N2-fixing bacteria are more sensitive to microtopography than nitrogen addition in degraded grassland.

Introduction: Soil bacteria play a crucial role in the terrestrial nitrogen (N) cycle by fixing atmospheric N2, and this process is influenced by both biotic and abiotic factors. The diversity of N2-fixing bacteria (NFB) directly reflects the efficiency of soil N fixation, and the diversity of NFB in degraded alpine meadow soil may change with different N fertilizing levels and varied slopes. However, how N addition affects the diversity of NFB in degraded alpine meadows, and whether this influence varies with slope, remain poorly understood. Methods: We conducted an N addition field experiment at three levels (2, 5, and 10 g N·m-2·a-1) to study the effects of N addition on soil NFB diversity on two different slopes in a degraded meadow on the Tibetan Plateau. Results: There were significant differences in the dominant bacterial species between the two slopes. The Chao1 index, species richness, and beta diversity of NFB did not differ significantly between slopes, but the Shannon index did. Interestingly, N addition had no effect on the diversity of NFB or the abundance of dominant bacteria. However, we did observe a significant change in some low-abundance NFB. The community composition and diversity of NFB were significantly positively correlated with slope and soil physicochemical properties (e.g., total potassium, pH, and total nitrogen). Conclusions: Our study highlights the variation in NFB communities among different slopes in degraded alpine meadows and their resilience to exogenous N addition. Our results also underscore the importance of considering the effects of micro-topography on soil microbial communities in future studies of alpine ecosystems.

Nitric Oxide Detection Using a Chemical Trap Method for Applications in Bacterial Systems.

Plant growth-promoting bacteria (PGPB) can be incorporated in biofertilizer formulations, which promote plant growth in different ways, such as fixing nitrogen and producing phytohormones and nitric oxide (NO). NO is a free radical involved in the growth and defense responses of plants and bacteria. NO detection is vital for further investigation in different agronomically important bacteria. NO production in the presence of KNO3 was evaluated over 1-3 days using eight bacterial strains, quantified by the usual Griess reaction, and monitored by 2,3-diaminonaphthalene (DAN), yielding 2,3-naphthotriazole (NAT), as analyzed by fluorescence spectroscopy, gas chromatography-mass spectrometry, and high-performance liquid chromatography. The Greiss and trapping reaction results showed that Azospirillum brasilense (HM053 and FP2), Rhizobium tropici (Br322), and Gluconacetobacter diazotrophicus (Pal 5) produced the highest NO levels 24 h after inoculation, whereas Nitrospirillum amazonense (Y2) and Herbaspirillum seropedicae (SmR1) showed no NO production. In contrast to the literature, in NFbHP-NH4Cl-lactate culture medium with KNO3, NO trapping led to the recovery of a product with a molecular mass ion of 182 Da, namely, 1,2,3,4-naphthotetrazole (NTT), which contained one more nitrogen atom than the usual NAT product with 169 Da. This strategy allows monitoring and tracking NO production in potential biofertilizing bacteria, providing future opportunities to better understand the mechanisms of bacteria-plant interaction and also to manipulate the amount of NO that will sustain the PGPB.

Bio-Organic Fertilizer: A Green Technology to Reduce Synthetic N and P Fertilizer for Rice Production.

Decomposed organic materials, in combination with plant growth-promoting bacteria (PGPB), are environmentally friendly and reduce synthetic fertilizer use in rice production. A bio-organic fertilizer (BoF) was prepared using kitchen waste (79%), chita-dhan (unfilled rice grain) biochar (15%), rock phosphate (5%), and a consortium of 10 PGPB (1%) to supplement 30% nitrogen and to replace triple superphosphate (TSP) fertilizer in rice production with an improvement of soil health. PGPB were local isolates and identified using 16S ribosomal RNA partial gene sequences as Bacillus mycoides, Proteus sp., Bacillus cereus, Bacillus subtilis, Bacillus pumilus, Paenibacillus polymyxa, and Paenibacillus spp. Isolates could fix N2 by 0.7-1.4 g kg-1, solubilize 0.1-1.2 g kg-1 phosphate, and produce 0.1-40 g kg-1 indoleacetic acid. The performance of BoF was evaluated by 16 field experiments and 18 farmers' field demonstration trials during the year 2017-2020 in different parts of Bangladesh. Performances of BoF were evaluated based on control (T1), full synthetic fertilizer dose of N, P, and K (T2), BoF (2 t ha-1) + 70% N as urea + 100% K as muriate of potash (T3), 70% N as urea + 100% P as TSP + 100% K as muriate of potash (T4), and 2 t ha-1 BoF (T5) treatments. At the research station, average grain yield improved by 10-13% in T3 compared with T2 treatment. Depending on seasons, higher agronomic N use efficiency (19-30%), physiological N use efficiency (8-18%), partial factor productivity (PFP)N (114-150%), recovery efficiency (RE)N (3-31%), N harvest index (HIN) (14-24%), agronomic P use efficiency (22-25%), partial factor productivity of P (9-12%), AREP (15-23%), and HIP (3-6%) were obtained in T3 compared with T2 treatment. Research results were reflected in farmers' field, and significant (P < 0.05) higher plant height, tiller, panicle, grain yield, partial factor productivity of N and P were obtained in the same treatment. Application of BoF improved soil organic carbon by 6-13%, along with an increased number of PGPB as compared with full synthetic fertilizer dose. In conclusion, tested BoF can be considered as a green technology to reduce 30% synthetic N and 100% TSP requirements in rice production with improved soil health.

