Communities In Grassland Soils Research Articles

Plant roots affect free-living diazotroph communities in temperate grassland soils despite decades of fertilization

Fixation of atmospheric N2 by free-living diazotrophs accounts for an important proportion of nitrogen naturally introduced to temperate grasslands. The effect of plants or fertilization on the general microbial community has been extensively studied, yet an understanding of the potential combinatorial effects on the community structure and activity of free-living diazotrophs is lacking. In this study we provide a multilevel assessment of the single and interactive effects of different long-term fertilization treatments, plant species and vicinity to roots on the free-living diazotroph community in relation to the general microbial community in grassland soils. We sequenced the dinitrogenase reductase (nifH) and the 16S rRNA genes of bulk soil and root-associated compartments (rhizosphere soil, rhizoplane and root) of two grass species (Arrhenatherum elatius and Anthoxanthum odoratum) and two herb species (Galium album and Plantago lanceolata) growing in Austrian grassland soils treated with different fertilizers (N, P, NPK) since 1960. Overall, fertilization has the strongest effect on the diazotroph and general microbial community structure, however with vicinity to the root, the plant effect increases. Despite the long-term fertilization, plants strongly influence the diazotroph communities emphasizing the complexity of soil microbial communities’ responses to changing nutrient conditions in temperate grasslands.

The direct and indirect drivers shaping RNA viral communities in grassland soils.

Climate change has been reshaping the soil environment as well as the residing microbiome. This study provides field-relevant information on how environmental and community factors collectively shape soil RNA communities and contribute to ecological understanding of RNA viral survival under various environmental conditions and virus-host interactions in soil. This knowledge is critical for predicting the viral responses to climate change and the potential emergence of biothreats.

Corpse decay of wild animals leads to the divergent succession of nrfA-type microbial communities.

Animal carcasses introduce large amounts of nitrates and ammonium into the soil ecosystem. Some of this ammonium is transformed from nitrite through the nrfA-type microbial community. However, it is unclear how nrfA-type microorganisms respond to the decomposition of corpses. This study applied high-throughput sequencing to characterize the ecological succession of nrfA-type microbial communities in grassland soil. Our results showed that Cyclobacterium and Trueperella were the predominant genera for nrfA-type communities in soil with a decomposing corpse (experimental group), while Cyclobacterium and Archangium were dominant in soil without a corpse (control group). The alpha diversity indexes and the resistance and resilience indexes of the microbial communities initially increased and then decreased during decomposition. Compared with the control group, nrfA-encoding community structure in the experimental group gradually became divergent with succession and temporal turnover accelerated. Network analysis revealed that the microbial communities of the experimental group had more complex interactions than those of the control groups. Moreover, the bacterial community assembly in the experimental group was governed by stochastic processes, and the communities of the experimental group had a weaker dispersal capacity than those of the control group. Our results reveal the succession patterns of the nrfA-type microbial communities during degradation of wild animal corpses, which can offer references for demonstrating the ecological mechanism underlying the changes in the nrfA-type microbial community during carcass decay. KEY POINTS: • Corpse decay accelerates the temporal turnover of the nrfA-type community in soil. • Corpse decay changes the ecological succession of the nrfA-type community in soil. • Corpse decay leads to a complex co-occurrence pattern of the nrfA-type community in soil.

Agricultural management and pesticide use reduce the functioning of beneficial plant symbionts.

Phosphorus (P) acquisition is key for plant growth. Arbuscular mycorrhizal fungi (AMF) help plants acquire P from soil. Understanding which factors drive AMF-supported nutrient uptake is essential to develop more sustainable agroecosystems. Here, we collected soils from 150 cereal fields and 60 non-cropped grassland sites across a 3,000 km trans-European gradient. In a greenhouse experiment, we tested the ability of AMF in these soils to forage for the radioisotope 33P from a hyphal compartment. AMF communities in grassland soils were much more efficient in acquiring 33P and transferred 64% more 33P to plants compared to AMF in cropland soils. Fungicide application best explained hyphal 33P transfer in cropland soils. The use of fungicides and subsequent decline in AMF richness in croplands reduced 33P uptake by 43%. Our results suggest that land-use intensity and fungicide use are major deterrents to the functioning and natural nutrient uptake capacity of AMF in agroecosystems.

