Microbial Activity In Rhizosphere Research Articles

Plant Species Mediate Rhizosphere Microbial Activity and Biodegradation Dynamics in a Riparian Soil Treated with Bensulfuron-methyl

A pot experiment was conducted to investigate microbial characteristics and the biodegradation process of bensulfuron-methyl (BSM) in a rhizosphere soils planted with different riparian plants. The results showed that microbial population decreased with BSM addition in the rhizosphere, especially for bacteria and fungi. The activities of the dehydrogenase (DHase) were stimulated firstly, due to BSM addition, but then were inhibited, and recovered to the initial level, while the activities of the phosphatase and urease showed obviously decreasing trend throughout the whole experiment. Rhizosphere soil substrate-induced respiration (SIR) was depressed by BSM, especially at the initial 14 days of incubation. Compared to Zizania aquatica and Phragmites australis, Acorus calamus showed a significantly (p < 0.05) higher DHase activity and larger SIR in the rhizosphere soils treated with BSM, which means that A. calamus can effectively alleviate inhibitory effect of the sulfonylurea herbicide addition on microbial activity. There were significant (p < 0.05) differences in microbial degradation dynamics of BSM in the rhizosphere soils among three kinds of riparian plants. A. calamus displayed a significantly (p < 0.05) higher degradation efficiency of BSM in the rhizosphere soils, followed by Z. aquatica and P. australis. The residual BSM concentration in A. calamus rhizosphere soil was 23.1 and 32.2% lower than that in Z. aquatica and P. australis rhizosphere soils, respectively, indicating a greater improvement effect on biodegradation of BSM in A. calamus rhizosphere soils.

Rhizosphere soil microbial activity under different vegetation types on the Loess Plateau, China

Severe soil and water loss has led to widespread land degradation of the Loess Plateau, China. During the past two decades, significant efforts have been made to revegetate degraded soil in the region. However, a better understanding of the ability of plants to promote soil microbial processes is important for successful revegetation. The current study was conducted at the Dunshan watershed on the Loess Plateau, with the aim of evaluating the effect of different vegetation types on the microbial properties of rhizosphere soil. Six vegetation types were evaluated: two natural grassland ( Artemisia capillaries and Heteropappus altaicus), two artificial (i.e. planted by humans) grassland ( Astragalus adsurgens and Panicum virgatum) and two artificial shrubland ( Caragana korshinskii and Hippophae rhamnoides) species; an area without vegetation cover was used as a control. The results indicated that the highest values of soil organic C, total nitrogen (TN), microbial biomass carbon (MBC), MBN, saccharase, urease, catalase and peroxidase were recorded for H. altaicus and A. capillaries; of basal respiration (BR), the MBN: TN ratio, alkaline phosphatase and polyphenol oxidase for A. adsurgens; and of the BR: MBC ratio for P. virgatum. A rhizosphere soil microbial index (RSMI) was obtained using principal component analysis, and metabolic quotient (BR/MBC), BR, urease, water-soluble C and cellulase were found to be most important for assessing rhizosphere soil quality. All revegetated soil showed higher RSMI compared with control soil, with soil under natural grassland species ( H. altaicus and A. capillaries) recording the highest value. It was concluded that plant species under arid climatic conditions have different effects on the microbial properties of rhizosphere soil owing to their different root exudates and plant debris. In addition, natural grasslands are most effective for revegetation of the Loess Plateau.

