to determine the contribution of root respiration to soil respiration. The highest soil respiration rate of 11-54 Μmol m^-2 s^-1 was found in August 2004 and the lowest soil respiration rate of 4.99 Μmol m^-2 s^-1 was found in April 2005. Within-site variation was smaller than seasonal change in soil respiration. Root biomass varied from 0.71 kg m^-2 in August 2004 to 102 in May 2005. Within-site variation in root biomass was larger than seasonal variation. Root respiration rate was highest in August 2004 (5.7 Μmol m^-2 s^-1) and lowest in October 2004 (1.7 Μmol m^-2 s^-1). Microbial respiration rate was highest in August 2004 (5.8 Μmol m^-2 s^-1) and lowest in April 2005 (2.59 Μmol m^-2 s^-1). We estimated that the contribution of root respiration to soil respiration ranged from 31% in October to 51% in August of 2004, and from 45% to 49% from April to June 2005.     

Positive correlation between VAM-induced changes in root exudation and mycorrhizosphere mycoflora

The influence of vesicular-arbuscular mycorrhizal (VAM) fungi on rhizosphere mycoflora was studied together with the possible mechanism involved in this process. Six combinations of VAM fungi and phosphorus fertilizer treatments were applied to Leucaena leucocephala roots and quantitative and qualitative observations were made periodically of the rhizosphere mycoflora and constituents of root exudates. The results obtained indicate that the presence of specific mycoflora in the rhizosphere of mycrorrhizal roots is mediated through root exudates rather than being an outcome of improved P nutrition.     

Partitioning the components of soil respiration: a research challenge

Little is known about the respiratory components of CO₂ emitted from soils and attaining a reliable quantification of the contribution of root respiration remains one of the major challenges facing ecosystem research. Resolving this would provide major advances in our ability to predict ecosystem responses to climate change. The merits and technical and theoretical difficulties associated with different approaches adopted for partitioning respiration components are discussed here. The way forward is suggested to be the development of non-invasive regression analysis validated by stable isotope approaches to increase the sensitivity of model functions to include components of rhizosphere microbial activity, changing root biomass and the dynamics of a wide range of soil C pools. Section Editor: A. Hodge     

Effects of defoliation and atmospheric CO2 depletion on nitrate acquisition,and exudation of organic compounds by roots of Festuca rubra

The aim of this study was to investigate the physiological basis of increased root exudation from Festuca rubra, in response to defoliation. The hypothesis, that assimilate supply to roots is a key determinant of the response of root exudation to defoliation was tested by imposing CO₂-deplete (< 50 mol mol^–1) atmospheres to F. rubra. This was done as a non-destructive means of preventing supply of new assimilate to roots of intact and defoliated plants. F. rubra was grown in axenic sand systems, with defoliation and CO₂-depletion treatments applied to plants at 14 and 35 days after planting. Root exudation and NO₃ ^– uptake were quantified throughout, and post-treatment uptake and allocation of N were determined from the distribution of ¹⁵N label, supplied as ¹⁵NO₃ ^–. Defoliation of F. rubra resulted in significantly (P <0.01) increased root exudation, CO₂-depletion did not result in increased exudation from plants of either age. When treatments were applied to F. rubra after 14 days, defoliation and CO₂-depletion each reduced NO₃ ^– uptake significantly (P <0.05). However, in older plants, uptake of NO₃ ^– was less sensitive to defoliation and CO₂-depletion. The results indicate that increased root exudation following defoliation is not related directly to reduced assimilate supply to roots. This was evident from the lack of effect of CO₂-depletion on root exudation, and the absence of correlation between root-C efflux and the rate of NO₃ ^– uptake. The physiological basis of increased exudation following defoliation remains uncertain, but may be dependent on physical damage, either directly or as a consequence of systemic responses to wounding.     