Sheep grazing impacts on soil methanotrophs and their activity in typical steppe in the Loess Plateau China

Soil methanotrophs are the only biological sink for atmospheric methane (CH4). The activity and the diversity of methanotrophic communities in grassland soils can be impacted by animal grazing. In this study, soils were collected from a 17-year running grazing intensity trial in a typical steppe in the Loess Plateau, China to examine the responses of CH4 oxidation rate and active methanotroph composition and abundance to different levels of sheep grazing. Soils were collected from summer grazed pastures at no (0 sheep ha−1), low (2.67 sheep ha−1), moderate (5.33 sheep ha−1) and high (8.67 sheep ha−1) levels of grazing. We found that all soils were able to oxidize atmospheric and high concentration (10%) CH4 through soil methanotrophs. Compared to ungrazed soils, the atmospheric CH4 oxidation rate and methanotroph abundance were higher in low and high grazed soils but lower in moderately grazed soils. Across all soils, the in-situ methanotroph community was dominated by upland soil cluster gamma methanotrophs and potentially contributed to the atmospheric concentration CH4 oxidation. Soil methanotroph community responses to the high 13CH4 concentration revealed an increase in methanotroph abundance and a transition of the active methanotroph community to traditional Type I (Methylocaldum and Methylobacter) and Type II (Methylocystis) methanotrophs. Methylocaldum responses to the high CH4 concentration were greater in highly grazed soil than in ungrazed soil. The activity of Methylobacter and Methylocystis under high CH4 concentrations was reduced by high grazing intensity. This study provides evidence that recent intensifications of animal grazing can result in particular community assembly of microbial methane oxidizers responsible for CH4 sink capabilities of grassland soils.

Drought and Plant Community Composition Affect the Metabolic and Genotypic Diversity of Pseudomonas Strains in Grassland Soils.

Climate and plant community composition (PCC) modulate the structure and function of microbial communities. In order to characterize how the functional traits of bacteria are affected, important plant growth-promoting rhizobacteria of grassland soil communities, pseudomonads, were isolated from a grassland experiment and phylogenetically and functionally characterized. The Miniplot experiment was implemented to examine the mechanisms underlying grassland ecosystem changes due to climate change, and it investigates the sole or combined impact of drought and PCC (plant species with their main distribution either in SW or NE Europe, and a mixture of these species). We observed that the proportion and phylogenetic composition of nutrient-releasing populations of the Pseudomonas community are affected by prolonged drought periods, and to a minor extent by changes in plant community composition, and that these changes underlie seasonality effects. Our data also partly showed concordance between the metabolic activities and 16S phylogeny. The drought-induced shifts in functional Pseudomonas community traits, phosphate and potassium solubilization and siderophore production did not follow a unique pattern. Whereas decreased soil moisture induced a highly active phosphate-solubilizing community, the siderophore-producing community showed the opposite response. In spite of this, no effect on potassium solubilization was detected. These results suggest that the Pseudomonas community quickly responds to drought in terms of structure and function, the direction of the functional response is trait-specific, and the extent of the response is affected by plant community composition.

