Soil pH drives the spatial distribution of bacterial communities along elevation on Changbai Mountain

The elevational patterns of diversity for plants and animals have been well established over the past century. However, it is unclear whether there is a general elevational distribution pattern for microbes. Changbai Mountain is one of few well conserved natural ecosystems, where the vertical distribution of vegetation is known to mirror the vegetation horizontal zonation from temperate to frigid zones on the Eurasian continent. Here, we present a comprehensive analysis of soil bacterial community composition and diversity along six elevations representing six typical vegetation types from forest to alpine tundra using a bar-coded pyrosequencing technique. The bacterial communities differed dramatically along elevations (vegetation types), and the community composition was significantly correlated with soil pH, carbon/nitrogen ratio (C/N), moisture or total organic carbon (TOC), respectively. Phylogenetic diversity was positively correlated with soil pH (P = 0.024), while phylotype richness was positively correlated with soil pH (P = 0.004), total nitrogen (TN) (P = 0.030), and negatively correlated with C/N ratio (P = 0.021). Our results emphasize that pH is a better predictor of soil bacterial elevational distribution and also suggest that vegetation types may indirectly affect soil bacterial elevational distribution through altering soil C and N status.     

Temperature sensitivity of soil respiration is affected by prevailing climatic conditions and soil organic carbon content: A trans-China based case study

Understanding the spatial variation of temperature sensitivity (i.e. Q₁₀) of soil respiration (R_s) and its controlling factors, is critical to improve the precision of carbon budget estimations at regional scales. In this study, data from 2-3 continuous years of R_s measurements over 15 ecosystems of ChinaFLUX were summarized to analyze the response of R_s to soil temperature. Moreover, we improved our dataset by collecting previously published Q₁₀ values from 34 ecosystems in China. The ecosystems studied were located in the main climatic zones of China, spanning from alpine via temperate to tropical. Spatial variations of Q₁₀ and its controlling factors were analyzed. The results showed that soil temperature at a 5 cm depth satisfactorily explained the seasonal variations in R_s of the 15 ChinaFLUX ecosystems (R² varying from 0.37 to 0.83). Based on the overall data, the Q₁₀ values of R_s in China ranged from 1.28 to 4.75. The spatial variations in Q₁₀ were primarily determined by soil temperature during measurement periods, soil organic carbon (SOC) content, and ecosystem type. Ecosystems in colder regions and with higher SOC content had relatively higher Q₁₀ values. Moreover, ecosystems of different vegetation types showed different Q₁₀ values. A temperature- and SOC-dependent function for Q₁₀ is suggested, which could be a valuable reference for improving the regional-scale models of R_s and ecosystem carbon cycles.     

Effect of invasive Acacia dealbata Link on soil microorganisms as determined by PCR-DGGE

Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain where it forms dense monospecific patches modifying the structure of different native ecosystems and threatening native aboveground biodiversity. In spite of the dramatic changes observed in the vegetation of invaded sites little is known about the consequences of invasion for soil microorganisms. To investigate the effect of A. dealbata invasion on the structure of soil fungi and bacteria communities, samples were taken from invaded and non-invaded areas from three different ecosystems in Northwest Spain: pine forest, shrubland and grassland. In each ecosystem type, soil samples were taken in areas of native vegetation, areas invaded by A. dealbata and in the transition zone between native and invaded vegetation. Soil microorganisms were analyzed in the different samples by PCR-DGGE using general primers for eubacteria and fungi. Soil analyses were also performed to evaluate the effect of A. dealbata invasion on soil fertility.The invasion by A. dealbata consistently increased soil N, C, organic matter and exchangeable P content in the three studied ecosystems. A clear effect of the invasion on the overall structure of microorganism communities was only observed in the shrubland where soil fungal communities in the invaded and transition areas clustered together and apart from the native soil. Significant differences in soil microorganisms richness and diversity between invaded and not invaded soils were only found in the grassland. Grassland invasion by A. dealbata lead to a significant increase of bacterial richness and to a significant reduction in fungal richness and diversity. Our results show that although the changes on soil chemistry due to A. dealbata invasion are consistent among the studied ecosystems, the effect on soil microorganisms depends on the ecosystem type affected by the invasion.     