Estimation of autotrophic and heterotrophic components of soil respiration by trenching is sensitive to corrections for root decomposition and changes in soil water content

This study aims to assess the effects of corrections for disturbances such as an increased amount of dead roots and an increase in volumetric soil water content on the calculation of soil CO₂ efflux partitioning. Soil CO₂ efflux, soil temperature and superficial soil water content were monitored in two young beech sites (H1 and H2) during a trenching experiment. Trenching induced a significant input of dead root mass that participated in soil CO₂ efflux and reduced the soil dissolved organic carbon content, while it increased superficial soil water content within the trenched plot. Annual soil CO₂ efflux in control plots was 528 g C m⁻² year⁻¹ at H1 and 527 g C m⁻² year⁻¹ at H2. The annual soil CO₂ efflux in trenched plots was 353 g C m⁻² year⁻¹ at H1 and 425 g C m⁻² year⁻¹ at H2. By taking into account annual CO₂ efflux from decaying trenched roots, the autotrophic contribution to total soil CO₂ efflux reached 69% at H1 and 54% at H2. The partitioning calculation was highly sensitive to the initial root mass estimated within the trenched plots. Uncertainties in the remaining root mass, the fraction of root C that is incorporated into soil organic matter during root decomposition, and the root decomposition rate constant had a limited impact on the partitioning calculation. Corrections for differences in superficial soil water content had a significant impact on annual respired CO₂ despite a limited effect on partitioning.     

Separating root and soil microbial contributions to soil respiration: A review of methods and observations

Forest soil respiration is the sum of heterotrophic (microbes, soil fauna) and autotrophic (root) respiration. The contribution of each group needs to be understood to evaluate implications of environmental change on soil carbon cycling and sequestration. Three primary methods have been used to distinguish hetero- versus autotrophic soil respiration including: integration of components contributing to in situ forest soil CO₂ efflux (i.e., litter, roots, soil), comparison of soils with and without root exclusion, and application of stable or radioactive isotope methods. Each approach has advantages and disadvantages, but isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots. Published data from all methods indicate that root/rhizosphere respiration can account for as little as 10 percent to greater than 90 percent of total in situ soil respiration depending on vegetation type and season of the year. Studies which have integrated percent root contribution to total soil respiration throughout an entire year or growing season show mean values of 45.8 and 60.4 percent for forest and nonforest vegetation, respectively. Such average annual values must be extrapolated with caution, however, because the root contribution to total soil respiration is commonly higher during the growing season and lower during the dormant periods of the year.     

Respiration of individual honeybee larvae in relation to age and ambient temperature

The CO₂ production of individual larvae of Apis mellifera carnica, which were incubated within their cells at a natural air humidity of 60–80%, was determined by an open-flow gas analyzer in relation to larval age and ambient temperature. In larvae incubated at 34 °C the amount of CO₂ produced appeared to fall only moderately from 3.89±1.57 µl mg^–1 h^–1 in 0.5-day-old larvae to 2.98±0.57 µl mg^–1 h^–1 in 3.5-day-old larvae. The decline was steeper up to an age of 5.5 days (0.95±1.15 µl mg^–1 h^–1). Our measurements show that the respiration and energy turnover of larvae younger than about 80 h is considerably lower (up to 35%) than expected from extrapolations of data determined in older larvae. The temperature dependency of CO₂ production was determined in 3.5-day-old larvae, which were incubated at temperatures varying from 18 to 38 °C in steps of 4 °C. The larvae generated 0.48±0.03 µl mg^–1 h^–1 CO₂ at 18 °C, and 3.97±0.50 µl mg^–1 h^–1 CO₂ at 38 °C. The temperature-dependent respiration rate was fitted to a logistic curve. We found that the inflection point of this curve (32.5 °C) is below the normal brood nest temperature (33–36 °C). The average Q₁₀ was 3.13, which is higher than in freshly emerged resting honeybees but similar to adult bees. This strong temperature dependency enables the bees to speed up brood development by achieving high temperatures. On the other hand, the results suggest that the strong temperature dependency forces the bees to maintain thermal homeostasis of the brood nest to avoid delayed brood development during periods of low temperature.Abbreviations m body mass - R rate of development or respiration - T_I inflexion point of a logistic (sigmoid) curve - T_L lethal temperature - T_O temperature of optimum (maximum) developmentCommunicated by G. Heldmaier     

Attractiveness of CO2 released by root respiration fades on the background of root exudates