Soil bacterial communities at the treeline in subtropical alpine areas

As a temperature-sensitive transition zone, the subtropical alpine region responds quickly to global warming. However, little is known about soil microbial communities at the treeline, where changes in vegetation in response to global warming are anticipated. Barcoded pyrosequencing of the 16S rRNA gene was used to investigate the bacterial communities of coniferous forest and grassland soils at the treeline of four different peaks above 3,000 m a.s.l. Although the forest soils were more acidic than the grassland soils, the other soil properties were highly variable with no consistent pattern in C and N contents and microbial biomass between two vegetation types. The Acidobacteria and α-Proteobacteria were the most abundant phylogenetic groups, although their relative abundances differed among the forest and grassland soil communities and between sites. The composition of bacterial communities or β-diversity, varied significantly between the sites and vegetation types. In contrast, α-diversity only differed significantly between sites. Two of the grassland sites and one forest had converted from forests and grassland, respectively within the last 60 years. The abundances of some genera, such as Acidobacteria Gp2 in the grasslands converted from forests, more closely resembled those of the other forest sites than the historically grassland sites, while the abundance of Acidobacteria Gp1 in the forest converted from grassland was more similar to the historically grassland sites. These results suggest a legacy effect for the transition of forest to grassland. Bacterial community structure also correlated significantly with soil pH, organic C and C/N. These results suggest that the present vegetation at the treeline influences soil bacterial community structure, although there is also a significant legacy effect on the abundance of certain bacterial groups.

Steeper spatial scaling patterns of subsoil microbiota are shaped by deterministic assembly process.

Although many studies have investigated the spatial scaling of microbial communities living in surface soils, very little is known about the patterns within deeper strata, nor is the mechanism behind them. Here, we systematically assessed spatial scaling of prokaryotic biodiversity within three different strata (Upper: 0-20cm, Middle: 20-40cm, and Substratum: 40-100cm) in a typical grassland by examining both distance-decay (DDRs) and species-area relationships (SARs), taxonomically and phylogenetically, as well as community assembly processes. Each layer exhibited significant biogeographic patterns in both DDR and SAR (p< .05), with taxonomic turnover rates higher than phylogenetic ones. Specifically, the spatial turnover rates, β and z values, respectively, ranged from 0.016±0.005 to 0.023±0.005 and 0.065±0.002 to 0.077±0.004 across soil strata, and both increased with depth. Moreover, the prokaryotic community in grassland soils assembled mainly according to deterministic rather than stochastic mechanisms. By using normalized stochasticity ratio (NST) based on null model, the relative importance of deterministic ratios increased from 48.0 to 63.3% from Upper to Substratum, meanwhile a phylogenetic based method revealed average βNTI also increased with depth, from -5.29 to 19.5. Using variation partitioning and distance approaches, both geographic distance and soil properties were found to strongly affect biodiversity structure, the proportions increasing with depth, but spatial distance was always the main underlying factor. These indicated increasingly deterministic proportions in accelerating turnover rates for spatial assembly of prokaryotic biodiversity. Our study provided new insights on biogeography in different strata, revealing importance of assembly patterns and mechanisms of prokaryote communities in below-surface soils.

Soil bacterial communities remain altered after 30years of agriculture abandonment in Pampa grasslands.

Old fields are spreading in the world because of agriculture abandonment, and they show a persistence of exotic plant species with little recovery towards the original vegetation composition. Soil biota may also differ between old fields and native grasslands, but were comparatively less studied than plant communities, despite their importance in biogeochemical processes. Here we compared soil bacterial communities of exotic-dominated old fields with those of remnants of native grasslands in the Inland Pampa, Argentina, using the 16S rRNA gene amplicon sequencing approach. We also characterized plant communities, soil physico-chemical properties, and soil respiration. We expected more diverse soil bacterial communities, with higher heterogeneity, in remnant grasslands than in old fields because of a more diverse and more heterogeneous plant community. However, our results showed that soil bacterial communities had higher Shannon diversity in old fields than in remnant grasslands, but richness was not significantly different. Also we found different bacterial community compositions between grasslands even at a low taxonomic level. On the other hand, old fields harbored less heterogeneous bacterial communities than remnants, and bacteria and plant beta diversity were correlated. Despite contrasting plant and bacterial composition between old fields and remnant grasslands, soil physico-chemical properties were quite similar between grasslands. Overall, our results showed that bacterial communities in grassland soils were associated with changes in plant communities after agricultural abandonment. Plant-microbial feedbacks might regulate plant and soil bacterial community assemblage in old fields, yet further research is needed to demonstrate this potential feedback mechanism.