The under-recognized dominance of Verrucomicrobia in soil bacterial communities

Verrucomicrobia are ubiquitous in soil, but members of this bacterial phylum are thought to be present at low frequency in soil, with few studies focusing specifically on verrucomicrobial abundance, diversity, and distribution. Here we used barcoded pyrosequencing to analyze verrucomicrobial communities in surface soils collected across a range of biomes in Antarctica, Europe, and the Americas (112 samples), as well as soils collected from pits dug in a montane coniferous forest (69 samples). Data collected from surface horizons indicate that Verrucomicrobia average 23% of bacterial sequences, making them far more abundant than had been estimated. We show that this underestimation is likely due to primer bias, as many of the commonly used PCR primers appear to exclude verrucomicrobial 16S rRNA genes during amplification. Verrucomicrobia were detected in 180 out of 181 soils examined, with members of the class Spartobacteria dominating verrucomicrobial communities in nearly all biomes and soil depths. The relative abundance of Verrucomicrobia was highest in grasslands and in subsurface soil horizons, where they were often the dominant bacterial phylum. Although their ecology remains poorly understood, Verrucomicrobia appear to be dominant in many soil bacterial communities across the globe, making additional research on their ecology clearly necessary.     

Fungal diversity in soils and historic wood from the Ross Sea Region of Antarctica

Microorganisms play a dominant role in Antarctic ecosystems, yet little is known about how fungal diversity differs at sites with considerable human activity as compared to those that are remote and relatively pristine. Ross Island, Antarctica is the site of three historic expedition huts left by early explorers to the South Pole, Robert F. Scott and Ernest Shackleton. The fungal diversity of these wooden structures and surrounding soils was investigated with traditional culturing methods as well as with molecular methodology including denaturing gradient gel electrophoresis (DGGE) using the internal transcribed spacer (ITS) regions of ribosomal DNA for identification. From historic wood and artifact samples and soils adjacent to the huts as well as soil samples obtained from the Lake Fryxell Basin, a remote Dry Valley location, and remote sites at Mt. Fleming and the Allan Hills, 71 fungal taxa were identified. The historic huts and associated artifacts have been colonized and degraded by fungi to various extents. The most frequently isolated fungal genera from the historic woods sampled include Cadophora, Cladosporium and Geomyces. Similar genera were found in soil samples collected near the huts. Sampling of soils from locations in the Transantarctic Mountains and Lake Fryxell Basin at considerable distances from the huts and with different soil conditions revealed Cryptococcus spp., Epicoccum nigrum and Cladosporium cladosporioides as the most common fungi present and Cadophora species less commonly isolated. DGGE revealed 28 taxa not detected by culturing including four taxa which possibly have not been previously described since they have less than 50% ITS sequence identity to any GenBank accessions. Fungi capable of causing degradation in the wood and artifacts associated with the expedition huts appear to be similar to those present in Antarctic soils, both near and at more remote locations. These species of fungi are likely indigenous to Antarctica and were apparently greatly influenced by the introduction of organic matter brought by early explorers. Considerable degradation has occurred in the wood and other materials by these fungi.     

Spatial distribution and physiology of biological soil crusts from semi-arid central Spain are related to soil chemistry and shrub cover

Despite the critical role of biological soil crusts (BSCs) in arid and semi-arid ecosystem function, few studies are found concerning the most important environmental variables affecting their distribution and physiology. This study seeks to determine soil and microenvironmental factors affecting the spatial distribution and pigment production of BSC-forming lichens and mosses in open patches of a semi-arid Mediterranean kermes oak thicket. We measured late-successional BSC cover, shrub cover, distance to nearest kermes oak (to test for effects of kermes oak thicket microenvironment on BSC), and pigment concentration of one lichen (Cladonia foliacea) and one moss (Pleurochaete squarrosa) species in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain). At the macroscale (>0.5 m), results showed that BSC distribution and pigments were tightly coupled to a suite of soil properties, in particular soil pH, Fe, and Ca. Specifically, soil pH had a positive relationship with the cover of five individual BSC-forming lichen species and was negatively related to pigment production in C. foliacea. When pH was excluded from the analysis, Ca appeared as the main soil variable and was correlated with total BSC cover and total lichen cover. The micronutrient Fe had a significant positive relationship with the concentration of eight pigments in P. squarrosa and was also coupled with the cover of two BSC-forming lichens. Manganese, previously proposed as a key limiting micronutrient for BSCs, affected lichen diversity in a negative way. At the microscale (∼0.5 m), kermes oak microenvironment, shrub cover, and moss cover were determinants of BSC distribution, and total lichen and total BSC cover were overrepresented on N and E-facing shrub microsites. Our findings suggest that soil chemical variability and microsite diversity created by neighbouring vegetation affect BSC distribution in complex and essential ways and that studies aiming to explore BSC-environment relationships should be conducted at various spatial scales. Studies based on species- or group-specific responses are, thus, inadequate to unveil the main factors determining the distribution of the diverse organisms that constitute BSCs and/or to propose potential tools aiming to restore BSC in arid and semiarid ecosystems.     