Effect of soil chemical fertilization on the diversity and composition of the tomato endophytic diazotrophic community at different stages of growth.

The aim of this work was to gain a more comprehensive and perspicacious view of the endophytic diazotrophic community (EDC) of tomato plant bacteria and assess the effects of chemical fertilization and the plant phenologic stage on the status of those microbes. When the EDC of stem and roots from tomato plants grown in a greenhouse with and without exogenous chemical fertilization was examined by pyrosequencing the nifH gene during the growth cycle, a high taxonomic and phylogenetic diversity was observed. The abundant taxa were related to ubiquitous endophytes such as Rhizobium or Burkholderia but also involved anaerobic members usually restricted to flooded plant tissues, such as Clostridium, Geobacter, and Desulfovibrio. The EDC composition appeared to be dynamic during the growth phase of the tomato, with the structure of the community at the early stages of growth displaying major differences from the late stages. Inorganic fertilization negatively affected the diversity and modified the profile of the predominant components of the EDC in the different growth stages. Populations such as Burkholderia and Geobacter plus the Cyanobacteria appeared particularly affected by fertilization.Our work demonstrates an extensive endophytic diazotrophic diversity, suggesting a high potential for nitrogen fixation. The effect of the phenologic stage and inorganic-chemical soil fertilization on the community structure indicated a dynamic community that responded to environmental changes. These findings contribute to a better understanding of endophytic associations that could be helpful in assisting to shape the endomicrobiome that provides essential benefits to crops.

"The Good, the Bad and the Double-Sword" Effects of Microplastics and Their Organic Additives in Marine Bacteria.

Little is known about the direct effects of microplastics (MPs) and their organic additives on marine bacteria, considering their role in the nutrient cycles, e.g., N-cycles through the N2-fixation, or in the microbial food web. To fill this gap of knowledge, we exposed marine bacteria, specifically diazotrophs, to pure MPs which differ in physical properties (e.g., density, hydrophobicity, and/or size), namely, polyethylene, polypropylene, polyvinyl chloride and polystyrene, and to their most abundant associated organic additives (e.g., fluoranthene, 1,2,5,6,9,10-hexabromocyclododecane and dioctyl-phthalate). Growth, protein overproduction, direct physical interactions between MPs and bacteria, phosphorus acquisition mechanisms and/or N2-fixation rates were evaluated. Cyanobacteria were positively affected by environmental and high concentrations of MPs, as opposed to heterotrophic strains, that were only positively affected with high concentrations of ~120 µm-size MPs (detecting the overproduction of proteins related to plastic degradation and C-transport), and negatively affected by 1 µm-size PS beads. Generally, the organic additives had a deleterious effect in both autotrophic and heterotrophic bacteria and the magnitude of the effect is suggested to be dependent on bacterial size. Our results show species-specific responses of the autotrophic and heterotrophic bacteria tested and the responses (beneficial: the "good," deleterious: the "bad" and/or both: the "double-sword") were dependent on the type and concentration of MPs and additives. This suggests the need to determine the threshold levels of MPs and additives concentrations starting from which significant effects can be observed for key microbial populations in marine systems, and these data are necessary for effective environmental quality control management.

Stable Isotope-Labeled Single-Cell Raman Spectroscopy Revealing Function and Activity of Environmental Microbes.