Cadmium-induced changes in soil biochemical characteristics of oat (Avena sativa L.) rhizosphere during early growth stages

A microcosm was assembled to physically separate soil from roots and was used to study both the impact of living roots on the soil–plant system during early stages of growth and plant responses to abiotic stress. Oat (Avena sativa L.) seedlings were grown in the microcosm unit for 44 days. Twenty-three days after planting, 0.154 mg CdSO4/g dry soil was added. Plants grown in Cd-treated microcosms showed considerable inhibition of shoot growth rates, and leaf chlorophyll content. Soil microbial biomass C and respiration increased with plant age, and most of the measured biochemical indicators decreased with increasing distance from the soil–root interface, thus demonstrating the rhizosphere effect, likely due to the quick assimilation of rhizodeposits by rhizosphere microflora. On the other hand, short-term Cd contamination sometimes had an inhibitory effect on soil respiration, qCO2, ATP content, and phosphatase activity, while stimulating microbial biomass, mainly at the rhizosphere level. The decrease in rhizosphere microbial activity observed after Cd application to soil may be due to a synergic effect of the metal directly on microbial cells and indirectly on plants, which reduced shoot growth rate and chlorophyll content, resulting in decreased availability of root exudates.

Differences in the Rhizosphere Microbial Activity and Community Composition of Commelina communis along a Copper Contamination Gradient

We studied the effects of copper (Cu) phytoremediation on microbial community composition under laboratory conditions. A Cu accumulator, Commelina communis, was grown on a soil containing different gradients of Cu. Results showed that the biomass of C. communis grown with Cu differed from that of the control. Concentrations of Cu in the shoots of C. communis were 73.6, 160.9, and 319.1 mg kg−1 under 200, 500, and 1000 mg kg−1 Cu treatments, respectively. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprint analysis revealed that additions of Cu decreased the number of bands in the soil with C. communis or without plants. The principal component analysis explained 52.7% of the variance for the different rhizospheres of soil and Cu treatments in the soil samples. These results indicated significant effects on soil bacteria activity and community composition in the rhizosphere of C. communis and provided a basis for further studies of metal-accumulator plant effects on soil microorganisms.

Movement of newly assimilated 13C carbon in the grass Lolium perenne and its incorporation into rhizosphere microbial DNA

One of the key processes that drives rhizosphere microbial activity is the exudation of soluble organic carbon (C) by plant roots. We describe an experiment designed to determine the impact of defoliation on the partitioning and movement of C in grass (Lolium perenne L.), soil and grass-sterile sand microcosms, using a (13)CO(2) pulse-labelling method. The pulse-derived (13)C in the shoots declined over time, but that of the roots remained stable throughout the experiment. There were peaks in the atom% (13)C of rhizosphere CO(2) in the first few hours after labelling probably due to root respiration, and again at around 100 h. The second peak was only seen in the soil microcosms and not in those with sterilised sand as the growth medium, indicating possible microbial activity. Incorporation of the (13)C label into the microbial biomass increased at 100 h when incorporation into replicating cells, as indicated by the amounts of the label in the microbial DNA, started to increase. These results indicate that the rhizosphere environment is conducive to bacterial growth and replication. The results also show that defoliation had no impact on the pattern of movement of (13)C from plant roots into the microbial population in the rhizosphere.

Effects of a humic acid and its size-fractions on the bacterial community of soil rhizosphere under maize ( Zea mays L.)

The effects of a humic acid (HA) and its size-fractions on plants carbon deposition and the structure of microbial communities in the rhizosphere soil of maize ( Zea mays L.) plants were studied. Experiments were conducted in rhizobox systems that separate an upper soil–plant compartment from a lower compartment, where roots are excluded from the rhizosphere soil by a nylon membrane. The upper rhizobox compartment received the humic additions, whereas, after roots development, the rhizosphere soil in the lower compartment was sampled and sliced into thin layers. The lux-marked biosensor Pseudomonas fluorescens 10586 pUCD607 biosensor showed a significant increase in the deposition of bioavailable sources of carbon in the rhizosphere of soils when treated with bulk HA, but no response was found for treatments with the separated size-fractions. PCR–DGGE molecular fingerprintings revealed that the structure of rhizosphere microbial communities was changed by all humic treatments and that the smaller and more bioavailable size-fractions were more easily degraded by microbial activity than the bulk HA. On the other hand, highly hydrophobic and strongly associated humic molecules in the bulk HA required additional plant rhizodeposition before their bio-transformation could occur. This work highlights the importance of applying advanced biological and biotechnological methods to notice changes occurring in plant rhizodeposition and rhizosphere microbial activity. Moreover, it suggests correlations between the molecular properties of humic matter and their effects on microbial communities in the rhizosphere as mediated by root exudation.

