Root-free Soil Research Articles

Root production and root mortality of winter barley and its implication with regard to phosphate acquisition

Winter barley was grown in a long-term fertilizer experiment (14 years) using two P treatments: (i) no P fertilization over the whole time (−P) and (ii) an annual fertilization of 44 kg P ha−1 (+P). The objective of the study was to investigate the influence of the P supply on total root production and root mortality (i.e., root turnover) and to assess the benefit of a more rapid root turnover on P acquisition. Shoot development and grain yield was reduced in the ‘−’ treatment, whereas the standing root system had nearly the same size as in the ‘+P’ treatment. Gross root growth was measured using the ‘ingrowth core method’. Mesh bags filled with root-free soil were buried into the rooting zone (0–30 cm) and root growth into the bags over periods of 2–3 weeks was determined. Assuming that no root mortality occured inside the bags during this short period, root length in the bags will be a measure of total root production. Total root production between April and June exceeded the size of the standing root system by a factor of 2 to 3 and was significantly higher at P deficiency. Root mortality as the difference between total root production and the size of the standing root system was also increased at P shortage. P uptake was calculated by using a mechanistic transport and uptake model. Calculations based on gross root growth and root mortality resulted in a higher uptake than calculations based on the development of the standing root system, although the length of the active roots were the same in both calculations. This was due to a better exploitation of undepleted soil areas by the growing root system. The root renewal by a continuous root growth and root mortality is discussed as a mechanism of P uptake efficiency.

Development of ectomycorrhizas in model beech–spruce ecosystems on siliceous and calcareous soil: a 4-year experiment with atmospheric CO2 enrichment and nitrogen fertilization

To study responses of forests to global change, model ecosystems consisting of beech and spruce trees were established in open top chambers. The ecosystems were exposed to four conditions for 4 years, each replicated four times: ambient and elevated CO2, and low and high nitrogen input. At the end of the trial, the trees were 6–8 years old. Each chamber contained two separate compartments with siliceous and calcareous soil. Here, we focus on the development of ectomycorrhizas in the topsoil layer. Ectomycorrhizal fungal biomass associated with the fine roots, estimated by using ergosterol content as a marker, was much higher in the siliceous compared to the richer calcareous soil. Also, in root-free soil samples, the level of ergosterol, indicative of the extraradical mycelium of ectomycorrhizal fungi as well as the mycelium of other fungi, was about six times higher in the siliceous than in the calcareous soil. Conditions of elevated atmospheric CO2 primarily affected ectomycorrhizas in the calcareous soil. Fungal biomass, calculated per soil volume of the top soil layer, increased significantly, as did the metabolic activity of the ectomycorrhizal fungi, measured as uptake of glucose and synthesis of trehalose. Conditions of nitrogen fertilization affected ectomycorrhizas in the siliceous, nutrient poor soil.

Analysis of bacterial communities in the rhizosphere of chrysanthemum via denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA as well as DNA fragments coding for 16S rRNA.

The effect of developing chrysanthemum roots on the presence and activity of bacterial populations in the rhizosphere was examined by using culture-independent methods. Nucleic acids were extracted from rhizosphere soil samples associated with the bases of roots or root tips of plants harvested at different stages of development. PCR and reverse transcriptase (RT) PCR were used to amplify 16S ribosomal DNA (rDNA) and 16S rRNA, respectively, and the products were subjected to denaturing gradient gel electrophoresis (DGGE). Prominent DGGE bands were excised and sequenced to gain insight into the identities of predominantly present (PCR) and predominantly active (RT-PCR) bacterial populations. The majority of DGGE band sequences were related to bacterial genera previously associated with the rhizosphere, such as Pseudomonas, Comamonas, Variovorax, and Acetobacter, or typical of root-free soil environments, such as Bacillus and Arthrobacter. The PCR-DGGE patterns observed for bulk soil were somewhat more complex than those obtained from rhizosphere samples, and the latter contained a subset of the bands present in bulk soil. DGGE analysis of RT-PCR products detected a subset of bands visible in the rDNA-based analysis, indicating that some dominantly detected bacterial populations did not have high levels of metabolic activity. The sequences detected by the RT-PCR approach were, however, derived from a wide taxonomic range, suggesting that activity in the rhizosphere was not determined at broad taxonomic levels but rather was a strain- or species-specific phenomenon. Comparative analysis of DGGE profiles grouped all DNA-derived root tip samples together in a cluster, and within this cluster the root tip samples from young plants formed a separate subcluster. Comparison of rRNA-derived bacterial profiles showed no grouping of root tip samples versus root base samples. Rather, all profiles derived from 2-week-old plant rhizosphere soils grouped together regardless of location along the root.