Microbial community size is a potential predictor of nematode functional group in limed grasslands

Agronomic management practices can impose structural change within soil biotic communities that may negatively impact soil processes including function and biodiversity. Thus, optimizing sustainable crop production that confers minimal impacts on the structure and function of soil biota is an imperative to deliver healthy, functional and resilient production systems. Liming is a management intervention to mitigate soil acidification with a generally positive effect on crop biomass. The application of lime changes soil pH, a known driver of microbial community composition, but it is unknown whether pH derived shifts in bacterial communities result in altered nematode communities. In this study we used qPCR, next generation sequencing and nematode directed T-RFLP to characterise microbial and nematode communities in a liming field trial with a control and three liming applications to incrementally increase pH by 0.5, 0.75 and 1 pH unit. We demonstrate over a 14-month experimental period an interaction between microbial and nematode communities in managed grasslands. Liming had a limited effect on nematode and microbial community structures. However bacterial and archaeal abundance as measured by 16S rRNA gene copy number was found to be a potential predictor of nematode functional group, based on recognized trophic strategies, with increased abundance of omnivorous and predatory nematodes, that are known to prey upon bacterivorous nematodes, with a concomitant increase of 16S rRNA gene copy number. Thus, indirectly suggesting suppression of the bacterial and archaeal community in the presence of bacterivorous nematodes. Where populations of bacterivorous nematodes were highest the relative abundance of both predatory and omnivorous nematodes was lowest. Thus, this study demonstrates clear connectivity between soil microbial and nematode communities in grassland soil.

Mineralization and nitrification: Archaea dominate ammonia-oxidising communities in grassland soils

In grasslands, N mineralization and nitrification are important processes and are controlled by several factors, including the in situ microbial community composition. Nitrification involves ammonia oxidising archaea (AOA) and bacteria (AOB) and although AOA and AOB co-exist in soils, they respond differently to environmental characteristics and there is evidence of AOA/AOB niche differentiation. Here, we investigated temporal variation in N mineralization and nitrification rates, together with bacterial, archaeal and ammonia-oxidiser communities in grassland soils, on different geologies: clay, Greensand and Chalk. Across geologies, N mineralization and nitrification rates were slower in the autumn than the rest of the year. Turnover times for soil ammonium pools were <24 h, whilst several days for nitrate. In clay soils, bacterial, archaeal, AOA, and AOB communities were clearly distinct from those in Chalk and Greensand soils. Spatially and temporally, AOA were more abundant than AOB. Notably, Nitrososphaera were predominant, comprising 37.4% of archaeal communities, with the vast majority of AOA found in Chalk and Greensand soils. AOA abundance positively correlated with nitrate concentration, whereas AOB abundance correlated with ammonium and nitrite concentrations, suggesting that these N compounds may be potential drivers for AOA/AOB niche differentiation in these grassland soils.

Evidence for niche differentiation of nitrifying communities in grassland soils after 44 years of different field fertilization scenarios

Long-term nitrogen (N) fertilization imposes strong selection on nitrifying communities in agricultural soil, but how a progressively changing niche affects potentially active nitrifiers in the field remains poorly understood. Using a 44-year grassland fertilization experiment, we investigated community shifts of active nitrifiers by DNA-based stable isotope probing (SIP) of field soils that received no fertilization (CK), high levels of organic cattle manure (HC), and chemical N fertilization (CF). Incubation of DNA-SIP microcosms showed significant nitrification activities in CF and HC soils, whereas no activity occurred in CK soils. The 44 years of inorganic N fertilization selected only 13C-ammonia-oxidizing bacteria (AOB), whereas cattle slurry applications created a niche in which both ammonia-oxidizing archaea (AOA) and AOB could be actively 13C-labeled. Phylogenetic analysis indicated that Nitrosospira sp. 62-like AOB dominated inorganically fertilized CF soils, while Nitrosospira sp. 41-like AOB were abundant in organically fertilized HC soils. The 13C-AOA in HC soils were affiliated with the 29i4 lineage. The 13C-nitrite-oxidizing bacteria (NOB) were dominated by both Nitrospira- and Nitrobacter-like communities in CF soils, and the latter was overwhelmingly abundant in HC soils. The 13C-labeled nitrifying communities in SIP microcosms of CF and HC soils were largely similar to those predominant under field conditions. These results provide direct evidence for a strong selection of distinctly active nitrifiers after 44 years of different fertilization regimes in the field. Our findings imply that niche differentiation of nitrifying communities could be assessed as a net result of microbial adaption over 44 years to inorganic and organic N fertilization in the field, where distinct nitrifiers have been shaped by intensified anthropogenic N input.