Microorganisms play a key role in driving the global element (C, N, H, P, and S) cycling. However, the function and activity of environmental microbes remain largely elusive because the vast majority of them are yet uncultured. Recent achievements in single cell stable isotope-labeled Raman spectroscopy enable direct investigation of function and activity of individual microbes in complex environmental communities. Here, this protocol describes a workflow to investigate environmental microbes in soil and water by combining 15N, 2D, and 13C stable isotope labeling with different single-cell Raman techniques, including normal Raman, resonance Raman (RR), and surface-enhanced Raman spectroscopy (SERS). Their applications in investigating functional bacteria driving the N and C cycles, and metabolically active cells are described.

Integrated Effects of Co-Inoculation with Phosphate-Solubilizing Bacteria and N2-Fixing Bacteria on Microbial Population and Soil Amendment Under C Deficiency.

The inoculation of beneficial microorganisms to improve plant growth and soil properties is a promising strategy in the soil amendment. However, the effects of co-inoculation with phosphate-solubilizing bacteria (PSB) and N2-fixing bacteria (NFB) on the soil properties of typical C-deficient soil remain unclear. Based on a controlled experiment and a pot experiment, we examined the effects of PSB (M: Bacillus megaterium and F: Pseudomonas fluorescens), NFB (C: Azotobacter chroococcum and B: Azospirillum brasilence), and combined PSB and NFB treatments on C, N, P availability, and enzyme activities in sterilized soil, as well as the growth of Cyclocarya Paliurus seedlings grow in unsterilized soil. During a 60-day culture, prominent increases in soil inorganic N and available P contents were detected after bacteria additions. Three patterns were observed for different additions according to the dynamic bacterial growth. Synergistic effects between NFB and PSB were obvious, co-inoculations with NFB enhanced the accumulation of available P. However, decreases in soil available P and N were observed on the 60th day, which was induced by the decreases in bacterial quantities under C deficiency. Besides, co-inoculations with PSB and NFB resulted in greater performance in plant growth promotion. Aimed at amending soil with a C supply shortage, combined PSB and NFB treatments are more appropriate for practical fertilization at intervals of 30-45 days. The results demonstrate that co-inoculations could have synergistic interactions during culture and application, which may help with understanding the possible mechanism of soil amendment driven by microorganisms under C deficiency, thereby providing an alternative option for amending such soil.

Respuesta de Fragaria mexicana y comunidades microbianas rizosféricas al aumento de temperature / Response of Fragaria Mexicana and rhizophere microbial communitites to temperature increase

Resumen Para los próximos 100 años se pronostica un incremento en la temperatura del planeta de casi 4 °C, lo cual pondrá en riesgo las especies que no logren adaptarse. En esta investigación se determinaron las respuestas morfofisiológicas de F. mexicana y los cambios en la población de bacterias fijadoras de nitrógeno atmosférico (BFN) asociadas a sus raíces, debido a dos condiciones ambientales con diferencias medias de temperatura de 5.1 oC: invernadero (temperatura alta, TA) y campo abierto (temperatura baja, TB); y con cuatro tratamientos: sin inocular (T), inoculadas con la cepa ocho (CP8), cepa cuatro (CP4) y con ambas cepas (CP8 + 4). Las BFN fueron aisladas de la rizosfera de F. mexicana y transformadas genéticamente con reporteros, para cuantificar la población al final del experimento. Se midió el peso seco de la parte aérea y la raíz, la tasa de asimilación de CO2 (A), el inicio de la floración, el número de flores y frutos; y la persistencia de las bacterias fijadoras de N atmosférico (BFN). Además, se evaluó la concentración de NO3, NH4, P y materia orgánica (MO) en el sustrato, al inicio y final del experimento. Las plantas sometidas a alta temperatura presentaron mayor peso seco de la parte aérea y fotosíntesis; con una disminución en el peso seco de la raíz, y en el número de flores y frutos. La MO en el sustrato disminuyó, mientras que la disponibilidad de NO3, NH4 y P aumentó. El incremento de temperatura y la mayor presencia de N en el substrato provocaron reducción en la población de BFN. Estos resultados sugieren que temperaturas altas estimulan el crecimiento de F. mexicana y tienen un impacto negativo sobre su reproducción y en las BFN asociadas a sus raíces.(AU)