The use of the ingrowth core method for measuring root production of arable crops — influence of soil conditions inside the ingrowth core on root growth

The ingrowth core method can be used to measure root gross growth (i.e. root production). A mesh bag filled with root free soil is buried into the root zone. After about 14 days, the bag is pulled out and root length inside the core can be determined. An objection against this method is the inability to obtain the same soil conditions inside the bag as outside, which can result in different root growth pattern in the ingrowth core compared to the bulk soil. To study this, mesh bags were buried in a stand of oilseed rape and were filled with soil at different nitrate, phosphate, moisture, and bulk density levels. Results showed that root growth was only influenced by a high nitrate content and a high soil density in the cores, which resulted in higher and lower root length densities (RLD), respectively. In a long-term ingrowth experiment similar root length densities in the cores and in the bulk soil were measured, indicating that there were no root growth enhancing or impeding conditions inside the ingrowth cores. The conclusion is drawn, that the ingrowth core method gives reliable results, provided the N content and the soil density inside the bags are comparable to the bulk soil. Die „ingrowth core“-Methode zur Messung der Wurzelproduktion von Nutzpflanzen — Der Einfluss der Bodenbeschaffenheit innerhalb des „ingrowth core“ auf das Wurzelwachstum Die „ingrowth core“-Methode wird genutzt, um das Bruttowachstum von Wurzeln zu bestimmen. Hierfür werden Schläuche aus Netzgewebe (Strümpfe) mit wurzelfreiem Boden gefüllt und im Bestand vergraben. Nach ca. 14 Tagen werden die Strümpfe herausgezogen und die Wurzellänge darin bestimmt. Ein möglicher Einwand gegen diese Methode ist, dass innerhalb der Strümpfe nicht dieselben Bodeneigenschaften vorliegen wie im Restboden. Dies könnte das Wurzelwachstum verändern. Um dies zu prüfen, wurden Strümpfe in einen Rapsbestand eingebracht. Dabei wurden der N-, P- und Wassergehalt, sowie die Bodendichte variiert. Ein Einfluss auf das Wurzelsystem konnte nur bei einem hohen N-Gehalt und einer hohen Bodendichte gefunden werden, die das Wurzelwachstum anregten bzw. behinderten. In einem weiteren Versuch, in dem der Boden innerhalb der Strümpfe über einen langen Zeitraum durchwurzelt werden konnte, stellten sich innerhalb und außerhalb der Strümpfe vergleichbare Wurzellängendichten ein. Auch dies zeigt, dass innerhalb der „ingrowth cores“ keine generelle Förderung oder Hemmung des Wurzelwachstums gegeben war. Es wird der Schluss gezogen, dass die „ingrowth core“-Methode geeignet ist, das Bruttowachstum von Wurzeln zu messen, wenn sich der N-Gehalt und die Bodendichte innerhalb der Strümpfe nicht zu stark vom Restboden unterscheiden.

