Root Compartment Research Articles

Effects of the mycorrhizal fungus Glomus intraradices on uranium uptake and accumulation by Medicago truncatula L. from uranium-contaminated soil

Phytostabilization strategies may be suitable to reduce the dispersion of uranium (U) and the overall environmental risks of U-contaminated soils. The role of Glomus intraradices, an arbuscular mycorrhizal (AM) fungus, in such phytostabilization of U was investigated with a compartmented plant cultivation system facilitating the specific measurement of U uptake by roots, AM roots and extraradical hyphae of AM fungi and the measurement of U partitioning between root and shoot. A soil-filled plastic pot constituted the main root compartment (CA) which contained a plastic vial filled with U-contaminated soil amended with 0, 50 or 200 mg KH2PO4−P kg–1soil (CB). The vial was sealed by coarse or fine nylon mesh, permitting the penetration of both roots and hyphae or of just hyphae. Medicago truncatula plants grown in CA were inoculated with G. intraradices or remained uninoculated. Dry weight of shoots and roots in CA was significantly increased by G. intraradices, but was unaffected by mesh size or by P application in CB. The P amendments decreased root colonization in CB, and increased P content and dry weight of those roots. Glomus intraradices increased root U concentration and content in CA, but decreased shoot U concentrations. Root U concentrations and contents were significantly higher when only hyphae could access U inside CB than when roots could also directly access this U pool. The proportion of plant U content partitioned to shoots was decreased by root exclusion from CB and by mycorrhizas (M) in the order: no M, roots in CB > no M, no roots in CB > M, roots in CB > M, no roots in CB. Such mycorrhiza-induced retention of U in plant roots may contribute to the phytostabilization of U contaminated environments.

Silicon nutrition of tomato and bitter gourd with special emphasis on silicon distribution in root fractions

AbstractTwo plant species, tomato (Lycopersicon esculentum Mill.) and bitter gourd (Momordica charantia), were used for in‐depth studies on the dynamics of silicon (Si) uptake and translocation to the shoots and compartmentation of Si in the roots. The experiments were conducted under controlled environmental conditions in nutrient solutions, which were partly amended with 1 mM Si in the form of silicic acid. At harvest, xylem exudates were collected, and Si concentrations and biomass of roots and shoots were determined. Mass flow of Si was calculated based on the Si concentration of the nutrient solution and transpiration determined in a parallel experiment. Plant roots were subjected to a fractionated Si analysis, allowing attributing Si to different root compartments. Silicon concentrations in the roots compared to the shoots were higher in tomato but lower in bitter gourd. A more ready translocation from the roots to the shoots in bitter gourd was in agreement with Si concentrations in the xylem exudates which were higher than in the external solution. In tomato, the xylem‐sap Si concentration was lower than in the nutrient solution. Calculated Si mass flow to the root exceeded Si uptake in tomato, which was consistent with the measured accumulation of Si in the root water‐free space (WFS). In contrast, Si concentration in the root WFS was lower than in the nutrient solution in bitter gourd, reflecting the calculated Si depletion at the root surface based on the comparison of Si mass flow and Si uptake. Within the roots, more than 80% of the total Si was located in the cell wall and less than 10% in the cytoplasmic fractions in tomato. In bitter gourd, between 60% and 70% of the total root Si was attributed to the cell‐wall fraction whereas the proportion of the cytoplasmic fraction reached more than 30%. Our results clearly confirm that tomato belongs to the Si excluders and bitter gourd to the Si‐accumulator plant species for which high Si concentrations in the cytoplasmic root fraction appear to be characteristic.