Abstract The earth could experience a warming of 4 °C in the next one hundred years. This would put at risk the plants that can´t adapt. Fragaria mexicana is an endemic plant of temperate forest of Mexico. The response of this wild strawberry to temperature increasing has not been studied and could play an important role for event of global warming. This study determinate the morphological and physiological responses of F. mexicana and changes in the N2-fixing bacteria (BFN) population on its roots, due to two environmental conditions with differences of temperature 5.1 °C: greenhouse (high temperature, TA) and open land (low temperature, TB); and with four free-living nitrogen-fixing bacteria (BFN) treatments: non-inoculated (T), inoculated with strain eight (CP8), strain four (CP4), and strains eight and four mixed (CP8 + 4). BFN were isolated from the rhizosphere of F. mexicana and were genetically transformed with reporters to quantify the population at the end of the experiment. NO3, NH4 and P and organic matter (MO) in the substrate were determined at beginning and finish of the experiment. Shoot and root dry weight, photosynthetic rate, flowering and fructification starting, flowers and fruits number, were measured. Shoot dry weight and photosynthetic rate were lower in TB than TA, decreasing 3.1 g and 0.94 µmol m-2 s-1, respectively. Root dry weight was 3.0 g less in TA compared with TB. Number flowers decrease in 40.89 % and number fruits in 38.11 % on TA than TB. F. mexicana plants start flowering 14 days previously in TB than TA. MO in the substrate decrease in TA, while the concentration of NO3, NH4 and P, increased. Population of BFN was lower in TA. Results obtained indicated that higher temperature promotes the growth of F. mexicana and reduce its reproduction and BNF population associated with its roots.(AU)

Elevated level of arsenic negatively influences nifH gene expression of isolated soil bacteria in culture condition as well as soil system.

Comprehensive studies on the effect of arsenic (As) on free-living diazotrophs that play a crucial role in soil fertility by nitrogen fixation are still scanty. Here, we isolated three free-living bacteria from rice field with potential nitrogen-fixing ability and investigated the impact of As on their nifH gene expression and extracellular polysaccharide (EPS) production in culture condition and soil system. 16S rRNA sequence analysis showed that the isolated bacteria were affiliated to ß-Proteobacteria, γ-Proteobacteria and Firmicutes. As(III) exposure to bacterial isolates followed by RT-qPCR analysis revealed that elevated levels of As reduced the expression of nifH gene in selective bacteria, both in culture medium and soil condition. We also noticed reduced production of EPS under higher concentration of As. All the three bacteria showed high tolerance to As(III), able to oxidize As and exhibited significant plant growth-promoting traits. This investigation indicated that an environment exposed with higher concentration of As might perturbed the activity of free-living diazotrophs in agricultural soil system.

Putative Nitrogen-Fixing Bacteria Associated With the Rhizosphere and Root Endosphere of Wheat Plants Grown in an Andisol From Southern Chile.

Acidic ash derived volcanic soils (Andisols) support 50% of cereal production in Chile. Nitrogen (N) is essential for cereal crops and commonly added as urea with consequent environmental concerns due to leaching. Despite the relevance of N to plant growth, few studies have focused on understanding the application, management and ecological role of N2-fixing bacterial populations as tool for improve the N nutrition of cereal crops in Chile. It is known that N2-fixing bacteria commonly inhabits diverse plant compartments (e.g., rhizosphere and root endosphere) where they can supply N for plant growth. Here, we used culture-independent and dependent approaches to characterize and compare the putative N2-fixing bacteria associated with the rhizosphere and root endosphere of wheat plants grown in an Andisol from southern Chile. Our results showed significantly greater bacterial loads in the rhizosphere than the root endosphere. Quantitative PCR results indicated that the copy number of the 16S rRNA gene ranged from 1012~1013 and 107~108 g-1 sample in rhizosphere and root endosphere, respectively. The nifH gene copy number ranged from 105~106 and 105 g-1 sample in rhizosphere and root endosphere, respectively. The total culturable bacteria number ranged from 109~1010 and 107~108 CFU g-1 sample in rhizosphere and 104~105 and 104 CFU g-1 sample in root endosphere using LB and NM-1 media, respectively. Indirect counts of putative N2-fixing bacteria were 103 and 102~103 CFU g-1 sample in rhizosphere and root endosphere using NFb medium, respectively. Sequencing of 16S rRNA genes from randomly selected putative N2-fixing bacteria revealed the presence of members of Proteobacteria (Bosea and Roseomonas), Actinobacteria (Georgenia, Mycobacterium, Microbacterium, Leifsonia, and Arthrobacter), Bacteroidetes (Chitinophaga) and Firmicutes (Bacillus and Psychrobacillus) taxa. Differences in 16S rRNA and putative nifH-containing bacterial communities between rhizosphere and root endosphere were shown by denaturing gradient gel electrophoresis (DGGE). This study shows a compartmentalization between rhizosphere and root endosphere for both the abundance and diversity of total (16S rRNA) and putative N2-fixing bacterial communities on wheat plants grown in Chilean Andisols. This information can be relevant for the design and application of agronomic strategies to enhance sustainable N-utilization in cereal crops in Chile.