Uptake of P from apatite by Pinus sylvestris seedlings colonised by different ectomycorrhizal fungi

The role of ectomycorrhizal (EM) fungi in increasing apatite dissolution was tested in a pot system with Pinus sylvestris (L.) seedlings growing in a sand/peat mixture. Non-mycorrhizal seedlings and seedlings inoculated with one of three different isolates of EM fungi were grown for 210 days in pots divided into a root-containing and a root-free compartment. The EM mycelium was allowed to colonise the root-free compartment, to which apatite had been added as a P source in half of the pots. All mineral nutrients except P were supplied in the form of a balanced nutrient solution. Seedlings grown with apatite as their P source grew significantly better and had higher P concentrations (1.1-1.5 mg/g) compared with seedlings growing without any P source (0.6-1.0 mg/g), indicating that they were able to use apatite-P. A weathering budget based on P uptake indicated that 6.7-18.9 mg apatite was weathered per pot which corresponds to 0.3-0.9% of the added apatite. A similar budget based on Sr uptake indicated that the apatite weathering rate was lower (0.13-0.3%). One Suillus variegatus isolate and an unidentified fungus had a significant positive influence on the dissolution of apatite, while another S. variegatus isolate had colonised roots poorly and did not influence weathering significantly. Oxalic acid was detected in root-free soil and was especially abundant in compartments colonised by S. variegatus. The concentration of oxalic acid was correlated to the concentration of phosphate in the soil solution of root-free soil, indicating that oxalic acid influences apatite dissolution.

Occurrence of salt, pH, and temperature-tolerant, phosphate-solubilizing bacteria in alkaline soils

An ecological survey was conducted to characterize 4800 bacterial strains isolated from the root-free soil, rhizosphere, and rhizoplane of Prosopis juliflora growing in alkaline soils. Of the 4800 bacteria, 857 strains were able to solubilize phosphate on plates. The incidence of phosphate-solubilizing bacteria (PSB) in the rhizoplane was highest, followed by rhizosphere and root-free soil. Eighteen bacterial strains out of 857 PSB were able to produce halo at 30 degrees C in a plate assay in the presence of 5% salt (NaCl) and solubilize tricalcium phosphate in National Botanical Research Institute's phosphate growth medium (NBRIP) broth, in the presence of various salts, pHs, and temperatures. Among the various bacteria tested, NBRI4 and NBRI7 did not produced halo in a plate assay at 30 degrees C in the absence of salt. Contrary to indirect measurement of phosphate solubilization by plate assay, the direct measurement of phosphate solubilization in NBRIP broth assay always resulted in reliable results. The phosphate solubilization ability of NBRI4 was higher than in the control in the presence of salts (NaCl, CaCl2, and KCl) at 30 degrees C. Phosphate solubilization further increased in the presence of salts at 37 degrees C as compared with 30 degrees C. At 37 degrees C, CaCl2 reduced phosphate solubilization ability of NBRI4 compared with the control. The results indicated the role of calcium salt in the phosphate solubilization ability of NBRI4.http://link.springer-ny. com/link/service/journals/00284/bibs/39n2p89.html

Improved establishment and development of three cactus species inoculated withAzospirillum brasilensetransplanted into disturbed urban desert soil

Survival and development of cactus transplants in urban, disturbed areas of the desert near La Paz, Baja California Sur, Mexico, was monitored. Young plants of three species of pachycereid cacti (Pachycereus pringlei, Stenocereus thurberi, and Lophocereus schottii) inoculated with the plant growth promoting bacterium Azospirillum brasilense in an eroded area (a dirt road) had a high survival rate and developed more rapidly compared with uninoculated control plants during a 3.5-year period after transplantation. Soil erosion in the inoculated experimental area diminished. Small, but significant soil accumulated in association with the growth of cactus roots into the wind-deposited dust. One demonstrated mechanism for stabilizing dust was by the upward growth of small roots during the rainy season into the deposited dust. Azospirillum brasilense survived well in the rhizospheres of these cacti for 2 years, but not in root-free soil. This study demonstrated the feasibility of using bacterial inoculation of cacti to enhance their establishment in disturbed areas, with the potential to stabilize soil.Key words: Azospirillum, beneficial bacteria, cactus, plant inoculation, plant growth promoting bacteria, PGPR, soil erosion, soil reclamation.

