Bacterial interactions in the rhizosphere of seagrass communities in shallow coastal lagoons

Rooted phanerogam communities in the shallow intertidal and subtidal coastal zone represent productive and healthy ecosystems. Inorganic nutrients are assimilated into seagrass biomass. Much of the organic matter resulting from moribund seagrass is rapidly mineralized, principally by bacteria. The microbial community of the rhizosphere is also highly active due to the supply of organic matter released during photosynthesis. This active sediment community plays an important role through carbon, nitrogen and phosphorous cycling in maintaining the stability and productivity of seagrass meadows. Over the last two decades, however, seagrass meadows in European coastal areas have declined due to increasing pollution. As eutrophication advances a trasition occurs from rooted phanerogram dominated communities to planktonic algal blooms and/or cyanobacterial blooms. Such changes represent the decline of a stable, high biodiversity habitat to an unstable one dominated by a few species. These changes of community structure can occur rapidly once the internal nutrient and organic matter control cycles are exceeded. A field investigation was undertaken to establish the spatial distribution of bacterial populations of Zostera noltii colonized and uncolonized sediment in the Bassin d'Arcachon, France. Bacteria were enumerated using both plate count and MPN techniques for different functional groups as well as determining the total bacterial populations present. Nitrogen fixation, ammonification, sulphate reduction rates, as well as alkaline phosphatase activity were also determined. Colonization of the Z. noltii roots and rhizomes was studied by light and scanning electron microscopy. Results confirmed that higher bacterial populations were present in the rhizosphere of Z. noltii compared to uncolonized sediments. Furthermore, electron microscopy identified the rhizome as the main site of colonization for a diverse range of morphological groups of bacteria. Sulphate reducing bacteria were identified as the key group of bacteria involved in N-fixation in the rhizosphere of Z. noltii. The data will be discussed in relation to the role played by the rhizosphere microflora in supplying and mobilising nutrients in Z. noltii.     

Seasonal variation in rates of heterotrophic nitrogen fixation (acetylene reduction) in Zostera noltii meadows and uncolonised sediments of the Bassin d'Arcachon, south-west France

Nitrogen fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Z. noltii and uncolonised sediments of the Bassin d'Arcachon, south-west France, using both slurry and whole core techniques. Measured rates using the slurry technique in Z. noltii colonised sediments were consistently higher than those determined in isolated cores. This was probably due to the release of labile organic carbon sources during preparation of the slurries. Thus, in colonised sediments the whole core technique may provide a more accurate estimate of in situ activity. Acetylene reduction rates measured by the whole core technique in colonised sediments were 1.8 to 4-fold greater, dependent upon the season, in the light compared with those measured in the dark, indicating that organic carbon released by the plant roots during photosynthesis was an important factor regulating nitrogen fixation. In contrast acetylene reduction rates in uncolonised sediments were independent of light.Addition of sodium molybdate, a specific inhibitor of sulphate reduction inhibited acetylene reduction activity in Z. noltii colonised sediments by > 80% as measured by both slurry and whole core techniques irrespective of the light regime, throughout the year inferring that sulphate reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. A mutualistic relationship between Z. noltii and nitrogen fixing SRB in the rhizosphere, based on the exchange of organic carbon and fixed nitrogen is proposed. In uncolonised sediments sodium molybdate initially severely inhibited acetylene reduction rates, but the level of this inhibition declined over the course of the year. These data indicate that the nitrogen fixing SRB associated with the Zostera roots and rhizomes were progressively replaced by an aerobic population of nitrogen fixers associated with the decomposition of this recalcitrant high C:N ratio organic matter.Acetylene and sulphate reduction rates in the seagrass beds showed distinct summer maxima which correlated with a reduced availability of NH ₄ ⁺ in the sediment and the growth cycle of Z. noltii in the Bassin. Overall, these data indicate that acetylene reduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by release of organic carbon from the plant roots and maintenance of low ammonium concentrations in the root zone due to efficient ammonium assimilation.Nitrogen fixation rates determined from acetylene reduction rates measured by the whole core technique ranged from 0.1 to 7.3 mg N m^–2 d^–1 in the Z. noltii beds and between 0.02 and 3.7 mg N m^–2 d^–1 in uncolonised sediments, dependent upon the season. Nitrogen fixation in the rhizosphere of Z. noltii was calculated to contribute between 0.4 and 1.1 g N m^–2 y^–1 or between 6.3 and 12% of the annual fixed nitrogen requirement of the plants. Heterotrophic nitrogen fixation therefore represents a substantial local input of fixed nitrogen to the sediments of this shallow coastal lagoon and contributes to the overall productivity of Z. noltii in this ecosystem.     