Earthworms as colonizers of natural and cultivated soil environments

For cultivated soils, the important function of earthworms as ecosystem engineers and their major contribution to the composition and functioning of soil ecosystems with a varying species diversity has been extensively addressed. However, the role of earthworms as colonizers of virgin, uncultivated soil in the process of soil formation has been little researched and long underrated. To better understand this role, the following questions need to be considered: (1) what makes an early colonizer successful, what are its characteristics, and which species are the most successful and under what circumstances are they successful?; (2) what are the limiting factors in these colonization processes with respect to environmental conditions and also to interspecific interactions?; (3) what do earthworms contribute to the further colonization by other soil animals?; and (4) how do they impact the soil itself and what could therefore be the consequences for soil management and restoration?These questions have recently been addressed from the perspective of new (or ‘alien’) earthworm species invading ecosystems, suggesting a massive influx of species, competitive to the originally present fauna. This idea is, however, contrary to colonization, which suggests a gradual exploration of a previously uninhabited area. Unlike recent research, this review approaches colonization primarily as a spatial dispersal process and part of natural succession processes, and is mainly illustrated with examples of Palearctic species, either in Europe or introduced elsewhere. To begin, the various stages of colonization: dispersal, establishment, population growth and interspecies relations are analysed. Next, the colonization processes, the possible limiting environmental factors and the sequence of the appearance and establishment of species are described. Dispersal rates and sequences of colonization by different earthworm species are given for different soil ecosystems. For colonization, limiting environmental factors such as pH, soil type and heavy metal contents as well as the presence of organic matter seem to play a more important role than inherent ecological characteristics like r/K selection. Finally, the role of earthworms in the early colonization of soils that are earthworm-free because of non-cosmopolitan distribution, drained former sea bottom, permanently water-logged soils or anaerobic, acid peaty soils are reviewed. If we understand the role of earthworms in succession, we will be able to improve their role in soil restoration and soil management.     

Short-range spatial variability of soil physico-chemical variables related to earthworm clustering in a neotropical gallery forest

In this study, we investigated the spatial distribution of an earthworm community together with the heterogeneity of selected soil properties in a gallery forest (GF) of the Colombian “Llanos”. We performed fine-scale spatial variability by intensively sampling 100 points distributed in the nodes of a regular grid with 5 m inter-sample distance. Non-parametric statistics were used and included SADIE analysis and partial Mantel test, in addition to geostatistics (semi-variograms) and correlogram computation. Our results indicated that the spatial distribution of earthworms was characterized by areas of presence (patches) and absence (gaps), although the general pattern was random at the scale of this study (<5 m), while soil physico-chemical characteristics showed a clumped spatial distribution. Contrary to previous results reported for the nearby savanna, a significant spatial association was found for two competing endogeic species Andiodrilus sp. and Glossodrilus sp. in the GF. Semi-variograms of soil environmental factors were adjusted to model families most commonly used (spherical and linear), and correlograms for earthworms showed significant positive and negative spatial autocorrelation for lag distances <15 m and >30 m, respectively. Partial Mantel test revealed specific significant relationships between soil variables and some species. The earthworm community of the GF displayed a random structure in a spatially clumped soil environment, and our results suggested that spatial distribution observed for some species could be the result of preferential selection of soil environmental factors. In other words, soil heterogeneity contributed to the formation of population patches for some earthworm species. The variability of suitable sites (resource availability patchiness) exerted an influence in the spatial distribution of earthworms at the scale used in this study, and we identified the spatial scale at which both environmental heterogeneity could influence and express earthworm impact on soil properties.     