Diazotrophic bacteria in root-free soil and in the root zone of pine ( Pinus sylvestris L.) and oak ( Quercus robur L.)

Studies on the occurrence, identity and potential nitrogenase activity of diazotrophic bacteria in soil and the root zone of Scots pine ( Pinus sylvestris L.) and common oak ( Quercus robur L.) were carried out. Diazotrophic bacteria were more numerous in soil and the root zone of oak than in those of pine. Most of the bacteria isolated from the roots of both trees belonged to the genera Pseudomonas and Bacillus. The majority of these bacteria had nitrogenase activity (tested by the acetylene reduction assay method) within the range from 4 to 20 nmoles C 2H 4 per culture per hour. No clear relationships between the taxonomic identity and/or source of isolation of the bacteria studied and their nitrogenase activity were found. Biotin, p-aminobenzoic acid (PABA) and yeast extract, applied both separately and in combination, significantly (at least several-fold) stimulated the activity of N 2 fixation in three selected (nitrogenase highly active) bacterial strains, as compared to a control without growth factors.

Rhizosphere colonization pattern of Agrobacterium radiobacter strain G12A, an antagonistic rhizobacterium to the potato cyst nematode Globodera pallida

The colonizing behaviour and survival of the rhizosphere bacterium Agrobacterium radiobacter G12A, an effective antagonist of the potato cyst nematode Globodera pallida, was investigated at the soil-root interface. A rifampicin resistant mutant that was used for reisolation after spray application onto tuber pieces, was quantitatively recovered 7, 14, 21, and 56 days after planting from the rhizosphere, rhizoplane, root tip, stem base, the initial tuber piece and from root-free soil mix. After 21 days A. radiobacter G12A constituted 10 to 30% of the total numbers of detectable bacteria on potato tubers and in the rhizosphere. Feeding groups of rhizoplane-colonizing microorganisms increased in density after the introduction of A. radiobacter G12A at 15 and 20 °C, but not significantly. All groups of microorganisms tested (except flourescent Pseudomonads at 25 °C) increased in the potato rhizoplane, some of them significantly, when temperature was increased from 15 °C to 25 °C.

Effects of Pseudomonas fluorescens DF57 on growth and P uptake of two arbuscular mycorrhizal fungi in symbiosis with cucumber

The effect of Pseudomonas fluorescens DF57 on growth and P uptake of two arbuscular mycorrhizal (AM) fungi in symbiosis with cucumber plants was studied in compartmentalised growth systems. Hyphae of Glomus intraradices Schenck & Smith (BEG87) or G. caledonium (Nicol. & Gerd.) Trappe & Gerdeman (BEG15) grew into lateral root-free compartments. Non-mycorrhizal plants served as control. The soil in half of the growth units of each mycorrhizal treatment was inoculated with P. fluorescens DF57. P. fluorescens DF57 enhanced hyphal length density of one of the AM fungi, G. caledonium, but this was not reflected in a higher hyphal transport of P from the root-free soil to the plant. The total P content was higher in plants grown in symbiosis with G. intraradices than in plants in the other treatments. G. caledonium and P. fluorescens DF57 had a synergistic effect in that total P content in plants inoculated with G. caledonium was higher in the presence than in the absence of P. fluorescens DF57.