Understanding the relationships between grazing intensity and the distribution of nitrifying communities in grassland soils

Nitrifying microbes are of critical importance in regulating efficient nitrogen (N) cycling, which plays a crucial role in plant productivity and maintaining soil sustainability. Long-term different intensities of grazing can strongly influence the microbial communities, while our understanding of the complex nitrifying community in the grazed grassland soil environment is still limited. To investigate whether and how long-term grazing with different intensities influence soil nitrifying communities, high-throughput sequencing and quantitative PCR analyses were performed on soil samples from permanent grassland soils under four grazing intensities: 0 (G0), 1.5 (G1), 6 (G2) and 9 (G3) sheepha−1. Results showed that the G3 treatment significantly reduced the soil nutrient content and increased the soil bulk density, changes that are not sustainable in the long run. The G1 treatment, on the other hand, significantly increased the soil nutrient content and would improve soil fertility. Some functional microbes were specifically enriched after long term grazing, like Nitrospirae (phylum) to Nitrospira (class) in the G2 samples and Chromatiales (order) to Nitrosococcus (genus) in the G3 soils. The numerically dominant Nitrosococcus watsonii lineage of ammonia oxidizing bacteria (AOB) was observed in this grassland soil. The redundancy analysis (RDA) together with the structural equation modeling (SEM) analysis showed that grazing intensity was important in mediating the distribution of soil microorganisms and affected nitrifying communities by impacting soil physicochemical characteristics (e.g., bulk density, NH4+-N). These results showed the shifts of nitrifying communities across different grazing intensities, and could aid in the determination of an optimal grazing intensity for these grazed grassland soils.

Soil pH is a major driver of soil diazotrophic community assembly in Qinghai-Tibet alpine meadows

The biological nitrogen input is essential for nutrient enrichment in natural grassland ecosystems. This process is carried out by diazotrophic microbes. However, the patterns and assembly processes of diazotrophic communities in grassland soil are not fully understood, especially in alpine meadows of high-altitude Qinghai-Tibet Plateau (QTP). In this study, the diversity patterns and community assembly processes of soil diazotrophic communities were investigated in QTP alpine meadows. The results showed that different diazotrophic taxa responded differentially to the increase of soil pH, and α-diversity increased linearly with pH values. Diazotrophic community turnover was enhanced by the increased variation in soil pH, moreover, it also showed strong response to mean annual temperature. Null model analysis showed that deterministic processes were most important in shaping the phylogenetic community structure and assembly of diazotrophs. Diazotrophic communities of each sampling site tended to be phylogenetically clustered, while communities at high pH soils (pH 7.00 to 8.00) showed more clustering than those of low pH (pH 5.02 to 7.00). The soil pH is a main driver in structuring soil diazotrophic community assembly by regulating the relative importance of deterministic and stochastic processes in QTP alpine meadows.

Land-use influences phosphatase gene microdiversity in soils.

Phosphorus cycling exerts significant influence upon soil fertility and productivity - processes largely controlled by microbial activity. We adopted phenotypic and metagenomic approaches to investigate phosphatase genes within soils. Microbial communities in bare fallowed soil showed a marked capacity to utilise phytate for growth compared with arable or grassland soil communities. Bare fallowed soil contained lowest concentrations of orthophosphate. Analysis of metagenomes indicated phoA, phoD and phoX, and histidine acid and cysteine phytase genes were most abundant in grassland soil which contained the greatest amount of NaOH-EDTA extractable orthophosphate. Beta-propeller phytase genes were most abundant in bare fallowed soil. Phylogenetic analysis of metagenome sequences indicated the phenotypic shift observed in the capacity to mineralise phytate in bare fallow soil was accompanied by an increase in phoD, phoX and beta-propeller phytase genes coding for exoenzymes. However, there was a remarkable degree of genetic similarity across the soils despite the differences in land-use. Predicted extracellular ecotypes were distributed across a greater range of soil structure than predicted intracellular ecotypes, suggesting that microbial communities subject to the dual stresses of low nutrient availability and reduced access to organic material in bare fallowed soils rely upon the action of exoenzymes.

