The effects of chronic nitrogen fertilization on alpine tundra soil microbial communities: implications for carbon and nitrogen cycling

Many studies have shown that changes in nitrogen (N) availability affect primary productivity in a variety of terrestrial systems, but less is known about the effects of the changing N cycle on soil organic matter (SOM) decomposition. We used a variety of techniques to examine the effects of chronic N amendments on SOM chemistry and microbial community structure and function in an alpine tundra soil. We collected surface soil (0-5 cm) samples from five control and five long-term N-amended plots established and maintained at the Niwot Ridge Long-term Ecological Research (LTER) site. Samples were bulked by treatment and all analyses were conducted on composite samples. The fungal community shifted in response to N amendments, with a decrease in the relative abundance of basidiomycetes. Bacterial community composition also shifted in the fertilized soil, with increases in the relative abundance of sequences related to the Bacteroidetes and Gemmatimonadetes, and decreases in the relative abundance of the Verrucomicrobia. We did not uncover any bacterial sequences that were closely related to known nitrifiers in either soil, but sequences related to archaeal nitrifiers were found in control soils. The ratio of fungi to bacteria did not change in the N-amended soils, but the ratio of archaea to bacteria dropped from 20% to less than 1% in the N-amended plots. Comparisons of aliphatic and aromatic carbon compounds, two broad categories of soil carbon compounds, revealed no between treatment differences. However, G-lignins were found in higher relative abundance in the fertilized soils, while proteins were detected in lower relative abundance. Finally, the activities of two soil enzymes involved in N cycling changed in response to chronic N amendments. These results suggest that chronic N fertilization induces significant shifts in soil carbon dynamics that correspond to shifts in microbial community structure and function.     

Response of soil bacterial communities to moisture and grazing in the Tibetan alpine steppes on a small spatial scale

As the Third Pole of the world, the Tibetan Plateau provides a typical alpine grassland environment for soil bacteria with its unique frigid and arid climate. Owing to clear changes in spatial moisture and increased grazing intensity, moisture and livestock grazing have become key factors influencing the microbial communities. Accordingly, we investigated the diversity and composition of soil bacteria in a selected alpine grassland within the dual gradients of moisture and grazing using high-throughput sequencing. Our results showed that grazing changed the soil bacterial diversity and composition, whereas moisture only influenced the relative abundance of the segmental community at the small spatial scale. Species richness was found to be increased by moderate grazing compared with that by high or low-grazing intensity. The relative abundance of dominant species and β-diversity of soil bacteria both showed differences with heavy, moderate, and low grazing. Some dominant bacteria were altered with the moisture content. However, there were no significant differences according to the moisture gradient in terms of the overall bacterial β diversity and composition. These results might be taken account into the small spatial scale as well as the compensation of grazing to moisture on this scale. This work provides new insights into the soil bacterial response to moisture gradients and grazing intensity in alpine steppe habitat.     

Insect-associated bacterial communities in an alpine stream

Hydrobiologia - The roles of macroinvertebrate and microbial communities in stream ecosystems are recognized to be important to energy flow and nutrient cycling. While the linkages of these major...     

Response of bacterial communities and plant-mediated soil processes to nitrogen deposition and precipitation in a desert steppe

Plant and Soil - Changes in nitrogen (N) and precipitation levels can substantially alter soil properties and plant growth, thereby altering soil microbial diversity and functionality. We used...     

Morphological analysis of alpine communities of the north-western Caucasus

The species composition of four alpine communities in the north-western Caucasus was subjected to a morphological analysis. The communities are an alpine lichen heath type (ALH), aFestuca varia grassland type (FVG), aGeranium-Hedysarum meadow type (GHM) and a snowbed community (SBC). Eighty-two species were studied, using the following morphological parameters: vegetative mobility, presence of rosettes, architectural model, life form according toRaunkiaer and life form according toSerebryakov. Representation spectra were calculated on the basis of species presence, above-ground phytomass proportion and species frequency. The results show that most alpine species have a low vegetative mobility; a semi-rosette growth form; a sympodial semi-rosette model of shoot formation and that they are characterised by the prevalence of hemicryptophytes. Differences between the four communities were found in the following features: tap-rooted and short-rhizome plants dominate in ALH and, moreover, dense-tussock plants are dominant in the phytomass; short-rhizome (species presence) and dense- tussock plants (phytomass) are dominant in FVG; short-rhizome and loose-tussock plants dominant in GHM; semi-shrub (phytomass basis) and loose-tussock plants are dominant of SBC. Tuberiferous, bulbiferous and monocarpic plants are not important in any of the communities. *** DIRECT SUPPORT *** A02DO006 00008     