Suppression of the biocontrol agent trichoderma harzianum by mycelium of the arbuscular mycorrhizal fungus glomus intraradices in root-free soil

Trichoderma harzianum is an effective biocontrol agent against several fungal soilborne plant pathogens. However, possible adverse effects of this fungus on arbuscular mycorrhizal fungi might be a drawback in its use in plant protection. The objective of the present work was to examine the interaction between Glomus intraradices and T. harzianum in soil. The use of a compartmented growth system with root-free soil compartments enabled us to study fungal interactions without the interfering effects of roots. Growth of the fungi was monitored by measuring hyphal length and population densities, while specific fatty acid signatures were used as indicators of living fungal biomass. Hyphal 33P transport and beta-glucuronidase (GUS) activity were used to monitor activity of G. intraradices and a GUS-transformed strain of T. harzianum, respectively. As growth and metabolism of T. harzianum are requirements for antagonism, the impact of wheat bran, added as an organic nutrient source for T. harzianum, was investigated. The presence of T. harzianum in root-free soil reduced root colonization by G. intraradices. The external hyphal length density of G. intraradices was reduced by the presence of T. harzianum in combination with wheat bran, but the living hyphal biomass, measured as the content of a membrane fatty acid, was not reduced. Hyphal 33P transport by G. intraradices also was not affected by T. harzianum. This suggests that T. harzianum exploited the dead mycelium but not the living biomass of G. intraradices. The presence of external mycelium of G. intraradices suppressed T. harzianum population development and GUS activity. Stimulation of the hyphal biomass of G. intraradices by organic amendment suggests that nutrient competition is a likely means of interaction. In conclusion, it seemed that growth of and phosphorus uptake by the external mycelium of G. intraradices were not affected by the antagonistic fungus T. harzianum; in contrast, T. harzianum was adversely affected by G. intraradices.

Effects of irrigation on litterfall, fine root biomass and production in a semideciduous lowland forest in Panama

The effects of irrigation on fine root biomass, root production and litterfall were measured at the community level, in a semideciduous lowland forest in Panama. Biomass of roots less than 2 mm in dia. in the first 10 cm of the soil (measured with soil cores), was higher in irrigated (1.80 Mg ha-1) than in non-irrigated plots (1.24 Mg ha-1). During the dry season, productivity of roots (measured with ingrowth cylinders filled with root-free soil), was higher in irrigated (1.6 g m-2 day-1) than in control plots (0.3 g m-2 day-1). In control plots, root productivity was highly seasonal. Maximum root growth into the root-free soil, occurred during the transitions from dry to wet, and from wet to dry season, possibly as a response to water and/or nutrient pulses. Litterfall was not significantly different between irrigated (3.8 g m-2 day-1) and control plots (3.7 g m-2 day-1). The results of this study show that root-productivity is limited by the water supply during the dry season, and that water by itself, is not a limiting factor for community-level litter production.

The use of fatty acid signatures to study mycelial interactions between the arbuscular mycorrhizal fungus Glomus intraradices and the saprotrophic fungus Fusarium culmorum in root-free soil

The saprotrophic fungus Fusarium culmorum, Penicillium hordei, Rhizoctonia solani and Trichoderma harzianum and the arbuscular mycorrhizal fungus Glomus intraradices were examined for content of phospholipid fatty acids (PLFA) and neutral lipid fatty acids (NLFA). The AM fungus differed from the saprotrophic fungi especially by its content of the fatty acid 16:1ω5 which was absent in the saprotrophs. The fatty acid 18:2ω6,9 was the dominant fatty acid of the saprotrophic fungi while it was negligible in mycelium of G. intraradices. Specificity in content of fatty acids made it possible to quantify G. intraradices. and F. culmorum simultaneously in soil. Furthermore, a compartmented growth system made it possible to study mycelial interactions in the absence of roots. We measured hyphal spread of both fungi, hyphal 33P transport of G. intraradices and sporulation of F. culmorum. The two fungi did not interact according to the parameters used in this study. We conclude that fatty acid signatures may be a valuable tool when studying interactions between AM fungi and other fungi in root-free soil.