Plants are endangered at their roots by soil-dwelling rhizophagous insects. These below-ground living herbivores may orient to the source of carbon dioxide (CO₂), an ubiquitous volatile released by respiring plant roots. Here, we studied the interaction of CO₂ and other plant root-derived chemical stimuli with regard to the chemical orientation of the polyphagous larvae of Melolontha melolontha L. (Scarabaeidae). A soil arena was developed that enabled both determination of the actual soil CO₂ concentration and the behavioural response of an insect to (a) CO₂ gradients per se, (b) chemical stimuli released from respiring, undamaged roots of plants potted into vermiculite in this arena and (c) combinations of CO₂ gradients and root-derived stimuli. In a root-free arena, larvae of M. melolontha oriented to the source of synthetic CO₂. However, similar CO₂ gradients generated by host plant roots did not attract the larvae. Neither did a synthetic CO₂ gradient combined with aqueous extracts from rhizospheres with undamaged plant roots elicit an attractive effect. Our data suggest that orientation of cockchafer larvae within CO₂ gradients generated by respiring roots is ‘masked’ by an aqueous extract from a rhizosphere with undamaged roots. The results emphasise that effects of behaviour modifying plant-derived compounds need to be investigated against the background of naturally co-occurring chemicals. The significance of our results for orientation of soil living insects is discussed with respect to abiotic conditions in natural soil and the role of soil microorganisms for the attractiveness of plant roots.     

An estimation of CO<Subscript>2</Subscript> fixation capacity in mangrove forest using two methods of CO<Subscript>2</Subscript> gas exchange and growth curve analysis

In many coastal areas of South-East Asia, attempts have been made to revive coastal ecosystem by initiating projects that encourage planting of mangrove trees. Compared to the terrestrial trees, mangrove trees possess a higher carbon fixation capacity. It becomes a very significant option for clean development mechanism (CDM) program. However, a reliable method to estimate CO₂ fixation capacity of mangrove trees has not been established. Acknowledging the above fact, we decided to set up an estimation method for the CDM program, using gas exchange analysis to estimate mangrove productivity, we put into consideration the net CO₂ fixation of reforested Kandelia candel (5-, 10-, and 15-year-old stand). This was estimated by gas exchange analysis and growth curve analysis. In growth curve analysis, we drew a growth curve of a single stand using data of above- and below-ground biomass. In the gas exchange analysis, we calculated CO₂ fixation capacity by (1) measuring respiration rate of each organ of stand and calculating respiratory CO₂ emission from above- to below-ground biomass. (2) Measuring the single-leaf photosynthetic rate in response to light intensity and calculating the photosynthetic CO₂ absorption. (3) We also developed a model for the diurnal changes in temperature, and monthly averages based on one-day estimation of CO₂ absorption and emission, which we corrected by this model in order to estimate the net CO₂ fixation capacity in response to temperature. Comparing the biomass accumulation of the two methods constructed for the same forest, the above-ground biomass accumulation of 10-year-old forest (34.3 ton ha⁻¹ yr⁻¹) estimated by gas exchange analysis was closely compared to those of growth curve analysis (26.6 ton ha⁻¹ yr⁻¹), suggesting that the gas exchange analysis was capable of estimating mangrove productivity. The validity of the estimated CO₂ fixation capacity by the gas exchange analysis and the growth curve analysis was also discussed.     

CO2 transported in xylem sap affects CO2 efflux from Liquidambar styraciflua and Platanus occidentalis stems, and contributes to observed wound respiration phenomena

The [CO₂] in the xylem of tree stems is typically two to three orders of magnitude greater than atmospheric [CO₂]. In this study, xylem [CO₂] was experimentally manipulated in saplings of sycamore (Platanus occidentalis L.) and sweetgum (Liquidambar styraciflua L.) by allowing shoots severed from their root systems to absorb water containing [CO₂] ranging from 0.04% to 14%. The effect of xylem [CO₂] on CO₂ efflux to the atmosphere from uninjured and mechanically injured, i.e., wounded, stems was examined. In both wounded and unwounded stems, and in both species, CO₂ efflux was directly proportional to xylem [CO₂], and increased 5-fold across the range of xylem [CO₂] produced by the [CO₂] treatment. Xylem [CO₂] explained 76–77% of the variation in pre-wound efflux. After wounding, CO₂ efflux increased substantially but remained directly proportional to internal stem [CO₂]. These experiments substantiated our previous finding that stem CO₂ efflux was directly related to internal xylem [CO₂] and expanded our observations to two new species. We conclude that CO₂ transported in the xylem may confound measurements of respiration based on CO₂ efflux to the atmosphere. This study also provided evidence that the rapid increase in CO₂ efflux observed after tissues are excised or injured is likely the result of the rapid diffusion of CO₂ from the xylem, rather than an actual increase in the rate of respiration of wounded tissues.     

