Development of soil microbial communities during tallgrass prairie restoration

Soil microbial communities were examined in a chronosequence of four different land-use treatments at the Konza Prairie Biological Station, Kansas. The time series comprised a conventionally tilled cropland (CTC) developed on former prairie soils, two restored grasslands that were initiated on former agricultural soils in 1998 (RG₉₈) and 1978 (RG₇₈), and an annually burned native tallgrass prairie (BNP), all on similar soil types. In addition, an unburned native tallgrass prairie (UNP) and another grassland restored in 2000 (RG₀₀) on a different soil type were studied to examine the effect of long-term fire exclusion vs. annual burning in native prairie and the influence of soil type on soil microbial communities in restored grasslands. Both 16S rRNA gene clone libraries and phospholipid fatty acid analyses indicated that the structure and composition of bacterial communities in the CTC soil were significantly different from those in prairie soils. Within the time series, soil physicochemical characteristics changed monotonically. However, changes in the microbial communities were not monotonic, and a transitional bacterial community formed during restoration that differed from communities in either the highly disturbed cropland or the undisturbed original prairie. The microbial communities of RG₉₈ and RG₀₀ grasslands were also significantly different even though they were restored at approximately the same time and were managed similarly; a result attributable to the differences in soil type and associated soil chemistry such as pH and Ca. Burning and seasonal effects on soil microbial communities were small. Similarly, changing plot size from 300 m² to 150 m² in area caused small differences in the estimates of microbial community structure. In conclusion, microbial community structure and biochemical properties of soil from the tallgrass prairie were strongly impacted by cultivation, and the microbial community was not fully restored even after 30 years.     

Macroinvertebrates in North American tallgrass prairie soils: effects of fire, mowing, and fertilization on density and biomass

The responses of tallgrass prairie plant communities and ecosystem processes to fire and grazing are well characterized. However, responses of invertebrate consumer groups, and particularly soil-dwelling organisms, to these disturbances are not well known. At Konza Prairie Biological Station, we sampled soil macroinvertebrates in 1994 and 1999 as part of a long-term experiment designed to examine the effects and interactions of annual fire, mowing, and fertilization (N and P) on prairie soil communities and processes. For nearly all taxa, in both years, responses were characterized by significant treatment interactions, but some general patterns were evident. Introduced European earthworms (Aporrectodea spp. and Octolasion spp.) were most abundant in plots where fire was excluded, and the proportion of the total earthworm community consisting of introduced earthworms was greater in unburned, unmowed, and fertilized plots. Nymphs of two Cicada genera were collected (Cicadetta spp. and Tibicen spp.). Cicadetta nymphs were more abundant in burned plots, but mowing reduced their abundance. Tibicen nymphs were collected almost exclusively from unburned plots. Treatment effects on herbivorous beetle larvae (Scarabaeidae, Elateridae, and Curculionidae) were variable, but nutrient additions (N or P) usually resulted in greater densities, whereas mowing usually resulted in lower densities. Our results suggest that departures from historical disturbance regimes (i.e. frequent fire and grazing) may render soils more susceptible to increased numbers of European earthworms, and that interactions between fire, aboveground biomass removal, and vegetation responses affect the structure and composition of invertebrate communities in tallgrass prairie soils.     

Regulation of nitrogen mineralization and nitrification in Southern Appalachian ecosystems: Separating the relative importance of biotic vs. abiotic controls

Long-term measurements of soil nitrogen (N) transformations along an environmental gradient within the Coweeta Hydrologic Laboratory basin in western North Carolina showed a strong seasonal pattern and suggested that vegetation community type—through its influence on soil properties—was an important regulating factor. Our objective was to determine the relative effects of biotic vs. abiotic factors on soil N transformations. During the 1999 and 2000 growing seasons we transplanted soil cores from each of the five gradient plots to all other gradient plots for their 28-day in situ incubation. N mineralization and nitrification rates in soils from the northern hardwood (NH) site were significantly increased when soils were transplanted to warmer sites. N mineralization rates also increased in transplanted soil from the dry mixed-oak/pine site to a wetter site. Multiple regression analysis of N mineralization from all five sites found that biotic (total soil N and C:N ratios) and climatic factors (moisture and temperature) regulate N mineralization. Regression analyses of individual sites showed that N mineralization rates responded to variation in temperature and moisture at only the high elevation northern hardwood site and moisture alone on the dry warm mixed-oak/pine site. N mineralization was unrelated to temperature or moisture at any of the other sites. Results indicate that soil properties plus climatic conditions affect soil N transformations along the environmental gradient at Coweeta. Environmental controls were significant only at the extreme sites; i.e., at the wettest and warmest sites and soils with highest and lowest C and N contents. The high degree of temperature sensitivity for the northern hardwood soils indicates potentially large responses to climatic change at these sites.     

Influence of soil properties on the vertical movement of genetically-marked Pseudomonas fluorescens through large soil microcosms

Summary Vertical translocation of the introduced transposon Tn5-tagged Pseudomonas fluorescens cells was studies after irrigation of 50-cm long soil columns of loamy sand. The soil in the columns was slowly brought to saturation using groundwater, and enough water was then slowly added to permit collection of the percolated water. Introduced bacteria were transported to lower soil layers to a significantly higher degree in undisturbed soil cores than in repacked cores; water transport was hampered in both core types due to high soil bulk densities. Soil bulk density affected the degree of transport of the introduced cells; progressively more cells were translocated to deeper soil layers and into the percolation water at decreasing soil bulk densities. Repeated percolation of soil at a bulk density of 1.25 caused an increase in Tn5-tagged cell numbers in the lower soil layers and in the percolated water. Further, cells initially introduced into a dry (5.3% moisture) soil were translocated to a lesser extent than cells introduced into a wetter (13% moisture) soil. Finally, wheat roots enhanced the water-induced transport of introduced cells to the 40- and 50-cm deep soil layers and into the effluent, but not to the remaining soil layers. Large soil columns such as those used in the present study are useful in assessing the transport and survival of introduced bacterial cells in soils under a variety of simulated environmental conditions.     