Estimating root respiration, microbial respiration in the rhizosphere, and root-free soil respiration in forest soils

We hypothesized that respiration measurements made using both the basal and excised-root respiration methods would allow us to quantify three important components of soil respiration: root respiration ( R root), microbial respiration in the rhizosphere ( R rhizo), and root-free soil respiration ( R rfs). Root respiration determined by the basal method was approximately one-third greater than root respiration determined by the excised-root method (52 versus 32% of total soil respiration, respectively). Results from a decomposition model constructed for the root C fractions (easily decomposable, slowly decomposable, and recalcitrant) showed that the easily and slowly decomposable C fractions disappeared approximately 3 months after the basal respiration measurements began. Since these C fractions contained the majority of the C source for rhizosphere microorganisms (i.e., rhizodeposition), microbial respiration in the rhizosphere must have been severely reduced, indicating that respiration measured after 3 months must be dominated by microorganisms in the root-free soil ( R rfs); this indicated that the basal method actually measured root-free soil respiration. This allowed us to fractionate the components of soil respiration based on the relationship: total soil respiration ( R total)= R root+ R rhizo+ R rfs; subtracting R root(excised-root method) and R rfs(basal method) from R total gave us an estimate of microbial respiration in the rhizosphere; and the contribution of these three components to R total: R root=32%, R rhizo=20%, and R rfs=48%. These results are important because they suggest a way that soil respiration can be separated into at least three functionally different components, and they show that microbial respiration in the rhizosphere is a significant sink for photosynthetically-fixed C in forests.

DNA-probing for genes coding for denitrification, N2-fixation and nitrification in bacteria isolated from different soils.

Bacteria isolated from different layers of four soils of the Cologne area were analyzed for denitrifying, nitrifying and N2-fixing isolates by colony hybridization using gene probes. In the soils tested, the percentage of denitrifying bacteria among the total population isolated was 3-8% (in one case exceptionally 15%) and thus small. Denitrifying bacteria were particularly enriched in the upper layer (depth approximately 5 cm) and were present only in low amounts at 25 cm depth in two gleysol soils. Nitrate apparently did not determine the distribution of denitrifying bacteria in these soils. The potential denitrification activity of different soil layers coincided with the distribution pattern of isolates assessed by DNA-probing. The total number of bacteria and of denitrifying isolates was considerably higher in or at the roots of plants than in the bulk, root-free soil adjacent to the plants. The percentage of the isolated aerobic N2-fixing bacteria varied between 0-3%, and these bacteria could be isolated mainly from the upper 5 cm layer. A small portion of the isolates hybridized with the probe coding for part of one subunit of ammonia monooxygenase from Nitrosomonas europaea. The investigation showed that DNA-probing can provide useful information about the relative distribution of denitrifying and N2-fixing bacteria in different soils and their layers.

Potential to improve transfer of N in intercropped systems by optimising host-endophyte combinations

Possibilities for improving N transfer from N2-fixing plants to non-N2-fixing plants by mycorrhiza have been investigated. Initially, the genetic variability with respect to N uptake was assessed by screening five varieties of chicory (Cichorium intybus L.), four of peas (Pisum sativum L.) and three of red clover (Trifolium pratense L.) in combination with eight isolates of arbuscular mycorrhizal fungi. The most promising plant - fungi combinations identified through the cultivar screening were used to optimise conditions for N transfer between intercropped N2-fixing plants (peas and clover) and non-N2-fixing chicory. In the first experiment, the recovery of fixed legume N was investigated using three cultivars, of chicory intercropped with pea variety, and inoculated with one of four mycorrhizal isolates. Roots of the N2-fixing pea and the non-N2-fixing chicory were separated by a root-free soil layer in a three-compartment container. A section of the legume roots was forced to grow into a separate compartment which received four split applications of 15N. The percentage of N in the chicory derived from transfer ranged between 3% and 50%. In a second experiment one chicory variety was intercropped with one red clover variety and inoculated with four mycorrhizal isolates respecetively. A harvest regime was chosen in which the shoots were harvested from intercropped plants at 3,4.5 and 6 months of age. At three months the percentage of N in the chicory derived from transfer ranged between 15% and 18% and at a plant age of 4.5 months from 46 to 77%. At six months the percentage of N in the chicory roots derived from transfer of legume N ranged from 20 to 34% and varied with fungal isolate. Our results show that there is potential for improving N transfer in intercropped plant systems through the methodological selection of suitable plant and mycorrhizal partners.