A novel method of quantifying root exudation in the presence of soil microflora

A microcosm is described in which root exudation may be estimated in the presence of microorganisms. Ryegrass seedlings are grown in microcosms in which roots were spatially separated from a microbial inoculant by a Millipore membrane. Seedlings grown in the microcosms were labelled with [¹⁴C]-CO₂, and the fate of the label within the plant and rhizosphere was determined. Inoculation of the microcosms with Cladosporium resinae increased net fixation of the [¹⁴C] label compared to plants grown under sterile conditions. Inoculation also increased root exudation. The use of the microcosm was illustrated and its applications discussed.     

Effects of HgCl2 on principal parameters of exudation from maize detached root system

To elucidate the role of aquaporins in the control of the root pressure, we tested the effects of HgCl₂ (aquaporin blocker) at concentrations from 10^?8 to 10^?2 M on the exudation rate (J _w). Experiments were performed with detached roots of 5–7-day-old etiolated maize (Zea mays L.) seedlings. HgCl₂ suppressed exudation by 50–70% at the concentration of 2 × 10^?5 M. At the concentration of 2.5 × 10^?4 M, HgCl₂ reduced J _w during first 20–40 min, but in 2 h, it activated exudation by ten and more times. In this case, the root and osmotic pressures of the exudates increased by 1.5 times. The hydraulic conductance reduced approximately by 30% during first 30 min and increased severalfold in 2 h. The temperature coefficient (Q₁₀) of J _w attained 14 in 2 h. At the concentration of 10^?2 M, HgCl₂-induced acceleration of exudation was replaced by its inhibition essentially immediately. We suggested that a driving force for HgCl₂-induced stimulation of the J _w might be an increase in the osmotic component of the root pressure or the involvement of its nonosmotic (so-called metabolic) component. The results allow a supposition that aquaporins are involved in the control of water transport in the root.     

Genetic control of root exudation

The literature on genetics of root exudation and on genotypic differences in qualitative and quantitative composition of root exudates in crop and native plant species was critically assessed. Differences in exudation have been reported for genotypes that differ in tolerance to nutrient deficiencies, ion toxicities, and pathogen attack. The exudation profile of a limited number of genotypes (frequently only two genotypes with the contrasting response to the environmental stress) have been reported to date. Little is known about the variability in larger samples of the germplasm or about actual genetics behind differential qualitative and quantitative composition of root exudates. Changing the exudation profile of a given genotype may be achieved by manipulating the biosynthetic capacity and by increasing the capacity of the plasma membrane to transport the specific compound out into the rhizosphere. Overexpression of the bacterial citrate synthase gene in the cytoplasm of tobacco plants resulted in exudation of large quantities of citrate into the rhizosphere and partial alleviation of the aluminium (Al) toxicity stress. A similar strategy of transforming plants with citrate synthase gene is being tried as a way of improving plant capacity to extract phosphorus (P) from soils with notoriously low P availability.More research into the genetic basis of qualitative and quantitative differences in root exudation is warranted. Understanding the genetic control of root exudation, followed by manipulation of qualitative and quantitative composition of root exudates, will result in better adaptation of plants to environmental conditions and a greater yield of crops.     

Soybean root respiration assessed from short-term14C-activity changes

During and immediately after labelling of soybeans (Glycine max. L.) in the field by exposure to¹⁴CO₂, its respiratory deposit into the soil atmosphere, and its liberation from the soil were used in conjunction with estimates of below-ground plant biomass to apportion total soil respiration. Root respiration of soybean plants at stage V₆ was estimated at 4 mg CO₂.(g root)^–1.h^–1. Soil biota, during the same time, contributed 35% of total soil respiration.Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 10700. Funded in part by USDA Grant SE 83-CRSR-2-2309.     