Rhizosphere soil microbial community structure and microbial activity in set-aside and intensively managed arable land

The effects of plant species which frequently occur in set-aside arable land on rhizosphere soil properties were assessed and compared to rhizosphere soil of Secale cereale (Rye) grown on an intensively managed arable soil (sandy Cambisol, Saxony, NE- Germany). On a 6 year old set-aside arable land rhizosphere soil samples were taken under Agropyron repens, Cirsium arvense and Rumex acetosa, the most frequent plant species, and under the leguminous plant species Vicia villosa. Phospholipid fatty acid analysis (PLFA) has been used to characterise the structure of the soil microbial community. Carbon mineralisation rates as well as gross ( 15 N isotope pool dilution method) and net nitrogen mineralisation rates were determined as indicator of microbial activity. In intensive managed plots a rhizosphere effect was obvious in higher nutrient contents, gross N mineral- isation rates and higher relative abundances of fungi and protozoa in Rye rhizosphere compared to bulk soil. Plant species altered rhizosphere microbial activity. Lowest gross N mineralisation and gross NH4 con- sumption rates were detected in Rye rhizosphere soil. Both rates revealed high positive correlations with dissolved organic carbon (extracted with KCl) and soil pH. The rhizosphere soil microbial communities of the three dominant plant species of the set-aside arable land (Agropyron, Cirsium, Rumex) were more similar to each other than to Vicia grown on the same set-aside plots and Rye grown on intensive managed plots. The highest number of non-identified PLFAs detected in Vicia rhizosphere soil suggests that microbial diversity was highest. Differences in quantity and quality of Vicia rhizodeposition, especially higher N contents, seem to induce the higher microbial activity and different microbial community structure. The rhizo- sphere soil of the dominant plant species on the set- aside and intensively managed arable land reflected the differences in bulk soil properties (obtained in a previous study) between the two management systems (e.g. pH, gross N mineralisation, metabolic quotient, PLFA marker characteristic of Gbacteria and fungi).

Field Note: Phytoremediation of Chlorinated Ethenes in Seepline Sediments: Tree Selection

Phytoremediation of chlorinated ethene (CE)-contaminated water was investigated at the Savannah River Site in Aiken, SC, USA. Perchloroethylene (PCE) and trichloroethylene (TCE) are present where CE-contaminated groundwater currently outcrops in seepline soils. Results of constructed and planted test cells, filled with soil from a noncontaminated seepline area and supplied with CE-contaminated groundwater (48 ppb) in the field for one season are presented. These test cells were planted with loblolly pines, hybrid poplars, coyote willow, and sweet gum. Cis-dichloroethylene (cDCE), a byproduct from rhizosphere microbial activity, was detected in the soils as well as some tree tissues. All trees tested were found to uptake both PCE and TCE (5–50 pbb/gm dry wt).

Biofertilizers improve plant growth, fruit yield, nutrition, metabolism and rhizosphere enzyme activities of Pomegranate ( Punica granatum L.) in Indian Thar Desert

Production of horticultural crops has undergone enormous change in recent years due to development of innovative technologies including integrated nutrient management practices using biofertilizers. The present study represents the positive response of biofertilizers in nursery seedlings followed by their transplantation in harsh field conditions of Indian Thar Desert. Nursery and field experiments were carried out to assess the effectiveness of selected N 2-fixing bacteria and AM fungi alone or in combination, on the growth and biomass production of Punica granatum. In both experiments, the combined treatment of Azotobacter chroococcum and Glomus mosseae was found to be the most effective. Besides enhancing the rhizosphere microbial activity and concentration of various metabolites and nutrients, these bioinoculants helped in better establishment of pomegranate plants under field conditions. A significant improvement in the plant height, plant canopy, pruned material and fruit yield was evident in 5-year-old pomegranate plants in field conditions. In view of the above results, use of biofertilizer technology may be adopted for the establishment and development of other horticultural plant species in arid regions.