Microbial assay of N2 fixation rate, a simple alternate for acetylene reduction assay.

Nitrogen is an essential element for living creatures in every ecosystem but nitrogen cannot be absorbed by the plant itself directly from the atmosphere, so for nitrogen, plant depends on both free living and symbiotic microbes present in the soil. Nitrogen fixation potentiality of the soil thus reveals its fertility with respect to nitrogen. Researchers developed and modified techniques for measuring nitrogen fixation rate of the soil and acetylene reduction assay (ARA) technique became the most popular till now. At the same time this technique has few limitations especially for the researchers from third world country due to lack of special infrastructure in the laboratory and the most required instrument for this technique, gas chromatograph machine, is very expensive. Any alternation of this technique is deserved highly for the researchers from the developing countries. The present work/method explained a new approach for determination of nitrogen fixation rate and this new method was named as "Microbial bio-assay". In this technique nitrogen fixers were cultured in specific medium and condition and after required time of interval the amount of nitrogen fixed by them were calculated. Exploration of soil of the Sundarban mangrove ecosystem was performed regarding the microbial N2 fixing capacity of that particular ecosystem. •The nitrogen fixation rate measured by acetylene reduction assay (ARA) was 1.13 times lower than the N2 fixation rate measured by microbial bio-assay.•Microbial bio-assay can be used as an alternate of ARA method to measure N2 fixation rate. The rates of N2 fixation measured by both two methods were positively correlated with the population of N2 fixing bacteria present in the soil of that particular ecosystem (R2 = 0.85, p < 0.005, n = 85 for microbial bio-assay and R2 = 0.78, p < 0.005, n = 85 for ARA).

Nitrogen fixing bacterial diversity in a tropical estuarine sediments.

Microorganisms play a significant role in biogeochemical cycles, especially in the benthic and pelagic ecosystems. Role of environmental parameters in regulating the diversity, distribution and physiology of these microorganisms in tropical marine environment is not well understood. In this study, we have identified dinitrogen (N2) fixing bacterial communities in the sediments by constructing clone libraries of nitrogenase (nifH) gene from four different stations in the Cochin estuary, along the southeastern Arabian Sea. N2 fixing bacterial clones revealed that over 20 putative diazotrophs belong to alpha-, beta-, gamma-, delta- and epsilon- proteobacteria and firmicutes. Predominant genera among these were Bradyrhizobium sp. (α-proteobacteria), Dechloromonas sp. (ß-proteobacteria); Azotobactor sp., Teredinibacter sp., Methylobacter sp., Rheinheimera sp. and Marinobacterium sp. (γ-proteobacteria); Desulfobacter sp., Desulfobulbus sp. and Desulfovibrio sp. (δ -proteobacteria); Arcobacter sp. and Sulfurospirillum sp. (ε-proteobacteria). Nostoc sp. was solely identified among the cyanobacterial phylotype. Nitrogen fixing Sulfate reducing bacteria (SRBs) such as Desulfobulbus sp., Desulfovibrio sp., Desulfuromonas sp., Desulfosporosinus sp., Desulfobacter sp., were also observed in the study. Most of the bacterial nifH sequences revealed that the identities of N2 fixing bacteria were less than 95% similar to that available in the GenBank database, which suggested that the sequences were of novel N2 fixing microorganisms. Shannon-Weiner diversity index of nifH gene ranged from 2.95 to 3.61, indicating an inflated diversity of N2 fixing bacteria. Canonical correspondence analysis (CCA) implied positive correlation among nifH diversity, N2 fixation rate and other environmental variables.