Assessment of Bacterial Communities and Predictive Functional Profiling in Soils Subjected to Short-Term Fumigation-Incubation.

Previous investigations observed that when soil was fumigated with ethanol-free CHCl3 for 24h and then incubated under appropriate conditions, after the initial flush of CO2 was over, soil organic carbon (SOC) mineralization continued at the same rate as in the non-fumigated soil. This indicates that, following fumigation, the much diminished microbial population still retained the same ability to mineralize SOC as the much larger non-fumigated population. We hypothesize that although fumigation drastically alters the soil bacterial community abundance, composition, and diversity, it has little influence on the bacterial C-metabolic functions. Here, we conducted a 30-day incubation experiment involving a grassland soil and an arable soil with and without CHCl3 fumigation. At days 0, 7, and 30 of the incubation, the bacterial abundances were determined by quantitative PCR, and the bacterial community composition and diversity were assessed via the 16S rRNA gene amplicon sequencing. PICRUSt was used to predict the metagenome functional content from the sequence data. Fumigation considerably changed the composition and decreased the abundance and diversity of bacterial community at the end of incubation. At day 30, Firmicutes (mainly Bacilli) accounted for 70.9 and 94.6% of the total sequences in the fumigated grassland and arable soil communities, respectively. The two fumigated soil communities exhibited large compositional and structural differences during incubation. The families Paenibacillaceae, Bacillaceae, and Symbiobacteriaceae dominated the bacterial community in the grassland soil, and Alicyclobacillaceae in the arable soil. Fumigation had little influence on the predicted abundances of KEGG orthologs (KOs) assigned to the metabolism of the main acid esters, saccharides, amino acids, and lipids in the grassland soil community. The saccharide-metabolizing KO abundances were decreased, but the acid ester- and fatty acid-metabolizing KO abundances were elevated by fumigation in the arable soil community. Our study suggests functional redundancy regarding the bacterial genetic potential associated with SOC mineralization.

Response of microbial communities from an apple orchard and grassland soils to the first-time application of the fungicide tetraconazole

The aim of the study was to assess the impact of the triazole fungicide tetraconazole applied at the field rate (FR) and at ten-fold the FR (10FR) on microorganisms in orchard soil with a long-term history of fungicides application and in grassland soil that had not previously been treated with pesticides. To ascertain this impact, the microbial activity determined by fluorescein diacetate (FDA) hydrolysis, the culturable number of bacteria, fungi and tetraconazole-resistant fungi, and the phospholipid microbial biomass and the structural and functional biodiversity assessed by the PLFA and Biolog approaches, respectively, were examined under laboratory conditions during 28-day experiment. The response of soil microorganisms to the fungicide tetraconazole, which had never been used before in these soils, depended on the management of the soils. In apple orchard soil that had been treated with FR or 10FR tetraconazole, a decrease in microbial activity was still observed on the 28th day after the application of the fungicide. In contrast, a significant impact of tetraconazole on the number of bacteria was still observed at the end of experiment in grassland soil. Results of principal component analysis (PCA) indicated that the application of tetraconazole significantly changed the structure of the microbial communities in the orchard soil. In addition, analysis of the Biolog profiles revealed a decrease in the catabolic activity of the microbial communities in grassland soil that had been treated with tetraconazole at both rates over time. The evaluation of the structural and functional diversity of microbial communities using PCA appears to be the most valuable monitoring tool for assessing the impact of tetraconazole application on soil microorganisms.