Soil mycoflora in tomato fields

Density and species richness of fungal communities in soils ofFusarium infested and non-infested tomato-growing localities were studied by comparison of rhizoplanes, rhizospheres, and root-free soils. The rhizosphere soils harbored the highest counts of fungi, followed by root-free soil and rhizoplanes in both localities. Species richness was high in the rhizosphere and root-free soil but distinctly low in the rhizoplane. The population density of the zhizosphere and the rhizoplane showed a significant difference between infested and non-infested localities.

Nematophagous fungi in the rhizosphere of agricultural crops

In a field experiment the rhizosphere effect of barley, pea and white mustard on the nematode-trapping fungi were investigated throughout a growing season. The densities of nematode-trapping fungi were slightly increased in the rhizospheres compared to the root-free soil. Pea rhizosphere had the greatest numbers of species of nematophagous fungi with an average of 2.4 species recovered from 0.1 g material, and in white mustard and barley rhizospheres and root-free soil less than 1.7 species were recovered from 0.1 g. Arthrobotrys oligospora was the most common species in both soil and rhizosphere. In a pot experiment the rhizosphere effect of pea and barley on nematophagous fungi was investigated in 5 different agricultural soils. Pea rhizosphere increased the densities of nematode-trapping fungi, and up to 780 propagules of nematode-trapping fungi g −1 rhizosphere soil were found, which was around 19 times higher than in the root-free soil. The number of nematodes were 6–290 times higher in the pea rhizosphere than in the root-free soil. Barley rhizosphere had little effect on the densities of nematode-trapping fungi, while the number of nematodes increased by 3–13 times. The most commonly detected species were A. oligospora, Stylopage sp. and network-forming Monacrosporium species, but there were no big differences in the species composition between the rhizosphere and the root-free soil.

Is available carbon limiting microbial respiration in the rhizosphere?

It is widely known that the carbon availability in the rhizosphere is much higher than in the bulk soil. However, studies have failed to show whether microbial respiration in the rhizosphere is carbon-limited. Precise and timely measurements are lacking. We measured carbon availability index (basal respiration divided by substrate-induced respiration), and water soluble carbon in soils sampled at four spatial points (rhizoplane, rhizosphere, bulk soil and root-free soil) in the rhizosphere continuum in greenhouse and field experiments. Carbon availability index and water soluble carbon were inversely related to the relative distance from the root surface, with several times higher concentrations in the rhizoplane soils. Microbial respiration was not limited by available carbon in the rhizoplane and in the rhizosphere.

Use of Cluster and Discriminant Analyses to Compare Rhizosphere Bacterial Communities Following Biological Perturbation

We present an approach to comparing the diversity and composition of bacterial communities from different habitats and for identifying which members of a community are most affected by an introduced bacterium. We use this method to explore both previously published and new data from field and growth chamber experiments in which we isolated heterotrophic bacteria from samples of root-free soil, roots of nontreated soybean seedlings, and from the roots of soybean seedlings grown from Bacillus cereus UW85n1-treated seeds. We characterize bacterial isolates for 40 physiological attributes, and grouped the isolates hierarchically using two-stage density-linkage cluster analysis. Multivariate analysis of variance and discriminant analysis of the relative frequencies of the clusters in the soil and rhizosphere habitats were then used to determine whether there were differences among the bacterial communities from the various habitats, and which of the clusters were most useful in discriminating among the communities. We used rarefied estimates of richness as a measure of community diversity in the various habitats. Introduction of UW85n1 affected the composition and/or diversity of rhizosphere communities in three of four experiments.