Distribution and abundance of fungi in the soils of Taylor Valley, Antarctica

The occurrence and distribution of culturable fungi in Taylor Valley, Antarctica was assessed in terms of soil habitat. Soil transects throughout the valley revealed differential habitat utilization between filamentous and non-filamentous (yeast and yeast-like) fungi. In addition, there were significant differences in species distribution patterns with respect to soil pH, moisture, distance from marine coastline, carbon, chlorophyll a, salinity, elevation and solar inputs. Filamentous fungal abundance is most closely associated with habitats having higher pH, and soil moistures. These close associations were not found with yeast and yeast-like fungi demonstrating that yeast and yeast-like fungi utilize a broader range of habitat. An intensive survey of the Victoria Land is necessary to gain a better understanding of their role in soil functioning and nutrient cycling processes.     

Earthworm communities in alluvial forests: Influence of altitude,vegetation stages and soil parameters

In many terrestrial ecosystems, soil parameters usually regulate the distribution of earthworm communities. In alluvial ecosystems, few studies have investigated the impact of periodic floods and alluvium deposition on soil fauna. In this context, we assumed that earthworm communities may vary depending on altitude (alpine, subalpine, mountain and hill levels), forest successional stage (post-pioneer to mature forests) and some soil parameters. Our results demonstrated that the composition of earthworm communities differed depending on altitudinal gradients. No earthworm was found at the alpine level while maximum density and biomass were observed at the hill level mainly due to the contribution of anecic species. A total of 27 species and subspecies were found over the three sampling sites, and Lumbricus moliboeus was discovered for the first time in carbonated soils. Soil texture had a major effect on epigeics that were often associated with coarse sandy texture in contrast to anecics which preferred deep soils and mature forest stages, which in combination provided the highest carbon content and the finest soil texture. In our study, carbonated fluviosols (Fluvisols according to the World Reference Base) were recorded; fluviosols typiques with well-structured A layers were generally found in mature or intermediate forest stages while most of fluviosols juveniles with heterogeneous texture were observed principally in post-pioneer forests. We conclude that in alluvial ecosystems, earthworm communities were highly dependent first on soil parameters, then altitude and to a lesser extent forest successional stages. Changes in earthworm communities tend to reflect a gradient of alluvial dynamics thus reinforcing the potential role of earthworms as bioindicators in natural and/or semi natural alluvial ecosystems.     

Fate and effects of insect-resistant Bt crops in soil ecosystems

Recent applications of biotechnology, especially genetic engineering, have revolutionized crop improvement and increased the availability of valuable new traits. A current example is the use of the insecticidal Cry proteins from the bacterium, Bacillus thuringiensis (Bt), to improve crops, known as Bt crops, by reducing injury from various crop pests. The adoption of genetically modified (GM) crops has increased dramatically in the last 11 years. However, the introduction of GM plants into agricultural ecosystems has raised a number of questions, including the ecological impact of these plants on soil ecosystems. Crop residues are the primary source of carbon in soil, and root exudates govern which organisms reside in the rhizosphere. Therefore, any change to the quality of crop residues and rhizosphere inputs could modify the dynamics of the composition and activity of organisms in soil. Insect-resistant Bt crops have the potential to change the microbial dynamics, biodiversity, and essential ecosystem functions in soil, because they usually produce insecticidal Cry proteins through all parts of the plant. It is crucial that risk assessment studies on the commercial use of Bt crops consider the impacts on organisms in soil. In general, few or no toxic effects of Cry proteins on woodlice, collembolans, mites, earthworms, nematodes, protozoa, and the activity of various enzymes in soil have been reported. Although some effects, ranging from no effect to minor and significant effects, of Bt plants on microbial communities in soil have been reported, using both culturing and molecular techniques, they were mostly the result of differences in geography, temperature, plant variety, and soil type and, in general, were transient and not related to the presence of the Cry proteins. The respiration (i.e., CO₂ evolution) of soils cultivated with Bt maize or amended with biomass of Bt maize and other Bt crops was generally lower than from soils cultivated with or amended with biomass of the respective non-Bt isolines, which may have been a result of differences in chemical composition (e.g., the content of starch, soluble N, proteins, carbohydrates, lignin) between Bt plants and their near-isogenic counterparts. Laboratory and field studies have shown differences in the persistence of the Cry proteins in soil, which appear to be the result primarily of differences in microbial activity, which, in turn, is dependent on soil type (e.g., pH, clay mineral composition, other physicochemical characteristics), season (e.g., temperature, water tension), crop species (e.g., chemical composition, C:N ratio, plant part), crop management practices (e.g., till vs. no-till), and other environmental factors that vary with location and climate zones. This review discusses the available data on the effects of Cry proteins on below-ground organisms, the fate of these proteins in soil, the techniques and indicators that are available to study these aspects, and future directions.     