Formononetin associated with phosphorus influences soybean symbiosis with mycorrhizal fungi and Bradyrhizobium / Formononetina associada com fósforo influencia a simbiose de soja com fungos micorrízicos e Bradyrhizobium

Arbuscular mycorrhizal fungi play an important role on nutrient supply to plants, specially P. However, the availability of inoculants for large-scale usage in agriculture is still limited because these organisms are obligatory symbionts. The use of symbiosis stimulants such as flavonoids can be an alternative to improve the beneficial effects of mycorrhiza for plant nutrition. The aim of this study was to evaluate the effect of the isoflavonoid biostimulant formononetin (7-hydroxy, 4'-methoxy isoflavone) in combination with three levels of phosphorus fertilization on mycorrhizal colonization, nodulation, and productivity of soybean, under field conditions. A 3 x 4 factorial scheme (levels of P: 0, 60 and 120 kg ha-1 P2O5 and doses of formononetin: 0, 25, 50 and 100 g ha-1) was used with five replicates. The following parameters were quantified at full bloom: plant height, shoot dry weight, nodule number, nodule dry weight, mycorrhizal colonization, and shoot N and P concentrations. Productivity was also evaluated at the end of the crop cycle. Formononetin stimulated mycorrhizal colonization at lower levels of P (0 and 60 kg ha-1), with colonization increasing from 56 to 64%. When applied with 60 kg ha-1 P2O5, formononetin increased soybean productivity to values similar to those observed when 120 kg ha-1 de P2O5, was applied. At doses above 50 g ha-1, formononetin applied to the seeds can reduce the need of P fertilization by 50%.

Os Fungos micorrízicos arbusculares desempenham papel importante no fornecimento de nutrientes para as plantas, especialmente P. No entanto, a disponibilidade de inoculantes com esses fungos, para o uso em larga escala na agricultura é ainda limitada, porque estes organismos são simbiontes obrigatórios. O uso de estimulantes simbióticos, como os flavonóides, podem ser uma alternativa para melhorar os efeitos benéficos da micorrrização na nutrição das plantas. O objetivo neste estudo foi avaliar o efeito do isoflavonóide bioestimulante formononetina (7-hidroxi, 4'-metoxi isoflavona) em combinação com três níveis de adubação fosfatada sobre a colonização micorrízica, a nodulação e a produtividade da soja, em condições de campo. Um esquema fatorial 3 x 4 (níveis de P: 0, 60 e 120 kg ha-1 de P2O5 e doses de formononetina: 0, 25, 50 e 100 g ha-1) foi utilizado, com cinco repetições. Os seguintes parâmetros foram quantificados em plena floração: altura da planta, matéria seca da parte aérea, número e matéria seca de nódulos, colonização micorrízica, e concentrações de N e P na parte aérea das plantas. A produtividade também foi avaliada no final do ciclo da cultura. A Formononetina estimulou a colonização micorrízica em níveis mais baixos de P (0 e 60 kg ha- 1), com aumentos de 56-64%. Quando aplicado com 60 kg ha-1 de P2O5, a formononetina aumentou a produtividade da soja, alcançando valores semelhantes aos observados quando foi aplicado 120 kg ha-1 de P2O5. Em doses acima de 50 g ha- 1, a formononetina aplicada na semente pode reduzir a necessidade de fertilização fosfatada em 50%.

Effect of combined microbes on plant tolerance to Zn-Pb contaminations.

The presence and composition of soil microbial communities has been shown to have a large impact on plant-plant interactions and consequently plant diversity and composition. The goal of the present study was to evaluate impact of arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria, which constitutes an essential link between the soil and the plant's roots. A greenhouse pot experiment was conducted to evaluate the feasibility of using selected microbes to improve Hieracium pilosella and Medicago sativa growth on Zn-Pb-rich site. Results of studies revealed that biomass, the dry mass of shoots and roots, increased significantly when plants were inoculated with mycorrhizal fungi and nitrogen-fixing bacteria. The addition of Azospirillum sp. and Nostoc edaphicum without mycorrhiza suppressed plant growth. Single bacterial inoculation alone does not have a positive effect on M. sativa growth, while co-inoculation with AMF improved plant growth. Plant vitality (expressed by the performance index) was improved by the addition of microbes. However, our results indicated that even dry heat sterilization of the substratum created imbalanced relationships between soil-plant and plants and associated microorganisms. The studies indicated that AMF and N2-fixers can improve revegetation of heavy metal-rich industrial sites, if the selection of interacting symbionts is properly conducted.

INFLUENCE OF EXUDATES OF AMMONIUM-RESISTANT N_2 -FIXING BACTERIA ON THE GROWTH OF RICE SEEDLING / 微生物学通报

By using cup culture, influence of different strains (ammonium-resistant N2-fixing type, wild type, none N2-fixing type) of Klebsiella oxytoca on the growth of rice seedling was compared. It was discovered that ammonium-resistant N2 -fixing bacteria could excrete some plant growth promoting substance, which could be adsorbed by cation resin. It' s activity wouldn't be affected at 80℃. At optimal concentration, the weight of rice root and seedling were increased by