Rhizosphere bacterial community composition responds to arbuscular mycorrhiza, but not to reductions in microbial activity induced by foliar cutting.

Differences in bacterial community composition (BCC) between bulk and rhizosphere soil and between rhizospheres of different plant species are assumed to be strongly governed by quantitative and qualitative rhizodeposit differences. However, data on the relationship between rhizodeposit amounts and BCC are lacking. Other soil microorganisms, e.g. arbuscular mycorrhizal fungi (AMF), may also influence BCC. We simulated foliar herbivory (cutting) to reduce belowground carbon allocation and rhizodeposition of pea plants grown either with or without AMF. This reduced soil respiration, rhizosphere microbial biomass and bacteriovorous protozoan abundance, whereas none of these were affected by AMF. After labelling plants with (13)CO(2), root and rhizosphere soil (13)C enrichment of cut plants were reduced to a higher extent (24-46%) than shoot (13)C enrichment (10-24%). AMF did not affect (13)C enrichment. Despite these clear indications of reduced rhizosphere carbon-input, denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes PCR-amplified targeting DNA and RNA from rhizosphere soil did not reveal any effects of cutting on banding patterns. In contrast, AMF induced consistent differences in both DNA- and RNA-based DGGE profiles. These results show that a reduction in rhizosphere microbial activity is not necessarily accompanied by changes in BCC, whereas AMF presence inhibits proliferation of some bacterial taxa while stimulating others.

EFFECT OF BACTERIAL INOCULATION, MOLYBDENUM AS WELL AS SOME SOIL AMENDMENTS ON SOYBEAN.

Lysimeters experiment was carried out in a wire house study effect of microbial inoculation using three active local strains of Bradyrhizobium japonicum, Azotobacter chroococcum and Thiobacillus thioparus, some soil amendments addition as compost, sulfur, rock phosphate on soybean (Glycine max L.) cv. Giza 21. The seeds were dreseed by potassium molybdate. In rhizospheric region total count of bacteria, actinomycetes and fungi at 45, 70 and 95 days from sowing were estimated. Also true nodules formation rate, biological yield as root, shoot length, height and dry matter weight were studied. Economical yield as; number of pods, weights and weight of 100 grains as seed index were studied. In addition, determinations of total nitrogen, phosphorus and potassium in seeds and soil properties before sowing and after harvesting were done. The results obtained cleared that useful effect of combined inoculation by three bacteria under study and interaction with soil amendments addition, which resulted increase in nodulation rate (biological nitrogen fixation process), rhizosphere microbial activity followed availability of phosphorus from rock phosphate, so affect that on plant productivity, seed contain of total nitrogen, phosphorus and potassium. Also maintenance, restoring soil fertility due to these treatments. Generally, organic fertilizers with other treatments gave best results in all parameters from control. The results of this study recommended that application of combined biofertilization (Bradyrhizobium, Azotobacter and Thiobacillus) and soil amendments addition (compost, sulfur and rock phosphate) as well as seed dressing by potassium molybdate promoted rhizosphere microorganism’s proliferation, plant nutrients availability, nitrogen fixation process gains due these treatments on soybean prameters and productivity, also soil maintenance and restoring fertility.