ABA, Hydraulics, and Gas Exchange of Split-rooted Apple Trees

Approach-grafted 1-year-old `Gala'/M7 apple trees were grown with both tops for the remainder of the 2003 season in a greenhouse. Trees were supplied with >100% (control, PRD100) or 50% (PRD50, DI50) of daily ETc either applied to one root compartment only (PRD100, PRD50) or divided evenly across both root compartments (control and DI50). ETc was estimated from gravimetric measurements, and irrigation was switched between wet and dry root compartments several times throughout the experiment. Soil moisture was measured both gravimetrically (tripod) and volumetrically (time-domain reflectometry). Predawn leaf water potential (υpd) and single leaf gas exchange (photosynthesis, stomatal conductance, and transpiration) were recorded daily, and sap flow in stems and roots was monitored continuously using the heat-pulse technique. Leaves were collected for abscisic acid (ABA) determination following gas exchange measurements. Regardless of irrigation placement (i.e., PRD or DI), both 50% ETc treatments experienced similar declines in υpd and single leaf gas exchange relative to control levels. In addition, ABA concentrations were similar for PRD50 and DI50, and were significantly higher than the control and PRD100 treatments. PRD100 trees had similar υpd as control trees; however, gas exchange was reduced >25% compared to the control. Bulk leaf ABA concentration did not differ significantly from control levels and does not by itself explain the down regulation of stomata with PRD100.

(457) An Approach-grafted, Split-rooted Apple System to Evaluate the Effects of Partial Rootzone Drying and Deficit Irrigation on Tree Water Relations

One-year-old `Gala'/M7 apple trees were potted into 30-L containers and approach-grafted about 45 cm above the graft union in late Spring 2003. Trees were grown with both tops for the remainder of the 2003 season in a greenhouse. In Apr. 2004, one of the tops was removed. Trees were fully watered by an overhead irrigation system until July 2004, when trees were subjected to one of four irrigation regimes: control received >100% of ETc applied evenly to the two pots; PRD100 received >100% ETc applied to one pot only; and two regimes received 50% ETc applied to either one (PRD50) or both pots (DI50). Both gravimetric (tripod) and volumetric (time-domain reflectometry) soil moisture measurements were taken daily prior to and after irrigations. In addition, heavy isotope H2O (18O) was applied to one of the two root compartments and analyzed in the leaves to further determine the validity of the model. Sap flow was monitored in six split-rooted trees using miniaturized heat-pulse probes inserted into the stem above the graft union and into each of the two root systems below the graft union. Under fully irrigated conditions, root sap flow was proportional to root trunk cross-sectional area, and was not a function of root system origin (i.e., roots of mother plant with original top remaining or roots of daughter plant with original top detached). Water uptake from a previously dried root zone was rapid when the irrigated side was switched, but much more gradual when the other side was maintained wet. Interactions between soil moisture and sap flow in relation to factors governing canopy demand will be presented.

Effects of Essential Fatty Acid Deficiency on Periodontal Tissue Adaptation to Spontaneous Tooth Migration

Essential fatty acids (EFAs) play a significant role in bone metabolism. Herein we studied the adaptation of alveolar bone to physiologic tooth drift in young rats deprived of essential fatty acids from birth. Reductions in femur size and trabecular bone volume reflected body growth impairment. Along the alveolar wall, osteoclastic resorption and bone formation were depressed, disrupting the adaptive deformation of the tooth socket to ongoing migration. As a result, the periodontal ligament narrowed considerably, and further adaptation was achieved through root resorption. Essential fatty acid deficiency (EFAD), did not affect precursor recruitment or differentiation in the periodontal ligament (PDL), but caused redistribution of nonspecific-esterase (NSE)-positive osteoclast precursors and tartrate-resistant acid phosphatase (TRAP)-positive pre-osteoclasts between the bone compartment (which was depleted) and the root compartment (which was enriched). EFAD had also a marked effect on the PDL vasculature; the number of vessels was reduced, whereas their size was markedly increased. As a whole, our results show that EFAD disturbs alveolar bone adaptation to drift, but that a reaction (detrimental to root integrity) prevents root collision with the bone surface, thereby preserving the PDL as a source of precursor cells for bone and cementum homeostasis. Moreover, our results confirm that although alveolar bone resorption is arachidonic acid-dependent, the factors activating root resorption are different.

Facilitation of pentachlorophenol degradation in the rhizosphere of ryegrass ( Lolium perenne L.)