Evidence for NH4+ switch-off regulation of nitrogenase activity by bacteria in salt marsh sediments and roots of the grass Spartina alterniflora

The regulatory effect of NH4+ on nitrogen fixation in a Spartina alterniflora salt marsh was examined. Acetylene reduction activity (ARA) measured in situ was only partially inhibited by NH4+ in both the light and dark after 2 h. In vitro analysis of bulk sediment divided into sediment particles, live and dead roots, and rhizomes showed that microbes associated with sediment and dead roots have a great potential for anaerobic C2H2 reduction, but only if amended with a carbon source such as mannose. Only live roots had significant rates of ARA without an added carbon source. In sediment, N2-fixing mannose enrichment cultures could be distinguished from those enriched by lactate in that only the latter were rapidly inhibited by NH4+. Ammonia also inhibited ARA in dead and live roots and in surface-sterilized roots. The rate of this inhibition appeared to be too rapid to be attributed to the repression and subsequent dilution of nitrogenase. The kinetic characteristics of this inhibition and its prevention in root-associated microbes by methionine sulfoximine are consistent with the NH4+ switch-off-switch-on mechanism of nitrogenase regulation.     

Evidence for NH4+ switch-off regulation of nitrogenase activity by bacteria in salt marsh sediments and roots of the grass Spartina alterniflora.

The regulatory effect of NH4+ on nitrogen fixation in a Spartina alterniflora salt marsh was examined. Acetylene reduction activity (ARA) measured in situ was only partially inhibited by NH4+ in both the light and dark after 2 h. In vitro analysis of bulk sediment divided into sediment particles, live and dead roots, and rhizomes showed that microbes associated with sediment and dead roots have a great potential for anaerobic C2H2 reduction, but only if amended with a carbon source such as mannose. Only live roots had significant rates of ARA without an added carbon source. In sediment, N2-fixing mannose enrichment cultures could be distinguished from those enriched by lactate in that only the latter were rapidly inhibited by NH4+. Ammonia also inhibited ARA in dead and live roots and in surface-sterilized roots. The rate of this inhibition appeared to be too rapid to be attributed to the repression and subsequent dilution of nitrogenase. The kinetic characteristics of this inhibition and its prevention in root-associated microbes by methionine sulfoximine are consistent with the NH4+ switch-off-switch-on mechanism of nitrogenase regulation.     

Nitrogen fixation in seagrass meadows: Regulation, plant–bacteria interactions and significance to primary productivity

The rhizosphere sediments of seagrasses are generally a site of intense nitrogen fixation activity and this can provide a significant source of "new" nitrogen for the growth of the plants. In this paper, I review the data concerning nitrogen fixation in seagrass ecosystems, the transfer of the fixed nitrogen from the bacteria to the plants and its contribution to the overall productivity of seagrasses in different climatic zones.
The relationship between the plants and diazotrophic heterotrophic bacteria in the rhizosphere is discussed, particularly focusing on the potentially important role of nitrogen-fixing, sulphate-reducing bacteria. The regulation of nitrogen fixation rates in the rhizosphere by photosynthetically driven oxygen and fixed carbon release by the plant roots and rhizomes, and the availability of ammonium in the porewater, is assessed. Finally, the hypothesis that a mutualistic or symbiotic association exists between the seagrasses and heterotrophic nitrogen fixers in the rhizosphere, based on the mutual exchange of fixed carbon and nitrogen, is discussed.     