Spatial heterogeneity provides organic matter refuges for soil microbial activity in the Patagonian steppe, Argentina

In arid and semi-arid ecosystems that are frequently classified as water limited, it is unclear how spatial and temporal variability of vegetation and climate could affect microbially-mediated soil processes. Our objective was to determine how aboveground spatial heterogeneity creates characteristic soil conditions that modulate microbial growth and activity in a semi-arid Patagonian steppe. In particular, we explored how micro-environmental and biogeochemical soil characteristics generated by the native vegetation could control soil β-glucosidase activity. Both life-form (shrubs, grasses, mosses and bare soil) and season exerted strong controls on all measured abiotic (soil temperature and gravimetric soil water content, inorganic nitrogen, pH and total C and N) and biotic (microbial biomass C and β-glucosidase activity) soil characteristics. Partial correlation between β-glucosidase activity and extracellular organic C (EOC) was high across seasons (r = 0.5; P < 0.001) while soil water content did not correlate with soil enzymatic activity (r = 0.09; P > 0.05). We postulate that labile soil carbon rather than water availability functions as a principal limitation of microbial activity in this semi-arid ecosystem, and the distribution of this carbon is, in large part, determined by the patchy distribution of vegetation.     

Interactions between physical and biotic factors influence CO2 flux in Antarctic dry valley soils

Soil carbon dioxide (CO₂) flux is an integrative measure of ecosystem functioning representing both biotic and physical controls over carbon (C) balance. In the McMurdo Dry Valleys of Antarctica, soil CO₂ fluxes (approximately −0.1-0.15 μmol m⁻² s⁻¹) are generally low, and negative fluxes (uptake of CO₂) are sometimes observed. A combination of biological respiration and physical mechanisms, driven by temperature and mediated by soil moisture and mineralogy, determine CO₂ flux and, therefore, soil organic C balance. The physical factors important to CO₂ flux are being altered with climate variability in many ecosystems including arid forms such as the Antarctic terrestrial ecosystems, making it critical to understand how climate factors interact with biotic drivers to control soil CO₂ fluxes and C balances. We measured soil CO₂ flux in experimental field manipulations, microcosm incubations and across natural environmental gradients of soil moisture to estimate biotic soil respiration and abiotic sources of CO₂ flux in soils over a range of physical and biotic conditions. We determined that temperature fluctuations were the most important factor influencing diel variation in CO₂ flux. Variation within these diel CO₂ cycles was explained by differences in soil moisture. Increased temperature (as opposed to temperature fluctuations) had little or no effect on CO₂ flux if moisture was not also increased. We conclude that CO₂ flux in dry valley soils is driven primarily by physical factors such as soil temperature and moisture, indicating that future climate change may alter the dry valley soil C cycle. Negative CO₂ fluxes in arid soils have recently been identified as potential net C sinks. We demonstrate the potential for arid polar soils to take up CO₂, driven largely by abiotic factors associated with climate change. The low levels of CO₂ absorption into soils we observed may not constitute a significant sink of atmospheric CO₂, but will influence the interpretation of CO₂ flux for the dry valley soil C cycle and possibly other arid environments where biotic controls over C cycling are secondary to physical drivers.     

Photosynthetic pigments in soils from northern Victoria Land (continental Antarctica) as proxies for soil algal community structure and function

Although soil algae are among the main primary producers in most terrestrial ecosystems of continental Antarctica, there are very few quantitative studies on their relative proportion in the main algal groups and on how their distribution is affected by biotic and abiotic factors. Such knowledge is essential for understanding the functioning of Antarctic terrestrial ecosystems. We therefore analyzed biological soil crusts from northern Victoria Land to determine their pH, electrical conductivity (EC), water content (W), total and organic C (TC and TOC) and total N (TN) contents, and the presence and abundance of photosynthetic pigments. In particular, the latter were tested as proxies for biomass and coarse-resolution community structure. Soil samples were collected from five sites with known soil algal communities and the distribution of pigments was shown to reflect differences in the relative proportions of Chlorophyta, Cyanophyta and Bacillariophyta in these sites. Multivariate and univariate models strongly indicated that almost all soil variables (EC, W, TOC and TN) were important environmental correlates of pigment distribution. However, a significant amount of variation is independent of these soil variables and may be ascribed to local variability such as changes in microclimate at varying spatial and temporal scales. There are at least five possible sources of local variation: pigment preservation, temporal variations in water availability, temporal and spatial interactions among environmental and biological components, the local-scale patchiness of organism distribution, and biotic interactions.     