The phytoremediation of xenobiotics depends upon plant-microbe interactions in the rhizosphere, but the extent and intensity of these effects are currently unknown. To investigate rhizosphere effects on the biodegradation of xenobiotics, a glasshouse experiment was conducted using a specially designed rhizobox where ryegrass seedlings were grown for 53 days in a soil spiked with pentachlorophenol (PCP) at concentrations of 8.7±0.5 and 18±0.5 mg kg −1 soil. The soil in the rhizobox was divided into six separate compartments at various distances from the root surface. Changes in PCP concentrations with increasing distance from the root compartment of the rhizobox were then assessed. The largest and most rapid loss of PCP in planted soil was at 3 mm from the root zone where total PCP decreased to 0.20 and 0.65 mg kg −1, respectively with the two PCP treatments. The degradation gradient followed the order: near-rhizosphere>root compartment>far-rhizosphere soil zones for both concentrations where ryegrass was grown. In contrast, there was no difference in PCP concentration with distance in the unplanted soil. The increases in both soil microbial biomass carbon and the activities of soil urease and phosphatase were accompanied by the enhanced degradation of PCP, which was higher in the near-rhizosphere than far-rhizosphere soil. The results suggest that the effect of root proximity is important in the degradation of xenobiotics such as PCP in soil.

Growth, gas exchange, and nutrient status in pepper ( Capsicum annuum L.) grown in recirculating nutrient solution as affected by salinity imposed to half of the root system

Pepper plants grown in recirculating nutrient solution were exposed to NaCl-salinity (60 mM NaCl, 8 dS m −1) imposed either to the entire or to half of the root system and compared to plants supplied with a standard nutrient solution (1.9 dS m −1). The saline solution was obtained by adding NaCl to the standard nutrient solution. In the split-root treatment, the root compartment not exposed to salinity was supplied with raw water (0.38 dS m −1). Both the stem and the root dry weights were markedly restricted by salinity, irrespective of salinizing half or the entire root system. In the split-root treatment, the dry weight of the root compartment receiving raw water did not differ significantly from that exposed to salinity. The net photosynthesis and the leaf chlorophyll content were restricted by both salinity treatments, but the decrease was more marked when the entire root system was exposed to salinity. In contrast, the stomatal conductance and the transpiration rate were equally reduced, regardless of salinizing the entire or part of the root system. The leaf Na and Cl concentrations were raised by the NaCl-salinity, but only in one sampling date the increase was significantly higher when the entire root zone was exposed to salinity, as compared with salinization of half of the root system. Salinity reduced significantly the leaf K, Ca, and Mg uptake but not to levels that could cause nutrient deficiencies. These results indicate that pepper is susceptible to high salinity, predominantly due to reduced stomatal conductance. However, after long-term exposure to salinity the growth may be suppressed due also to inhibition of photosynthesis at chloroplast level. The adverse effects of high NaCl-salinity are hardly mitigated when only a part of the root system is salinized, which indicates that the response is governed by root exposure to high NaCl concentrations and not by inefficiency of the roots to take up water.

From single fine roots to a black alder forest ecosystem: How system behaviour emerges from single component activities

Based on empirical findings in a natural black alder ecosystem in Northern Germany we developed an individual based model that integrates components of a black alder ecosystem interacting on different levels of organisation. The factors determining seasonal fine root biomass development of forest ecosystems are not yet fully understood. We used an object oriented model approach to investigate this complex matter for black alder trees. Processes like growth, storage, respiration, transport, nutrient mineralisation and uptake as well as interactions among these factors are described on the level of functionally differentiated plant organs (fine roots, coarse roots, stem, branches, leaves) and soil units. The object structure of the model is determined by spatial relations between plant modules as well as between plant modules and their local environment modules. As results of model application we found that (i) on the organ level, spatio-temporal plasticity of (root) growth allocation is related to spatio-temporal variation of resource availability, (ii) on the plant level, balanced root:shoot growth appears in response to variation of available resources light and nutrients, (iii) on the population level, tree stand development (population structure, self-thinning) resulted from coexistence and competition between plant individuals. For the understanding of the root compartment it seems relevant that the model implementation of local scale fine root dynamics is consistent with a self-organised large scale spatial heterogeneity of fine root activity pattern. On the other hand, fine-root dynamics cannot be explained as a result of autonomous dynamics. A reference to above-ground processes is a necessary condition and the overall plant seems to act as an integrator providing boundary conditions for local activity pattern. At the same time fine-root characteristics are of some importance for properties on hierarchically higher levels, e.g. co-existence in a tree population or element cycling in the ecosystem. As a conclusion, modelling of the spatio-temporal dynamics of tree root systems appears as a paradigmatic example of scale and organisation level integrating processes.