NITROGEN FIXATION ASSOCIATED WITH NATURAL AND CULTIVATED STANDS OF TYPHA LATIFOLIA L. (TYPHACEAE)

Acetylene reduction by bacteria associated with Typha latifolia L. roots and rhizomes was studied in the field and in the laboratory. In situ studies indicated that the rate of acetylene reduction in a natural cattail population was ca. 4-fold higher than in a cultivated cattail stand. Nitrogenase activity was found to occur principally in the rhizosphere of roots and rhizomes with the greatest activity occurring in association with mature roots. Scanning electron and light microscopy, and 2, 3, 5 triphenyltetrazolium chloride reduction showed that bacterial associations were limited to the rhizoplane of this angiosperm. One diazotrophic bacterial genus was found to be associated with roots and rhizomes and was identified as the facultative anaerobe Bacillus. Contribution of free-living cyanobacteria to total nitrogen fixation in the natural stand was negligible. Calculations show that a natural stand of cattails may fix 18 kg nitrogen ha^−-1 yr^−-1 or ca. 8.2% of the total nitrogen present in the standing crop.     

Nitrogen Fixation Associated with Rinsed Roots and Rhizomes of the Eelgrass Zostera marina

Nitrogen fixation was associated with the rinsed roots and rhizomes of the seagrass, Zostera marina L. Nitrogenase activity (acetylene reduction) was greater on rhizomes compared to roots, and on older roots and rhizomes relative to younger tissue. Compared to aerobic assays, anaerobic or microaerobic conditions enhanced the rate of acetylene reduction by rhizomes with attached roots, with the highest activity (100 nanomoles per gram dry weight per hour) occurring at pO₂ = 0.01 atmosphere. Addition of glucose, sucrose, or succinate also increased the rate of acetylene reduction under anaerobic conditions, with glucose providing the most stimulation. In one experiment, comparison of acetylene reduction assays with ¹⁵N₂ incorporation yielded a ratio of about 2.6:1. Seagrass communities are thought to be limited by the availability of nitrogen and, therefore, nitrogenase activity directly associated with their roots and rhizomes suggests the possibility of a N₂-fixing flora which may subsidize their nutritional demand for nitrogen.     

Impact of sampling methods on sulfate reduction rates and dissolved organic carbon (DOC) concentrations in vegetated salt marsh sediments

The impact of sediment coring on measured rates of sulfate reduction(SRR) by the whole core ³⁵S-injection technique was assessed inmarshsediment vegetated by Spartina anglica. Simultaneously,therole of extraction method (centrifugation vs. sippers) for determination ofporewater DOC in vegetated sediment was evaluated. SRR was measuredinsitu with radiotracer injected directly into the sediment and in atime series from 1 to 24 h after coring. SRR incubations carriedout within 6 h (June) or 12 h (August) of coringyielded up to an order of magnitude higher rates than measured insitu. The enhancement of SRR was instantaneous but temporary andcorrelated with measured porewater DOC concentrations. Cores sampled fromrootedsediments should therefore not be used for sulfate reduction incubations withinthe first 12 h due to the effect of DOC leaching from roots cutduring the coring procedure. The labile fraction of leached DOC appears to beexhausted after a pre-incubation period of at least 12 h.Measurement of porewater DOC is also problematic in vegetated sediment.Porewater extraction by centrifugation of sediment may result in up to oneorderof magnitude higher DOC concentrations than in porewater obtained by anondestructive sipper technique. DOC is probably forced out of roots duringcentrifugation resulting in erroneously high porewater DOC concentrations.     