Effects of glyphosate and foliar amendments on activity of microorganisms in the soybean rhizosphere

A field study was conducted to determine the effects of glyphosate on microbial activity in the rhizosphere of glyphosate-resistant (GR) soybean and to evaluate interactions with foliar amendments. Glyphosate at 0.84 kg ae ha− 1 was applied GR soybean at the V4–V5 development stages. Check treatments included a conventional herbicide tank mix (2003 study only) and no herbicides (hand-weeded). Ten days after herbicide application, a commercially available biostimulant and a urea solution (21.0% N) were applied to soybean foliage at 33.5 mL ha− 1 and 9.2 kg ha− 1, respectively. Soil and plant samples were taken 0, 5, 10, 15, 20 and 25 days after herbicide application then assayed for enzyme and respiration activities. Soil respiration and enzyme activity increased with glyphosate and foliar amendment applications during the 2002 growing season; however, similar increases were not observed in 2003. Contrasting cumulative rainfall between 2002 and 2003 likely accounted for differences in soil microbial activities. Increases in soil microbial activity in 2002 suggest that adequate soil water and glyphosate application acted together to increase microbial activity. Our study suggests that general soil microbial properties including those involving C and N transformations are not sensitive enough to detect effects of glyphosate on rhizosphere microbial activity. Measurements of soil-plant-microbe relationships including specific microbial groups (i.e., root-associated Fusarium spp.) are likely better indicators of impacts of glyphosate on soil microbial ecology.

Effect of Fertilizer P and Farmyard Manure on Bioavailable P as Influenced by Rhizosphere Microbial Activities in Soybean-Wheat Rotation

ABSTRACT The efficiency of use of applied P may be improved through an understanding of soil P dynamics in relation to management practices in a cropping system. The objective of this study was to examine the influence of farmyard manure (FYM) and fertilizer P application on the activities of native arbuscular mycorrhizal fungi (AMF), inorganic P-solubilizing and organic-P mineralizing organisms, and selected enzyme activities in a soybean (Glycine max merril L.)-wheat (Triticum aestivum L.) rotation of five years duration. The results showed that the greatest microbial activities were measured when maximum crop growth occurred and that their activities increased significantly with the application of manure. AMF root colonization was twice higher in soybean than in wheat. The application of fertilizer P (44 kg P ha−1) reduced AMF colonization in roots, which was 15% lower than in control plots in both soybean and wheat. The combined application of fertilizer P and manure significantly improved the inorganic-P solubilizing activity of rhizosphere soil, which ranged from 0.87 to 1.53 μg TCP g−1 soil d−1 and organic-P mineralizing rate ranged from 0.07 to 0.31 μg inosine-triphosphate g−1 soil d−1. Root acid phosphatase was maximal at the early-growth stage of the crops and significantly increased with both manure and P application, but decreased as crops matured. However, rhizosphere soil alkaline phosphatase remained higher even after maturity than at vegetative stage. It was observed that after six cropping cycles of soybean-wheat; the labile organic phosphorus (Po) (NaHCO3-P) and moderately labile Po (NaOH-P) were relatively higher than stable Po (HCl-P). The bioavailable P was significantly correlated with the rates of inorganic-P solubilization and organic-P mineralization and with the activities of root phosphatase and rhizosphere soil alkaline phosphatase. These results demonstrate that qualitative P solubilizing/mineralizing organisms associated with AMF, and activity of enzymes mediated through microbes are a major factor in controlling organic and inorganic P in this management system.

Effect of Rhizodegradation in Diesel-Contaminated Soil Under Different Soil Conditions

In Order To Develop A Cultivation Technique For The Practical Use of Phytoremediation of Diesel-Contaminated Soil, We Evaluated The Rhizodegradation of Diesel-Contaminated Soil Using Italian Ryegrass. Experiments Were Conducted Under Two Different Soil Conditions That Were Expected To Reduce The influence of Diesel On The Plant. Under The First Condition, The initial Diesel Concentration Which Is Expressed in The Total Petroleum Hydrocarbon (Tph) Concentration Was Set To 0.80%. The Concentration Was Almost Half The Upper Limit For The Growth of Italian Ryegrass. Under The Second Condition, Zeolite Was Added To The Experimental Soil To Improve The Cation Exchange Capacity (Cec). in 152 Days Experiments, We Evaluated The Plant Growth Variables, Tph Concentration, Soil Dehydrogenase Activity (Dha) That Is Reflective of The Rhizosphere Microbial Activity, And The Aerobic Bacterial Count. The Results Suggest That The Tph Concentration in First Condition (0.80%) Could Not Bring About A Significant Recovery of Plant Growth. The Plant Growth Observed in First Condition Was Equal To ThatObserved in The Case of The Upper Limit Tph Concentration Used in Our Previous Study. However, Under The Second Condition,It Is Suggested That The Addition of Zeolite Could increase Plant Growth, Which Can in Turn Improve The Rhizodegradation Effect.