Photosynthesis and respiration of Griffithsia monilis (Rhodophyceae): Effect of light,salinity, and oxygen

The giant-celled alga Griffithsia monilis has a low light compensation point and saturates photosynthesis at 60–90 mol photons m^-2s^-1 (oxygen evolution and CO₂ fixation). Under dark and low light intensities ¹⁴C is preferentially incorporated into amino acids (mainly aspartate and alanine). With increasing light a gradual change was observed and, under light saturation, compounds of the anionic fraction (digeneaside and hexosephosphates) were the most strongly labeled compounds, together with the amino acids glycine and serine. To a large extent (30–40% of the total) ¹⁴C was fixed into EtOH-insoluble products, the hydrolysates of which consisted mainly of glucose and mannose. In the steady state the rates of photosynthesis and respiration decreased with increasing salinity. Changes in the rates after hyperosmotic shocks were less severe in cells adapted to high salinities. Photorespiration exists in Griffithsia: Glycine and serine are the major labeled compounds in O₂-saturated media.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Effects of elevated [CO2] on forest growth and carbon storage: a modelling analysis of the consequences of changes in litter quality/quantity and root exudation

Many researchers have proposed that the stimulus of plant growth under elevated [CO₂] observed in short-term experiments will be moderated in the longer term by a reduction in soil nitrogen (N) availability linked to decreased litter quality and/or increased litter production. However, these negative feedbacks may be offset to some extent by a stimulus in N fixation linked to increased root exudation. The aim of this modelling study is to examine how changes in litter quality/quantity and root exudation –- if they occur –- will affect the CO₂ responses of net primary productivity and ecosystem carbon (C) storage on different timescales. We apply a model of C and N cycling in forest ecosystems (G’DAY) to stands of Norway spruce (Picea abies, L. Cast) growing at a N-limited experimental site at Flakaliden, Sweden, and draw the following conclusions: (1) in the absence of changes in litter quality and root exudation, the short-term CO₂ stimulus of litter quantity leads to only a minimal CO₂ stimulus of productivity or C storage in the medium term (≈ 20 years) and long term (≈ 200 years), because of constraints on soil N availability; (2) increasing plant nitrogen use efficiency (via a decrease in the N:C ratio of new litter) makes little impact on these results; (3) a significant CO₂ response in the medium term requires a substantial decrease in the N:C ratio of older litter, when it is approaching stabilisation as soil organic matter, although the long-term CO₂ response remains small; and (4) an increase in N fixation leads to a small effect on productivity in the short term, but a very large effect on both productivity and C storage in the long term. These results suggest that soil N constraints on the long-term CO₂-fertilisation effect can be overcome to a significant extent only by increases in N acquisition, although only modest increases may be required. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ecophysiological characterization of carnivorous plant roots: Oxygen fluxes,respiration, and water exudation

Various ecophysiological investigations on carnivorous plants in wet soils are presented. Radial oxygen loss from roots of Droseraceae to an anoxic medium was relatively low 0.02 – 0.07 mol(O₂) m^{– 2} s^–1 in the apical zone, while values of about one order of magnitude greater were found in both Sarracenia rubra roots and Genlisea violacea traps. Aerobic respiration rates were in the range of 1.6 – 5.6 mol kg^–1 (f.m.) s^–1 for apical root segments of seven carnivorous plant species and 0.4 – 1.1 mol kg^–1 (f.m.) s^–1 for Genlisea traps. The rate of anaerobic fermentation in roots of two Drosera species was only 5 – 14 % of the aerobic respiration. Neither 0.2 mM NaN₃ nor 0.5 mM KCN influenced respiration rate of roots and traps. In all species, the proportion of cyanide-resistant respiration was high and amounted to 65 – 89 % of the total value. Mean rates of water exudation from excised roots of 12 species ranged between 0.4 – 336 mm 3 kg^–1 (f.m.) s^–1 with the highest values being found in the Droseraceae. Exudation from roots was insensitive to respiration inhibitors. No significant difference was found between exudation rates from roots growing in situ in anoxic soil and those kept in an aerated aquatic medium. Carnivorous plant roots appear to be physiologically very active and well adapted to endure permanent soil anoxia.