The formation of iron plaques on roots and rhizomes of the seagrass Cymodocea serrulata (R. Brown) Ascherson with implications for sulphide intrusion

Despite the fact that iron plaque formation is ubiquitous in aquatic macrophytes and has been known for several decades, there are few reports of plaque occurrence in seagrasses to date. Herein we present the first microscopical observation and chemical quantification of iron (Fe) plaques on the shoots, rhizomes and roots of the seagrass Cymodocea serrulata (R. Brown) Ascherson collected from intertidal seagrass beds in Thailand. Plaques were observed on shoot bases, rhizomes and roots with the highest concentrations of iron in the plaques from the roots, reaching an average of 509 μmol gDW⁻¹. Interestingly, the most negative stable sulphur isotope (δ³⁴S) values, indicating H₂S intrusion into the plants occurred in the sampling site with the most intense root oxidizing capacity, as indicated by a greater Fe plaque formation. These apparently contradictory findings may be attributed to oxidizing capacity of root tips and root hairs sufficient to promote Fe(III) deposition in the rhizosphere, preceding deposition of plaques on the roots. While this rhizosphere oxidation may result in a more efficient sulphide detoxification during the day photosynthetic phase, root tips and hairs may serve as vulnerable sites for sulphide intrusion at night. The presence of Fe plaque on C. serrulata roots and rhizomes reveals the complexity of seagrass–sediment interactions and deserves further attention to understand if this is a local phenomenon or a newly discovered adaptive mechanism in seagrasses.     

Ferric iron reduction by bacteria associated with the roots of freshwater and marine macrophytes.

In vitro assays of washed, excised roots revealed maximum potential ferric iron reduction rates of >100 micromol g (dry weight)(-1) day(-1) for three freshwater macrophytes and rates between 15 and 83 micromol (dry weight)(-1) day(-1) for two marine species. The rates varied with root morphology but not consistently (fine root activity exceeded smooth root activity in some but not all cases). Sodium molybdate added at final concentrations of 0.2 to 20 mM did not inhibit iron reduction by roots of marine macrophytes (Spartina alterniflora and Zostera marina). Roots of a freshwater macrophyte, Sparganium eurycarpum, that were incubated with an analog of humic acid precursors, anthroquinone disulfate (AQDS), reduced freshly precipitated iron oxyhydroxide contained in dialysis bags that excluded solutes with molecular weights of >1,000; no reduction occurred in the absence of AQDS. Bacterial enrichment cultures and isolates from freshwater and marine roots used a variety of carbon and energy sources (e.g., acetate, ethanol, succinate, toluene, and yeast extract) and ferric oxyhydroxide, ferric citrate, uranate, and AQDS as terminal electron acceptors. The temperature optima for a freshwater isolate and a marine isolate were equivalent (approximately 32 degrees C). However, iron reduction by the freshwater isolate decreased with increasing salinity, while reduction by the marine isolate displayed a relatively broad optimum salinity between 20 and 35 ppt. Our results suggest that by participating in an active iron cycle and perhaps by reducing humic acids, iron reducers in the rhizoplane of aquatic macrophytes limit organic availability to other heterotrophs (including methanogens) in the rhizosphere and bulk sediments.     

Relationship between porewater organic carbon content, sulphate reduction and nitrogen fixation (acetylene reduction) in the rhizosphere of Zostera noltii

Depth profiles of nitrogen fixation (acetylene reduction), sulphate reduction, NH ₄ ⁺ concentration and porewater volatile fatty acids concentrations were measured in Zostera noltii colonised sediments in the Bassin d'Arcachon, France in March 1994. Acetylene reduction activity (ARA) was detectable throughout sediment profiles. Addition of sodium molybdate (20 mmol l^–1) a specific inhibitor of sulphate reduction to slurries inhibited ARA by >75% inferring that sulphate-reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. The peak of ARA was coincident with that of sulphate reduction and a relatively constant relationship of 40 mole sulphate reduced per mole acetylene reduced was recorded throughout the profiles. From this ratio it was calculated that at least 17% of the ATP yield from sulphate reduction would be required to support the measured rates of nitrogen fixation (acetylene reduction).Acetate was the dominant constituent of the porewater volatile fatty acids pool, accounting for >90% of the total pool as measured by HPLC. Concentrations of porewater acetate recorded by HPLC were compared with those measured using an enzymatic technique and these data indicate that approximately 10% of the total porewater acetate pool was not available to microbial metabolism. Profiles of porewater acetate concentrations measured by both techniques were similar to those recorded for both ARA and sulphate reduction and thus acetate oxidation may fuel these activities.     