Rhizosphere Microbial Activity During Phytoremediation of Diesel-Contaminated Soil

To know microbial activity and diesel-removal efficiency influencing through plant roots, we examined the effect of the rhizosphere on phytoremediation of diesel-contaminated soils by alfalfa (Medicago sativa L.). Pots were treated with and without diesel and allowed to stabilize for 7 weeks, at which time four experimental/control groups were prepared: (1) planted diesel-contaminated soil, (2) unplanted diesel-contaminated soil, (3) planted uncontaminated soil, and (4) unplanted uncontaminated soil. Samples of rhizosphere and bulk soils were separately taken from all planted pots. After 7 weeks of alfalfa growth from seeds, the removal efficiencies in rhizosphere and bulk soil samples were 82.5% and 36.5∼ 59.4%, respectively. The total microbial activity was highest in diesel-contaminated rhizosphere soils. Significantly more culturable soil bacteria and hydrocarbon-degraders were found in diesel-contaminated rhizosphere soil versus unplanted and uncontaminated bulk soil, with a greater increase seen in hydrocarbon-degraders (172-fold) versus general soil bacteria (14-fold). DGGE (Denaturing Gel Gradient Electrophoresis) analysis revealed that the bacterial community structure was most highly influenced by the combined presence of diesel contamination and plant roots (39.13% similarity compared to the control), but that diesel contamination alone had a higher influence (42.31% similarity compared to the control) than the rhizosphere (50.00% similarity compared to the control). Sequence analysis and BLAST searches revealed that all samples were dominated by members of α -, γ -, δ - and ε -proteobacteria, and Chloroflexi. The rhizosphere samples additionally contained novel dominant members of α -proteobacteria and Cytophaga-Flexibacter-Bacteroides, while the diesel samples contained additional dominant α -proteobacteria and the rhizosphere plus diesel samples contained other ε -proteobacteria. Collectively, these findings indicate that the presence of plant roots (i.e., a rhizosphere) had a greater effect on bacterial activity in diesel contamination than did the absence of diesel contamination, whereas diesel contamination had a greater effect on bacterial community structure. These novel findings provide new insight into the mechanisms of phytoremediation.

Effect of Foliar Applied Plant Elicitors on Microbial and Nematode Populations in the Root Zone of Potato

Systemic acquired resistance (SAR) is a process whereby a plant that successfully resists a pathogen becomes highly resistant to subsequent infection not only by the original pathogen but also by a wide variety of pathogens. Most SAR research has focused on resistance in leaves, so much less is known about the effectiveness of foliar applications of SAR compounds in the protection of plant roots and associated microorganisms in soil. This study was conducted to determine if foliar SAR‐inducing applications (BTH or harpin) negatively impact the potato root system beneficial rhizosphere microbial populations and activity or influence pathogenic nematode populations. Foliar applications of benzo (1,2,3) thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH) and the microbial protein harpin applied in various combinations, timings, and rates showed no effects on microbial biomass, culturable bacteria, Pseudomonas populations, or N‐mineralization potentials over 2 years. No stimulatory or inhibitory effects on major bacterial populations were observed, indicating that SAR induction does not have a negative effect on general microbial populations or activities. BTH and harpin both reduced the numbers of lesion nematodes (Pratylenchus spp.) by potato harvest. BTH reduced root knot nematodes, Meloidogyne chitwoodi at the end of the season. In addition, BTH and high‐dose harpin (applied at the 4× rate) reduced the nematode infection index in comparison to the control. The SAR elicitors increased the population densities of nontarget free‐living nematodes in the soil compared to the control. Potato yields were not affected by plant elicitors but BTH and harpin both reduced the number of culled potatoes 26% compared to the control. Future studies are designed to determine if these plant elicitors have any direct effect on rhizosphere diversity or if plants with active defense pathways alter carbon flow and root exudates into the soil.