Long-term experimental warming reduces soil nematode populations in the McMurdo Dry Valleys, Antarctica

Climate models predict significant future warming in polar regions. In the McMurdo Dry Valleys, Antarctica, projected summer climate warming is expected to increase snow and glacial melt, resulting in higher stream discharge, rising lake levels, and an increase in areas of moist soil, but the potential influence of warming and associated changes in hydrology on the soil ecosystem is poorly understood. To examine the effects of soil warming and changes in the availability of liquid water on populations of soil invertebrates and their habitat, we established a full-factorial warming and water addition experiment at one experimental site in each of the three hydrologic basins of Taylor Valley, Antarctica, and measured responses over 8 years. We hypothesized that an increase in temperature and moisture together would enhance habitat suitability for soil invertebrates thereby increasing abundance, biomass and diversity of the soil animal communities. Instead, warming treatments had an overall negative effect on density and body size of the microbial-feeding nematode Scottnema lindsayae, the dominant animal in the dry valleys, which decreased by 42% in warmed plots. While experimental moisture additions as a single annual pulse had no effect on nematodes, the surface flooding of one site from rapid melting of upslope subsurface ice (the result of an unusual natural warming event) drastically altered soil moisture, salinity, and animal communities; mortality of S. lindsayae increased and densities decreased. This extreme soil wetting event also resulted in an increase in chlorophyll a and populations of Eudorylaimus spp, a nematode species that prefers moist to wet habitats and feeds on soil micro-algae. Our results suggest that warming in the dry valleys could significantly affect soil nematode populations and species composition both directly and indirectly by altering species-specific habitat suitability for soil biota.     

Controls on soil respiration in semiarid soils

Soil respiration in semiarid ecosystems responds positively to temperature, but temperature is just one of many factors controlling soil respiration. Soil moisture can have an overriding influence, particularly during the dry/warm portions of the year. The purpose of this project was to evaluate the influence of soil moisture on the relationship between temperature and soil respiration. Soil samples collected from a range of sites arrayed across a climatic gradient were incubated under varying temperature and moisture conditions. Additionally, we evaluated the impact of substrate quality on short-term soil respiration responses by carrying out substrate-induced respiration assessments for each soil at nine different temperatures. Within all soil moisture regimes, respiration rates always increased with increase in temperature. For a given temperature, soil respiration increased by half (on average) across moisture regimes; Q₁₀ values declined with soil moisture from 3.2 (at −0.03 MPa) to 2.1 (−1.5 MPa). In summary, soil respiration was generally directly related to temperature, but responses were ameliorated with decrease in soil moisture.     

Diurnal and seasonal carbon dioxide exchange and its components in temperate grasslands in the Netherlands — an outline of the methodology

A methodological outline is presented of a study into the diurnal and seasonal cycle of carbon fluxes within grassland ecosystems in the Netherlands in relation to their environment. At experimental sites Lelystad and Zegveld ?redominantlyLolium perenne L. at a clay and peat soil, respectively — measurements will be made on (1) net CO₂ assimilation of the grassland vegetation using infrared gas analysis; (2) carbon distribution within the plant using¹⁴C pulse labeling; and (3) carbon and CO₂ fluxes associated with root respiration and soil organic matter decomposition using¹⁴C pulse labeling. At both sites and at experimental site Cabauw additional measurements will be made on total CO₂ fluxes between the grassland vegetation and the lower part of the atmospheric boundary layer. For the analysis of the experimental results and generalisation of the relationships between carbon fluxes and environmental and plant factors use will be made of dynamic simulation models of grass growth and soil organic matter dynamics.     

Responses of soil respiration and its components to drought stress

Purpose

Climate change is likely to increase both intensity and frequency of drought stress. The responses of soil respiration (R _s) and its components (root respiration, R _r; mycorrhizal respiration, R _m; and heterotrophic respiration, R _h) to drought stress can be different. This work aims to review the recent and current literature about the variations in R _s during the period of drought stress, to explore potential coupling processes and mechanisms between R _s and driving factors in the context of global climate change.