Experimental study of solute transport and extraction by a single root in soil

The fate of 14C-2,4,6-trinitrotoluene ([U-14C]TNT) in soil/plant systems was studied using onion (Allium cepa L.) plants with only a single root. It was found that the single roots grew exponentially and that the rate of water uptake of the onion plants increased exponentially, as well. The concentration of [U-14C] in the roots at first increased and then appeared to reach a steady state, while the [U-14C] concentration in the leaves was found to increase linearly with time. The [U-14C] concentration in the rhizosphere increased gradually, while in the bulk soil it decreased slowly. The accumulation of [U-14C] in the rhizosphere is likely to difference between movement into the rhizosphere (through advective mass flow of soil water by root uptake) and its uptake into the roots. The distribution of 14C in the soil/plant system was found to be 60–85% in the soil solid phase, 7–11% in the soil liquid phase, <1% in the soil air phase, <1% in the root compartment, and <0.01% in the leaf compartment. The maximum RCF (root concentration factor) value for TNT and its derivates was found to be about 20, and the maximum TSCF (transpiration stream concentration factor) was 0.18. These values can be changed by a variety of factors in soil-plant systems

Uranium distribution and cycling in Scots pine ( Pinus sylvestris L.) growing on a revegetated U-mining heap

We determined the uranium distribution in soil and its allocation in compartments of 35-year-old Scots pine developed on a revegetated U-mining heap. The processes controlling the dynamics of U recycling were identified and further quantified in terms of annual fluxes. As pine developed, an acid humus layer emerged leading to weathering of the alkaline mining debris but this had little effect on U mobility in the soil profile. Increased U mobility mainly involved a translocation of U to metal-humus chelates in surface layers. The root compartment accounted for 99.3% of the U budget in tree, thus serving as an effective barrier which restricts U uptake. The current root uptake and transfer of U to upper parts of the tree amounted to about 3 g ha −1 y −1, i.e. less than 0.03% of the current NH 4-exchangeable U pool in the soil (0–30 cm). Allocation and translocation pattern made it clear that a dominant fraction of the translocated U moves passively with the ascent xylem sap, most likely as a soluble complex, and steadily accumulates in the needles. Consequently, 97% of the U annual uptake is returned to the soil through litterfall. At the studied site, the risk of U dissemination due to biomass turnover or trunk harvest was low when considered in relation to the current “exemption level” for U.

Closing the Carbon Budget of a Scots Pine forest in the Netherlands

The aim of this study was to close the carbon budget and reduce uncertainty in annual C balances for Scots pine (Pinus sylvestris) forests in The Netherlands. This was done by comparing estimates of the Net Ecosystem Exchange (NEE) as assessed by two different methods. The inventory based carbon budgeting method estimated the average NEE for 1997 – 2001 at 202 g C m−2 yr−1 (a sink) with a confidence interval of 138 – 271 g C m−2 yr−1. The estimate obtained by the eddy covariance method was 295 g C m−2 yr−1 on average for the same period, with a confidence interval of 224 – 366 g C m−2 yr−1. Uncertainties in the eddy covariance method were mostly related to gap filling of the data. Main uncertainties in the inventory-based method are related to the soil and the root compartment. The difference in NEE as obtained by two independent methods indicates that it is not straightforward to design a sound National System for monitoring and reporting of the total land area and for accounting of changes in forest area under the Kyoto Protocol, and that more effort is required in this field.