Response of marine microbial communities to anthropogenic stress

Marine microbial communities adapt rapidly to changingenvironmental conditions, including anthropogenicstress. Adaptation involves a wide range ofstrategies, including, (a) formation of resistant,dormant stages, (b) initiation of repair mechanisms, (c)immobilization of toxic chemicals, (d) active transportof chemicals out of the cell, (e) use of contaminantchemicals as carbon or energy sources, and (f)transformation of contaminants to less toxic or morevolatile forms.Adaptation responses are generally plasmid- orchromosomally-mediated and controlled throughinduction or derepression of a variety of biochemicalpathways. Characterization of microbial communityresponses at the molecular level provides biomarkersof contaminant exposure which in turn may be used toprovide an overall picture of ecosystem health. Thisreview will discuss the interactions betweenmicroorganisms and environmental contaminants and thepotential use of microbial biomarkers to assess thehealth of the microbial ecosystem.     

Contribution of nitrogen fixation to nitrogen nutrition in an alpine sedge community (Caricetum curvulae)

Summary In situ acetylene reduction assays (ARA) were carried out over two growing seasons at 2550 m in the upper alpine zone of the Tyrolean Central Alps of Austria. For comparative purposes, some Fabaceae species introduced into the upper alpine zone from lower elevation (2000 m) were subjected to ARA. At the end of the growing season the potted plants were transferred to the laboratory where their acetylene reducing activities were measured again. In situ nitrogenase activity is very low. The highest values were found in association with Leucanthemopsis alpina and Veronica bellidioides (150 and 217 nmol ethylene 24 h^-1 per pot respectively). Higher levels of activity were detected in pots transferred to the laboratory (maximum value 750 nmol ethylene 24 h^-1 per pot; assay temperature about 12°C higher than in the field) and in the Fabaceae transferred to the upper alpine zone (14×10³ nmol ethylene 24 h^-1 per pot of Trifolium badium and T. pallescens). Maximum nitrogen input in the field is in the range of 8 mg m^-2 a^-1. Therefore, under natural circumstances biological nitrogen fixation contributes only very small amounts of nitrogen to this alpine vegetation system, the process being inhibited by low soil temperatures. Possible alternative sources and patterns of N acquisition are discussed in relation to the overall nitrogen economics of plants of the upper alpine zone.     

Response of soybean-Rhizobium symbioses to mineral nitrogen

Summary The effect of mineral nitrogen on establishment and activity of symbioses between soybean and several strains ofRhizobium japonicum and on the establishment of nodules ofR. japonicum isolated from nodules of field crops is studied. All strains were highly susceptible to the effects of 200 ppm NO₃–N on the establishment of symbiosis; 50 ppm NO₃–N had little effect. Response of symbioses establishhed in the absence of mineral N to short term exposure to nitrate or ammonium varied significantly between strains. Nodule isolates from soybean crops growing in nitrifying soil were no less susceptible to the inhibitory effects of mineral N on nodule formation than a laboratory culture of the commercial inoculant strain.     

Microbial communities and activities in alpine and subalpine soils

Soil samples were collected along two slopes (south and north) at subalpine (1500–1900 m, under closed vegetation, up to the forest line) and alpine altitudes (2300–2530, under scattered vegetation, above the forest line) in the Grossglockner mountain area (Austrian central Alps). Soils were analyzed for a number of properties, including physical and chemical soil properties, microbial activity and microbial communities that were investigated using culture-dependent (viable heterotrophic bacteria) and culture-independent methods (phospholipid fatty acid analysis, FISH). Alpine soils were characterized by significantly ( P <0.01) colder climate conditions, i.e. lower mean annual air and soil temperatures, more frost and ice days and higher precipitation, compared with subalpine soils. Microbial activity (soil dehydrogenase activity) decreased with altitude; however, dehydrogenase activity was better adapted to cold in alpine soils compared with subalpine soils, as shown by the lower apparent optimum temperature for activity (30 vs. 37 °C) and the significantly ( P <0.01–0.001) higher relative activity in the low-temperature range. With increasing altitude, i.e. in alpine soils, a significant ( P <0.05–0.01) increase in the relative amount of culturable psychrophilic heterotrophic bacteria, in the relative amount of the fungal population and in the relative amount of Gram-negative bacteria was found, which indicates shifts in microbial community composition with altitude.     