Nitrogen‐dependent bacterial community shifts in root,rhizome and rhizosphere of nutrient‐efficient Miscanthus x giganteus from long‐term field trials

The perennial energy crop Miscanthus × giganteus is recognized for its extraordinary nitrogen‐use efficiency. While the remobilization of nitrogen (N) to the rhizome after the growth phase contributes to this efficiency, the plant‐associated microbiome might also contribute, as N‐fixing bacterial species had been isolated from this grass. Here, we studied established Miscanthus × giganteus plots in southern Germany that either received 80 kg N ha^?1 a^?1 or that were not N‐fertilized for 14 years. The bacterial communities of the bulk soil, rhizosphere, roots and rhizomes were analysed. Major differences were encountered between plant‐associated fractions. Nitrogen had little effect on soil communities. The roots and rhizomes showed less microbial diversity than soil fractions. In these compartments, Actinobacteria and N‐fixing symbiosis‐associated Proteobacteria depended on N. Intriguingly, N₂‐fixing‐related bacterial families were enriched in the rhizomes in long‐term zero N plots, while denitrifier‐related families were depleted. These findings point to the rhizome as a potentially interesting plant organ for N fixation and demonstrate long‐term differences in the organ‐specific bacterial communities associated with different N supply, which are mainly shaped by the plant.     

Measured and modelled differences in nutrient concentrations between rhizosphere and bulk soil in a Norway spruce stand

The gradient in soil characteristics from the bulk soil to the root surface is important to roots and to the organisms that live in the rhizosphere. Our ability to measure ion concentrations at the root surface is extremely limited, and models are largely untested. We used data from a well studied Norway spruce stand in SW Sweden to compare the measured difference in nutrient concentrations between rhizosphere and bulk soil with the difference predicted by a steady-state simulation model based on ecosystem budgets of nutrient uptake. The simulation model predicted depletion of NH₄, Ca, Mg, K in the rhizosphere, which shows that budgeted uptake rates were greater than the mass flow of bulk solution towards the root. In plots treated with ammonium sulphate, the model predicted an accumulation of S in the rhizosphere. In contrast, the observed rhizosphere concentrations were generally enriched in nutrients, relative to bulk soil. Collecting rhizosphere soil adhering to root surfaces may not be an appropriate method for describing the concentration gradient around the root. In addition, the simulation model omits some processes affecting conditions in the rhizosphere that are important to explaining nutrient uptake.     

Ferric Iron Reduction by Bacteria Associated with the Roots of Freshwater and Marine Macrophytes

In vitro assays of washed, excised roots revealed maximum potential ferric iron reduction rates of >100 μmol g (dry weight)⁻¹ day⁻¹ for three freshwater macrophytes and rates between 15 and 83 μmol (dry weight)⁻¹ day⁻¹ for two marine species. The rates varied with root morphology but not consistently (fine root activity exceeded smooth root activity in some but not all cases). Sodium molybdate added at final concentrations of 0.2 to 20 mM did not inhibit iron reduction by roots of marine macrophytes (Spartina alterniflora and Zostera marina). Roots of a freshwater macrophyte, Sparganium eurycarpum, that were incubated with an analog of humic acid precursors, anthroquinone disulfate (AQDS), reduced freshly precipitated iron oxyhydroxide contained in dialysis bags that excluded solutes with molecular weights of >1,000; no reduction occurred in the absence of AQDS. Bacterial enrichment cultures and isolates from freshwater and marine roots used a variety of carbon and energy sources (e.g., acetate, ethanol, succinate, toluene, and yeast extract) and ferric oxyhydroxide, ferric citrate, uranate, and AQDS as terminal electron acceptors. The temperature optima for a freshwater isolate and a marine isolate were equivalent (approximately 32°C). However, iron reduction by the freshwater isolate decreased with increasing salinity, while reduction by the marine isolate displayed a relatively broad optimum salinity between 20 and 35 ppt. Our results suggest that by participating in an active iron cycle and perhaps by reducing humic acids, iron reducers in the rhizoplane of aquatic macrophytes limit organic availability to other heterotrophs (including methanogens) in the rhizosphere and bulk sediments.     