Dendroremediation of trinitrotoluene (TNT). Part 2: fate of radio-labelled TNT in trees.

Problems of long-term existence of the environmental contaminant 2,4,6-trinitrotoluene (TNT) and necessities for the use of trees ('dendroremediation') in sustainable phytoremediation strategies for TNT are described in the first part of this paper. Aims of the second part are estimation of [14C]-TNT uptake, localisation of TNT-derived radioactivity in mature tree tissues, and the determination of the degree of TNT-degradation during dendroremediation processes. Four-year-old trees of hybrid willow (Salix spec., clone EW-20) and of Norway spruce (Picea abies) were cultivated in sand or ammunition plant soil (AP-soil) in wick supplied growth vessels. Trees were exposed to a single pulse application with water solved [U-14C]-TNT reaching a calculated initial concentration of 5.2 mg TNT per kg dry soil. Two months after application overall radioactivity and extractability of 14C were determined in sand/soil, roots, stem-wood, stem-bark, branches, leaves, needles, and Picea May sprouts. Root extracts were analysed by radio TLC. 60 days after [14C]-TNT application, recovered 14C is accumulated in roots (70% for sand variants, 34% for AP-soil variant). 15-28% of 14C remained in sand and 61% in AP-soil. 3.3 to 14.4% of 14C were located in aboveground tree portions. Above-ground distribution of 14C differed considerably between the angiosperm Salix and the gymnosperm Picea. In Salix, nearly half of above-ground-14C was detected in bark-free wood, whereas in Picea older needles contained most of the above-ground-14C (54-69%). TNT was readily transformed in tree tissue. Approximately 80% of 14C was non-extractably bound in roots, stems, wood, and leaves or needles. Only quantitatively less important stem-bark of Salix and Picea and May shoots of Picea showed higher extraction yields (up to 56%). Pulse application of [14C]-TNT provided evidence for the first time that after TNT-exposure, in tree root extracts, no TNT and none of the known metabolites, mono-amino-dinitrotoluenes (ADNT), diaminonitrotoluenes (DANT), trinitrobenzene (TNB) and no dinitrotoluenes (DNTs) were present. Extractable portions of 14C were small and contained at least three unknown metabolites (or groups) for Salix. In Picea, four extractable metabolites (or groups) were detected, where only one metabolite (or group) seemed to be identical for Salix and Picea. All unknown extractables were of a very polar nature. Results of complete TNT-transformation in trees explain some of our previous findings with 'cold analytics', where no TNT and no ADNT-metabolites could be found in tissues of TNT-exposed Salix and Populus clones. It is concluded that 'cold' tissue analysis of tree organs is not suited for quantitative success control of phytoremediation in situ. Both short rotation Salicaceae trees and conifer forests possess a dendroremediation potential for TNT polluted soils. The degradation capacity and the large biomass of adult forest trees with their woody compartments of roots and stems may be utilized for detoxification of soil xenobiotics.

Consequences of sodium exclusion for the osmotic potential in the rhizosphere – Comparison of two maize cultivars differing in Na+ uptake