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.

Afforestation of degraded soils with Pinus halepensis Mill.: Effects of inoculation with selected microorganisms and soil amendment on plant growth, rhizospheric microbial activity and ectomycorrhizal formation

Two techniques commonly used in soil restoration: (1) inoculation with selected microbial strains and (2) soil amendment with urban waste, were evaluated for afforestation of a degraded gypsiferous soil with Pinus halepensis Miller. One year after planting, survival of pines was not affected by any of these techniques, reaching values over 97% in all cases. Inoculation with two PGPR strains previously proven to enhance Pinus spp. growth, or with an ectomycorrhizal fungus separately did not affect pine growth or the nutrient content in needles. However, significant increments of pine growth due to soil amendment were detected. Most of the nutrients analysed in needles were found in higher amounts in plants grown in non-amended soil, except nitrogen that was higher in plants grown in the amended soil. The rhizospheric microbial activity was highly increased by soil amendment whereas inoculation with the different microorganisms had no effect on this parameter. The amendment of soil with urban waste drastically reduced the percentages of ectomycorrhizas on the pine roots and affected the frequency of the described ectomycorrhizal morphotypes compared with plants grown in the non-amended soil. Our results suggest that before undertaking a strategy for soil restoration, like plant inoculations with selected microorganisms or soil amendments, the autochthonous beneficial bacterial and fungal communities in the soil should be taken into account, as well as studies for adjusting the doses of amendments, to assure the establishment and development of plants and to keep the ecological equilibrium of soil.

Ammonium versus Nitrate Nutrition of Plants Stimulates Microbial Activity in the Rhizosphere

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NH4 + vs. NO3 ) nutrition (50 or 100 mg kg )1 soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NH4 + as the predominant N source for plants grown in NH4 + -treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NH4 + than under NO3 ) nutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NH4 + than under NO3 – supplied at 150 mg N kg )1 soil; whereas at both N levels applied, NH4 + compared to NO3 ) nutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH4 + vs. NO3 ) nutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NH4 + of the rhizosphere microbial activity was probably due to higher availability of root exudates under NH4 + than under NO3 ) nutrition.

Defoliation alters the relative contributions of recent and non‐recent assimilate to root exudation from Festuca rubra

ABSTRACTThe deposition of organic compounds from plant roots is a key determinant of rhizosphere microbial activity and community structure. Consequently, C‐flow from roots to soil is an important process in coupling plant and microbial productivity, via impacts on microbial nutrient cycling in soil. Experimentally, isotopic tracers (13C or 14C) are used to track C inputs to soil and microbial communities. However, in many such studies the relationship between labelled C‐flows and total C‐flows are not established, limiting the interpretative value of the results. In this study, we applied steady‐state near natural abundance 13CO2 labelling to determine the impact of partial defoliation of Festuca rubra on root exudation. This approach in axenic culture facilitated determination of the contribution of pre‐ and post‐defoliation assimilates both to root C‐flow and plant tissues. The results demonstrated that total root exudation was increased in the two days following defoliation. This was concurrent with reduced net CO2 assimilation and reduced allocation of post‐defoliation assimilates below‐ground and to active root meristems. Through determination of the δ13C of root exudates, it was established that the source of the increased root exudation was pre‐defoliation assimilate. However, this response was transient, with reduced deposition of pre‐ and post‐defoliation assimilates from roots during the period 2–4 d following defoliation. The results highlight the limitations of pulse‐labelling approaches as a means of quantifying impacts of treatments on root exudation, particularly where the treatment is likely to affect plant C‐partitioning or the balance between deposition to, and re‐mobilization from, C‐storage pools.