Structure and function of denitrifying and nitrifying bacterial communities in relation to the plant species in a constructed wetland

The community structure and potential activities of nitrifying and denitrifying bacteria were studied in the rhizosphere of Typha latifolia and Phragmites australis present in a free water system constructed wetland (CW). Potential nitrate reduction and nitrification activities were shown to be significantly higher in the rhizosphere when compared with the nonvegetated sediment. Higher rates were generally obtained for P. australis . The community structure of denitrifying bacteria in the rhizosphere differed from that found at the bulk sediment, as revealed by PCR-denaturing gradient gel electrophoresis (DGGE) of the nitrous oxide reductase encoding gene nosZ . Results also show a greater nosZ genotype diversification and suggest a plant species effect in rhizosphere samples obtained during events of low hydraulic retention times. Ammonia-oxidizing communities were less complex on the basis of PCR-DGGE analysis of the 16S rRNA gene. Retrieved sequences were all related to Nitrosomonas marina and Nitrosomonas ureae , being both present in rhizosphere and bulk sediment regardless of environmental changes. The results demonstrate the effect of vegetation on the functioning and structure of bacterial communities involved in the removal of nitrogen in the treatment cells of a CW and point to the use of vegetation coverage to promote nitrification or denitrification in particular areas.     

Microscale fumigation-extraction and substrate-induced respiration methods for measuring microbial biomass in barley rhizosphere

Changes in microbial biomass in the rhizosphere of young barley seedlings was studied. A fumigation-extraction (FE) method with measurement of ninhydrin-reactive nitrogen (NR-N) and a substrate-induced respiration (SIR) method were applied on a microscale to rhizosphere soil samples of approximately 0.1 g. Rhizosphere soil was defined as the soil adhering to the roots when they were carefully separated from the bulk soil. The rhizosphere soil was gently washed off the roots with either distilled water (FE) or with glucose solution (SIR). Shaking and mild sonication was used to disperse the soil without disrupting the roots. Fumigation was carried out by direct addition of liquid chloroform to the isolated soil. These techniques were proven to give reliable results under the experimental conditions of this investigation. Rhizosphere soil was isolated from segments of the roots representing different distances to the seed different root ages. In the rhizosphere of young barley seedlings, biomass NR-N increased significantly compared to the bulk soil from day 6 after sowing (average increases of 33–97%), especially where adventitious roots had developed. From this time, SIR rates were also significantly higher in the rhizosphere than in bulk soil (average increases 72–170%). The average ratio of SIR rate to biomass NR-N was found to be approximately 50% higher in the rhizosphere than in the bulk soil, which may indicate that a larger fraction of the microbial community is potentially active in the rhizosphere as compared to the bulk soil.     

Tight coupling of root-associated nitrogen fixation and plant photosynthesis in the salt marsh grass Spartina alterniflora and carbon dioxide enhancement of nitrogenase activity

The coupling of root-associated nitrogen fixation and plant photosynthesis was examined in the salt marsh grass Spartina alterniflora. In both field experiments and hydroponic assay chambers, nitrogen fixation associated with the roots was rapidly enhanced by stimulating plant photosynthesis. A kinetic analysis of acetylene reduction activity (ARA) showed that a five-to sixfold stimulation occurred within 10 to 60 min after the plant leaves were exposed to light or increased CO2 concentrations (with the light held constant). In field experiments, CO2 enrichment increased plant-associated ARA by 27%. Further evidence of the dependence of ARA on plant photosynthate was obtained when activity in excised roots was shown to decrease after young greenhouse plants were placed in the dark. Seasonal variation in the ARA of excised plant roots from field cores appears to be related to the annual cycle of net photosynthesis in S. alterniflora.     