AbstractAccording to the biphasic model of growth response to salinity, growth is first reduced by a decrease in the soil osmotic potential (Ψo), i.e., growth reduction is an effect of salt outside rather than inside the plant, and genotypes differing in salt resistance respond identically in this first phase. However, if genotypes differ in Na+ uptake as it has been described for the two maize cultivars Pioneer 3906 and Across 8023, this should result in differences in Na+ concentrations in the rhizosphere soil solution and thus in the concentration of salt outside the plant. It was the aim of the present investigation to test this hypothesis and to investigate the effect of such potential differences in soil Ψo caused by Na+ exclusion on plant water relations. Sodium exclusion at the root surface of intact plants growing in soil was investigated by sampling soil solution from the rhizosphere of two maize cultivars (Across 8023, Pioneer 3906). Plants were grown in a model system, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 μm mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). Across 8023 accumulated higher amounts of sodium in the shoot compared to the excluder (Pioneer 3906). The lower Na+ uptake in the excluder was partly compensated by higher K+ uptake. Pioneer 3906 not only excluded sodium from the shoot but also restricted sodium uptake more efficiently from roots relative to Across 8023. This was reflected by higher Na+ concentrations in the rhizosphere soil solution of the excluder 34 days after planting (DAP). The difference in Na+ concentration in rhizosphere soil solution between cultivars was neither due to differences in transpiration and thus in mass flow, nor due to differences in actual soil water content. As the lower Na+ uptake of the excluder (Pioneer 3906) was only partly compensated by increased uptake of K+, soil Ψo in the rhizosphere of the excluder was more negative compared to Across 8023. However, no significant negative effect of decreased soil Ψo on plant water relations (transpiration rate, leaf Ψo, leaf water potential, leaf area) could be detected. This may be explained by the fact that significant differences in soil Ψo between the two cultivars occurred only towards the end of the experiment (27 DAP, 34 DAP).

Turnover of Nitrogen‐15‐Labeled Fertilizer in Old Grassland

In this paper we follow the fate of single applications of 15N‐labeled fertilizer to old grassland, over a period of nearly 20 yr. In 1980 and 1981, 15N‐labeled N was applied to two of the treatments on the Park Grass Continuous Hay Experiment at Rothamsted, started in 1856. The labeled N was applied at the same rate (nominally 96 kg ha−1 yr−1) and in the same chemical form (NH4 or NO3) as the unlabeled N normally applied as fertilizer to the selected treatments. After 19 yr, 69.6% of the N applied in 1980 as 15NH4 had been harvested in successive cuts of herbage, with a further 16.5% remaining in the soil. For 15NO3, 64.3% had been harvested and 13.8% remained in the soil. The 15N data were then used to calculate annual inputs of nonfertilizer N, annual losses of N and N turnover times in old grassland, assuming that the selected treatments were under steady‐state conditions. The annual input of N from nonfertilizer sources (rain, dry deposition, N fixation by leguminous components of the herbage, etc.) was large: 39 kg N ha−1 yr−1 for the NH4 treatment and 31 kg for the NO3 treatment. Leguminous plants made up &lt;2% of the herbage in both the NH4 and NO3 treatments. The annual loss from the NH4 treatment was 19 kg N ha−1 yr−1 and 24 from the NO3 treatment. The gross turnover time of N in the root compartment (which included plant crowns) was 1.41 yr for the NH4 treatment and 0.42 yr for NO3 The gross turnover time of soil microbial N was 2.13 yr (NH4) and 1.83 yr (NO3): for humus N (i.e., soil N not in roots or microbial biomass) it was 181 yr (NH4) and 116 yr (NO3).

Combination of micro suction cups and time‐domain reflectometry to measure osmotic potential gradients between bulk soil and rhizosphere at high resolution in time and space

SummaryA prerequisite to investigate the importance of osmotic potential (Ψo) in relation to matric potential (Ψm) in the soil for water uptake is the existence of a method that measures the temporal and spatial dynamics of Ψo in the vicinity of roots. One method for measuring Ψoin situ is the collection of soil solution with micro suction cups, the spatial resolution of which is suitable for rhizosphere studies. A major drawback of soil solution sampling is the disturbance of soil solution equilibrium, which makes frequent measurements impossible, so another method is required to provide information on the temporal dynamics of Ψo. The time‐domain reflectometry (TDR) technique might be suitable as the signal attenuation (σ) shows a close linear correlation with the salt concentration for a known soil water content. The temporal resolution of the TDR technique is high and the measurement has no impact on soil solution equilibrium. However, the spatial resolution of the TDR technique is too coarse to be used on its own in rhizosphere studies.We used a combination of TDR (fine temporal resolution) and micro suction cups (fine spatial resolution) to measure Ψo in a model system with Zea mays grown in quartz substrates. Osmotic potential changed continuously with time, and a steep gradient between bulk soil and the root compartment developed during the 39‐day growing period. The steepest gradient measured over a distance of 6 mm across the nylon net, separating the bulk soil from the root compartment, was −365 kPa. The combination of both methods made it possible to extend the time interval between micro suction cup samplings and thus minimize the impact of sampling on soil solution equilibrium. Problems of separate calibration were avoided by calibrating the TDR measurements against the results obtained with the micro suction cups within the same experiment.