Results and discussion

The sensitivity of soil respiration and its components to drought stress depended on the ecosystems and seasonality. Drought stress depressed R _s in mesic and xeric ecosystems, while it stimulated R _s in hydric ecosystems. The reductions in supply and availability of substrate decreased both auto- and heterotrophic respirations, leading to the temporal decoupling of root and mycorrhizal respiration from canopy photosynthesis as well as C allocation. Drought stress also reduced the diffusion of soluble C substrate, and activities of extracellular enzymes, consequently, limited microbial activity and reduced soil organic matter decomposition. Drought stress altered Q ₁₀ values and broke the coupling between temperature and soil respiration. Under drought stress conditions, R _m is generally less sensitive to temperature than R _r and R _h. Elevated CO₂ concentration alleviated the negative effect of drought stress on soil respiration, principally due to the promotion of plant C assimilation subsequently, which increased substrate supply for respiration in both roots and soil microorganisms. Additionally, rewetting stimulated soil respiration dramatically in most cases, except for soil that experienced extreme drought stress periods. The legacy of drought stress can also regulate the response of soil respiration rate to rewetting events in terrestrial ecosystems through changing abiotic drivers and microbial community structure.

Conclusions and perspectives

There is increasing evidence that drought stress can result in the decoupling of the above- and belowground processes, which are associated with soil respiration. However, studies on the variation in rates of soil respiration and its components under different intensities and frequencies of drought stress over the ecosystems should be reinforced. Meanwhile, molecular phylogenetics and functional genomics should be applied to link microbial ecology to the process of R _s. In addition, we should quantify the relationship between soil respiration and global change parameters (such as warming and elevated [CO₂]) under drought stress. Models simulating the rates of soil respiration and its components under global climate change and drought stress should also be developed.     

Geographic distance and amorphous iron affect the abundance and distribution of <Emphasis Type="Italic">Geobacteraceae</Emphasis> in paddy soils in China

Purpose

Geobacteraceae are important dissimilatory Fe (III)-reducing microorganisms, influencing the cycling of metals, nutrients as well as the degradation of organic contaminants. However, little is known about their distribution, diversity, and abundance of Geobacteraceae and the effects of environment factors and geographic distance on the distribution and diversity of Geobacteraceae in paddy soils remain unclear. Therefore, the objectives of this study were to investigate the distribution, diversity, and abundance of Geobacteraceae in paddy soils and to determine key factors in shaping the Geobacteraceae distribution, environmental factors, geographic distance, or both and to quantify their contribution to Geobacteraceae variation.

Materials and methods

Illumina sequencing and quantitative real-time PCR using a primer set targeting 16S rRNA genes of bacteria affiliated with the family Geobacteraceae were employed to measure the community composition, diversity, and abundance patterns of 16S rRNA genes of Geobacteraceae in 16 samples collected from north to south of China. MRT, Mantel test, and VPA were used to analyze the relationship between communities of Geobacteraceae and environmental factors and geographic distance.

Results and discussion

Quantitative PCR showed that the abundance of 16S rRNA genes of Geobacteraceae ranged from (1.20?±?0.18)?×?10⁸ to 1.13?×?10⁹?±?2.25?×?10⁸ copies per gram of soil (dry weight) across different types of soils. Illumina sequencing results showed Geobacter was the dominant genus within the family of Geobacteraceae. Multivariate regression tree (MRT) analysis showed that soil amorphous iron contributed more (22.46 %) to the variation of dominant species of Geobacteraceae than other examined soil chemical factors such as pH (14.52 %), ammonium (5.12 %), and dissolved organic carbon (4.74 %). Additionally, more geographically distant sites harbored less similar communities. Variance partitioning analysis (VPA) showed that geographic distance contributed more to the variation of Geobacteraceae than any other factor, although the environmental factors explained more variation when combined. So, we detected the uneven distribution of Geobacteraceae in paddy soils of China and demonstrated that Geobacteraceae community composition was strongly associated with geographic distance and soil chemical factors including aFe, pH, Fe, DOC, C:N, and NO₃ ^?-N. These results greatly expand the knowledge of the distribution of Geobacteraceae in environments, particularly in terrestrial ecosystems.

Conclusions

Our results showed that geographic distance and amorphous iron played important roles in shaping Geobacteraceae community composition and revealed that both geographic distance and soil properties governed Geobacteraceae biogeography in paddy soils. Our findings will be critical in facilitating the prediction of element cycling by incorporating information on functional microbial communities into current biogeochemical models.