The macroinvertebrate communities of two contrasting Norwegian glacial rivers in relation to environmental variables

1. Macroinvertebrate communities in two Norwegian glacial rivers, one in the western fjords (Dalelva) and one in the eastern mountains (Leirungsåi), were investigated during three time periods in 1996 and 1997.
2. Channel stability variables (substratum heterogeneity/Pfankuch index/hydraulic stress) and water temperature accounted for 54% of the total inertia in the principal components analysis (PCA) ordination of environmental variables. The importance of these variables was confirmed by cluster analysis.
3. The two rivers were well separated in the ordinations, with Leirungsåi showing much greater heterogeneity. This is explained by differences in altitudinal range, terrestrial vegetation and the importance and nature of tributary inputs.
4. Channel stability and temperature were also important in determining faunal communities in the two glacial rivers, supporting the main determining variables in the conceptual model of glacial streams ( Milner & Petts, 1994 ). However, clear temporal differences were apparent in the data, the two rivers being more similar during the summer period of high discharge dominated by glacial meltwater. During spring and especially during autumn environmental conditions and the macroinvertebrate fauna differed both within and between rivers.
5. Diamesinae dominated in the upper reaches of both rivers, with Orthocladiinae becoming more common downstream. The dominance of Diamesinae persisted further down Dalelva because of the continued influence of glacial tributaries, whereas in Leirungsåi the influence of non-glacial tributaries led to a change towards a greater proportion of Orthocladiinae. Lakes modified macroinvertebrate communities in both river systems.     

Response of Anabaena species to different nitrogen sources

Nitrogenase activity, ammonia excretion and glutamine synthetase (GS) activity were examined in five strains of Anabaena (A. anomala ARM 314, A. fertilissima ARM 742, A. variabilis ARM 310, A. oryzae ARM 313 and A. oryzae ARM 570) in the presence of 2.5 mM NO3-N (KNO3), 2.5 mM NH-4-N [(NH4)2SO4] and diatomic nitrogen (N2). Ammonium-N was more inhibitory to nitrogenase activity as compared to NO3-N in all the strains. Maximum GS activity was exhibited in NO3-N medium, irrespective of the cyanobacterial strains studied. Uninduced release of ammonia was observed in all the species, with A. oryzae ARM 313 and Anabaena variabilis ARM 310 exhibiting maximum excretion of 0.25-0.31 and 0.27-1.23 mu moles NH4 mg Chl(-1) respectively on the 15th day of incubation. The glutamine synthetase activity of A. oryzae ARM 313 was relatively very high as compared to Anabaena variabilis ARM 310. There was no nitrate reductase activity in any of the Anabaena sp. grown on NH3-N or N2-N on the 15th day of incubation.     

Response of microbial communities of Lake Baikal to extreme temperatures

The survival rate, metabolic activity, and ability for growth of microbial communities of Lake Baikal have been first studied after exposure to extremely low temperatures (freeze-thawing) for different lengths of time. It has been shown that short-term freezing (1–3 days) inhibits the growth and activity of microbial communities. The quantity of microorganisms increased after 7-and 15-day freezing. In the periods of maximums, the total number of microorganisms in the test samples was twice as high as in the control. It was established that after more prolonged freezing the microorganisms required more time after thawing to adapt to new conditions. In the variants with 7-and 15-day freezing, the activities of defrosted microbial communities were three or more times higher than in the control. The survival rate and activity of Baikal microorganisms after freeze-thawing confirms the fact that the Baikal microbial communities are highly resistant to this type of stress impact.     

Response of microbial communities of Lake Baikal to extreme temperatures

The survival rate, metabolic activity, and ability for growth of microbial communities of Lake Baikal after exposure to extremely low temperatures (freeze-thawing) for different lengths of time have been first studied. It has been shown that short-term freezing (1-3 days) inhibits the growth and activity of microbial communities. The quantity of microorganisms increased after 7- and 15-day freezing. In the periods of maximums, the total number of microorganisms in the test samples was twice as high as in the control. It was established that after more prolonged freezing the microorganisms required more time after thawing to adapt to new conditions. In the variants with 7- and 15-day freezing, the activities of defrosted microbial communities were three or more times higher than in the control. The survival rate and activity of Baikal microorganisms after freeze-thawing confirms the fact that the Baikal microbial communities are highly resistant to this type of stress impact.