Root-induced changes of lead availability in the rhizosphere of Oryza sativa L

Abstract Geogenic, as well as anthropogenic heavy metals from distant sources, gradually increase the level of toxic metals in natural environments and these will be increasingly taken up by plants and transferred further up the food chain. To control any detrimental environmental impacts of soil contaminants, it is desirable to investigate the solubility and mobility of soil chemical contaminants under land-based agricultural systems. In this paper, a greenhouse experiment was conducted to assess the effects of rhizospheric dynamics on the bioavailability of lead. Rice (Oryza sativa L.) and Ultisol were used as the test materials. The results showed NH4OAc extractable Pb in rice rhizosphere was much higher than in bulk soil, which meant that the activation processes in the rhizosphere was significant and the amount of bioavailable Pb increased. The distribution of extractable Pb at a distance of 0–10 mm from the root compartment, however, varied and had a little depletion compared to that in the rhizosphere. The significant difference of Pb forms between the rhizosphere and bulk soil further demonstrated the root-induced changes of lead bioavailability in the rhizosphere might also be related to the various levels of Pb treatment and the simultaneity of Cd. It was observed the accumulation of soluble plus exchangeable Pb (SE-Pb) in the 500 mg kg−1 Pb treatment, and relatively stronger fixation of Pb to oxides in the 250 mg kg−1 Pb treatment. The activation of SE-Pb in rice rhizosphere was weaker in the 5 mg kg−1 Cd treatment and stronger in the 10 mg kg−1 Cd treatment than that in the treatment without Cd. The activation of lead in the rhizosphere of Oryza sativa L., suggested a potential risk of heavy metal accumulation and phytotoxicity in plants and soil ecosystem.

Possible processes releasing nonexchangeable potassium from the rhizosphere of maize

The source and the releasing processes of nonexchangeable K from the rhizosphere were evaluated by using a 0.01 M HCl sequential extraction that enables the detection of subtle depletion of nonexchangeable K in the rhizosphere. Rhizobox experiments were conducted in which maize (Zea mays L.) plants were grown on different K sources (non-allophanic Andosol, Fluvisol, biotite and orthoclase) for 17 days. Nonexchangeable K decreased significantly in the rhizosphere of Andosol, Fluvisol and biotite, but not of orthoclase. The width of depletion was about 0–1 mm from the root-accumulating compartment regardless of the K sources, and was much less than that of exchangeable (5–10 mm) and water-soluble (50 mm) K. Rhizosphere pHs were above 4.5 in any treatment. These results suggested that the main source of nonexchangeable K for maize was interlayer K in 2:1 type phyllosilicate, and that the releasing process involved was cation exchange of the K rather than mineral dissolution by protons. For the exchange of interlayer K, a decrease of solution K+ below a certain threshold is known as a prerequisite. But the concentration of solution K+ at the root compartment of Andosol or Fluvisol, estimated to be more than 100 μM, was relatively higher than the known thresholds. Moreover, the significant release of nonexchangeable K from biotite occurred only at the root compartment where marked depletion of solution K+ was not observed. We therefore suggest that the release of interlayer K from the rhizosphere can occur even without a marked depletion of solution K+ through the following processes; (1) accumulation of cations such as Ca2+, Mg2+ or Na+ in the rhizosphere, (2) their adsorption on 2:1 type clay minerals near the edge of interlayer but inaccessible and nonexchangeable by NH4 +, (3) concomitant removal of the NH4 +-nonexchangable K even above the known thresholds of solution K+, which is followed by the expansion of the interlayer space, and (4) further removal of the deeper K by repetition of the above processes.