Tight coupling of root-associated nitrogen fixation and plant photosynthesis in the salt marsh grass Spartina alterniflora and carbon dioxide enhancement of nitrogenase activity.

The coupling of root-associated nitrogen fixation and plant photosynthesis was examined in the salt marsh grass Spartina alterniflora. In both field experiments and hydroponic assay chambers, nitrogen fixation associated with the roots was rapidly enhanced by stimulating plant photosynthesis. A kinetic analysis of acetylene reduction activity (ARA) showed that a five-to sixfold stimulation occurred within 10 to 60 min after the plant leaves were exposed to light or increased CO2 concentrations (with the light held constant). In field experiments, CO2 enrichment increased plant-associated ARA by 27%. Further evidence of the dependence of ARA on plant photosynthate was obtained when activity in excised roots was shown to decrease after young greenhouse plants were placed in the dark. Seasonal variation in the ARA of excised plant roots from field cores appears to be related to the annual cycle of net photosynthesis in S. alterniflora.     

Diversity and composition of bacterial community in the rhizosphere sediments of submerged macrophytes revealed by 454 pyrosequencing

Freshwater lake sediments support a variety of submerged macrophytes that may host groups of bacteria exerting important ecological functions. We collected three kinds of commonly found submerged macrophyte species (Ceratophyllum demersum, Vallisneria spiralis and Elodea nuttallii) to investigate the bacterial community associated with their rhizosphere sediments. High-throughput 454 pyrosequencing and bioinformatics analyses were performed to examine the diversity and composition of the bacterial community. The results obtained indicated that the diversity of the bacterial community associated with the rhizosphere sediments of submerged macrophytes was significantly lower than that of the bulk sediment. Remarkable differences in the bacterial community composition between the rhizosphere and bulk sediments were also observed.     

Effects of ammonium sulphate application on the chemistry of bulk soil,rhizosphere, fine roots and fine-root distribution in a Picea abies (L.) karst. stand

The effect of ammonium sulphate application on the bulk and rhizosphere soil chemistry, elemental concentration of living fine roots (<2 mm in diameter), amounts of living and dead fine roots, root length density and specific root length density were investigated in a 28 year old Norway spruce stand in SW Sweden. The treatments started in 1988. Core samples of the LFH layer and mineral soil layers were sampled in control (C) and ammonium sulphate (NS) treatment plots in 1988, 1989 and 1990. Soil pH and NO₃-S and SO₄-S, Al, Ca, Mg, Mn and K concentrations were measured for both the bulk soil and rhizosphere soil.The pH-values of the bulk and rhizosphere soil decreased in 1989 and 1990 in NS plots compared to control plots, while the SO₄-S concentration increased. The Ca, Mg and K concentration increased in the NS treatment in almost all layers in the bulk and the rhizosphere soil. Ammonium ions may have replaced these elements in the soil organic matter. The NS treatment reduced Mg concentration in fine roots in all layers in 1990. The Al concentrations in the rhizosphere and bulk soil were higher in NS plots in all layers, except at 0–10 cm depth, both in 1989 and 1990. The Al content of living fine roots was higher in NS plots than C plots but the differences were not significant. The NS addition did not affect the P and K contents of fine roots in any soil layer, but the S concentrations of fine roots were significantly higher in NS plots in 1989 and 1990. The fine root necromass was higher in NS than in C in 1990, in the LFH layer, indicating a gradual decrease in the vitality of the fine roots. It was suggested that the NS treatment resulted in displacement of Mg and K from exchange sites in the LFH layer leading to leaching of these cations to the mineral soil. Further application of ammonium sulphate may damage the fine roots and consequently adversely affect the water and nutrient uptake of root systems.