The influence of environmental factors on protistan microorganisms in grassland soils along a land-use gradient

In this study, we investigated the effect of land use intensity, soil parameters and vegetation on protistan communities in grassland soils. We performed qualitative (T-RFLP) and quantitative (qPCR) analyses using primers specifically targeting the 18S rRNA gene for all Eukarya and for two common flagellate groups, i.e. the Chrysophyceae and the Kinetoplastea. Both approaches were applied to extracted soil DNA and RNA, in order to distinguish between the potentially active protists (i.e. RNA pool) and the total protistan communities, including potentially inactive and encysted cells (i.e. DNA pool). Several environmental determinants such as site, soil parameters and vegetation had an impact on the T-RFLP community profiles and the abundance of the quantified 18S rRNA genes. Correlating factors often differed between quantitative (qPCR) and qualitative (T-RFLP) approaches. For instance the Chrysophyceae/Eukarya 18S rDNA ratio as determined by qPCR correlated with the C/N ratio, whereas the community composition based on T-RLFP analysis was not affected indicating that both methods taken together provide a more complete picture of the parameters driving protist diversity. Moreover, distinct T-RFs were obtained, which could serve as potential indicators for either active organisms or environmental conditions like water content. While site was the main determinant across all investigated exploratories, land use seemed to be of minor importance for structuring protist communities. The impact of other parameters differed between the target groups, e.g. Kinetoplastea reacted on changes to water content on all sites, whereas Chrysophyceae were only affected in the Schorfheide. Finally, in most cases different responses were observed on RNA- and DNA-level, respectively. Vegetation for instance influenced the two flagellate groups only at the DNA-level across all sites. Future studies should thus include different protistan groups and also distinguish between active and inactive cells, in order to reveal causal shifts in community composition and abundance in agriculturally used systems.

Diversity and structure of a bacterial community in grassland soils disturbed by sheep grazing, in the Loess Plateau of northwestern China.

The relationship between disturbance, biodiversity, and ecosystem function has been a hot topic recently in international ecological research, and a universally applicable model remains elusive. In this study, we assessed the diversity and structure of a bacterial community in grassland soils along a disturbance gradient due to sheep grazing. Bacteria were identified based on 16S rDNA gene libraries prepared from a 12-year field experiment that included four grazing, intensity treatments: no grazing, light grazing, moderate grazing and heavy grazing in the Loess Plateau of northwestern China. We found that diversity indices of bacterial 16S rDNA increased with grazing intensity, suggesting that disturbance led to higher bacterial diversity. The bacterial community structure, measured as species composition, was also affected by grazing. In addition, the change in soil bacterial community composition was maximum under heavy grazing, based on the Sorensen similarity index. Overall, the relationship between disturbance and bacterial diversity is complex, therefore, more studies are required to determine the possibility of using microbial diversity as an indicator of ecosystem stability.

Impact of long-term N, P, K, and NPK fertilization on the composition and potential functions of the bacterial community in grassland soil.

Soil abiotic and biotic interactions govern important ecosystem processes. However, the mechanisms behind these interactions are complex, and the links between specific environmental factors, microbial community structures, and functions are not well understood. Here, we applied DNA shotgun metagenomic techniques to investigate the effect of inorganic fertilizers N, P, K, and NPK on the bacterial community composition and potential functions in grassland soils in a 54-year experiment. Differences in total and available nutrients were found in the treatment soils; interestingly, Al, As, Mg, and Mn contents were variable in N, P, K, and NPK treatments. Bacterial community compositions shifted and Actinobacteria were overrepresented under the four fertilization treatments compared to the control. Redundancy analysis of the soil parameters and the bacterial community profiles showed that Mg, total N, Cd, and Al were linked to community variation. Using correlation analysis, Acidobacteria, Bacteroidetes, and Verrucomicrobia were linked similarly to soil parameters, and Actinobacteria and Proteobacteria were linked separately to different suites of parameters. Surprisingly, we found no fertilizers effect on microbial functional profiles which supports functional redundancy as a mechanism for stabilization of functions during changes in microbial composition. We suggest that functional profiles are more resistant to environmental changes than community compositions in the grassland ecosystem.