Turnover of Nitrogen-15-Labeled Fertilizer in Old Grassland

In this paper we follow the fate of single applications of 15N-labeled fertilizer to old grassland, over a period of nearly 20 yr. In 1980 and 1981, 15N-labeled N was applied to two of the treatments on the Park Grass Continuous Hay Experiment at Rothamsted, started in 1856. The labeled N was applied at the same rate (nominally 96 kg ha−1 yr−1) and in the same chemical form (NH4 or NO3) as the unlabeled N normally applied as fertilizer to the selected treatments. After 19 yr, 69.6% of the N applied in 1980 as 15NH4 had been harvested in successive cuts of herbage, with a further 16.5% remaining in the soil. For 15NO3, 64.3% had been harvested and 13.8% remained in the soil. The 15N data were then used to calculate annual inputs of nonfertilizer N, annual losses of N and N turnover times in old grassland, assuming that the selected treatments were under steady-state conditions. The annual input of N from nonfertilizer sources (rain, dry deposition, N fixation by leguminous components of the herbage, etc.) was large: 39 kg N ha−1 yr−1 for the NH4 treatment and 31 kg for the NO3 treatment. Leguminous plants made up <2% of the herbage in both the NH4 and NO3 treatments. The annual loss from the NH4 treatment was 19 kg N ha−1 yr−1 and 24 from the NO3 treatment. The gross turnover time of N in the root compartment (which included plant crowns) was 1.41 yr for the NH4 treatment and 0.42 yr for NO3 The gross turnover time of soil microbial N was 2.13 yr (NH4) and 1.83 yr (NO3): for humus N (i.e., soil N not in roots or microbial biomass) it was 181 yr (NH4) and 116 yr (NO3).

Gradients in soil solution composition between bulk soil and rhizosphere – In situ measurement with changing soil water content

Soil solution composition changes with time and distance from the root surface as a result of mass flow, diffusion, plant nutrient uptake and root exudation. A model system was designed, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 μm mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). K+ concentration in the rhizosphere soil solution decreased during the initial growth stage (12 days after planting, DAP). Thereafter K+ accumulated with time, due to mass flow as the dominating process. The extend of K+ accumulation depended on the initial fertiliser application. As K+ concentrations in soil solution increase, not only as a result of transport exceeding uptake, but also as a result of decreasing soil water content, it is hypothesised that K concentration in soil solution is not the only trigger for the activity of K transporters in membranes, but ABA accumulation in roots induced by decreasing soil matric potentials may add to the regulation. A strong decrease of rhizosphere pH with time is observed as a result of H+ efflux from the roots in order to maintain cation-anion balance. In addition the K+ to Ca2+ ratio was altered continuously during the growing period, which has an impact on Ca2+ uptake and thus firmness of cell walls, apoplast pH, membrane integrity and activity of membrane transporters. The value of osmotic potential in the rhizosphere soil solution increased with time indicating decreasing soil water availability. Modelling approaches based on the data obtained with the system might help to fill in the time gaps caused by the low temporal resolution of soil solution sampling method.

Soil and rhizosphere microorganisms have the same Q10 for respiration in a model system

AbstractWe compared the Q10 relationship for root‐derived respiration (including respiration due to the root, external mycorrhizal mycelium and rhizosphere microorganisms) with that of mainly external ectomycorrhizal mycelium and that of bulk soil microorganisms without any roots present. This was studied in a microcosm consisting of an ectomycorrhizal Pinus muricata seedling growing in a sandy soil, and where roots were allow to colonize one soil compartment, mycorrhizal mycelium another compartment, and the last compartment consisted of root‐ and mycorrhiza‐free soil. The respiration rate in the bulk soil compartment was 30 times lower than in the root compartment, while that in the mycorrhizal compartment was six times lower. There were no differences in Q10 (for 5–15°C) between the different compartments, indicating that there were no differences in the temperature relationship between root‐associated and non‐root‐associated organisms. Thus, there are no indications that different Q10 values should be used for different soil organism, bulk soil or rhizosphere‐associated microorganisms when modelling the effects of global climate change.