Relating microarthropod community structure and diversity to soil fertility manipulations in temperate grassland

We aimed to identify patterns of diversity in a below-ground community of microarthropods (mites and Collembola) after 15 months of a nutrient (calcium and nitrogen) manipulation experiment, located at the Natural Environment Research Council (NERC) Soil Biodiversity Site in Scotland, UK. We found that microarthropod densities increased with elevated soil fertility, but we detected no concurrent change in the diversity of soil microarthropods (mites and Collembola combined). That microarthropod density increased concurrently with improvements in soil fertility and plant productivity suggests that soil microarthropod communities are predominately regulated by bottom-up forces, driven by increased energy transfer via plant inputs to soil, providing increased food resources for fauna. However, that we found no concurrent change in the diversity of soil microarthropods provides little support for the idea that the diversity of soil fauna is positively related to their population density, primary productivity or improvements in soil conditions resulting from nutrient manipulations. However, we did find that microarthropod communities of more fertile sites contained a greater proportion of predators suggesting that more energy was transferred to higher trophic levels under elevated soil fertility. Our findings suggest that unlike plant communities, soil faunal diversity may not be strongly regulated by competition in productive situations, since competitive exclusion might not occur due to increased predation. Whilst we conclude that soil microarthropod diversity at our study site has not been affected by the nutrient additions to date, in the longer term we predict that changes in community composition and diversity could arise, most likely through top-down regulation of the soil food web.     

Grazing‐induced alterations of soil hydraulic properties and functions in Inner Mongolia,PR China

Increasing grazing intensities of sheep and goats can lead to an increasing degradation of grasslands. We investigated four plots of different grazing intensities (heavily grazed, winter‐grazed, ungrazed since 1999, and ungrazed since 1979) in Inner Mongolia, PR China, in order to study the effects of trampling‐induced mechanical stresses on soil hydraulic properties. Soil water transport and effective evapotranspiration under “heavily grazed” and “ungrazed since 1979” were modeled using the HYDRUS‐1D model. Model calibration was conducted using data collected from field studies. The field data indicate that grazing decreases soil C content and hydrophobicity. Pore volume is reduced, and water‐retention characteristics are modified, the saturated hydraulic conductivity decreases, and its anisotropy (vertical vs. horizontal conductivity) is influenced. Modeling results revealed higher evapotranspiration on the ungrazed site (ungrazed since 1979) compared to the grazed site (heavily grazed) due to wetter soil conditions, more dense vegetation, litter cover, and decreased runoff and drainage, respectively. Grazing modified the partitioning of evapotranspiration with lower transpiration and higher evaporation at the grazed site owing to reduced root water uptake due to reduced evaporation and a patchy soil cover.     

Soil biota and soil properties in the surface rooting zone of mesquite (Prosopis glandulosa) in historical and recently desertified Chihuahuan Desert habitats

Summary The woody legume, mesquite (Prosopis glandulosa) has expanded from its historical habitats (playas and arroyos) to recently occupied grassland and dune habitats during the desertification of perennial grasslands in the Chihuahuan Desert. We studied historical and recently occupied sites, having hypothesized that the trophic structure and population density of soil microarthropods and nematodes associated with the surface root system of mesquite would differ in sites representing historical and recent habitats, and that the N mineralization potential would be lower in the recent habitats. Our results showed that net N mineralization potential did not differ significantly among the sites, even though soil nutrient concentrations and texture varied widely. Concentrations of organic C, N, and P were lowest in the recent dune habitat and highest at the playa. Very low concentrations of P in the dune and grassland soils implicated P as a limiting factor in these systems. The bacterial-feeding and omnivore-predator functional groups made up the largest fraction of the nematode community at most of the sites. The high density of plant-feeding nematodes at the playa indicated that herbivory is potentially most important at this site. Total microarthropod densities did not vary significantly among habitats, with Collembola densities highest in the mesquite dunes. Grazers were the dominant microarthropod functional group. While both C and N pool sizes were higher in the historical habitats, a higher substrate lability in the recent habitats appeared to support biota populations and N mineralization rates equivalent to those in the playa and arroyo. Differences in soil properties and biota among historical and recent mesquite habitats may be important for understanding the changes that have occurred in Chihuahuan Desert ecosystems during desertification.     

Soil microarthropod community testing: A new approach to increase the ecological relevance of effect data for pesticide risk assessment

In the present study, a new complementary approach combining the use of the natural soil microarthropod community and conventional test methods was used. The effects of soil contamination with the insecticide carbofuran on two geographically distinct microarthropod communities (Mediterranean and Tropical) were evaluated in their soils of origin under controlled laboratory conditions.After contamination of two agricultural soils from Portugal and Brazil, a gradient of concentrations was prepared. Soil cores were taken from the respective uncontaminated surrounding areas and the mesofauna of three cores was extracted directly to the test soil. After extracting the microarthropod communities to the test soil, these were incubated under laboratory conditions for 4 weeks, after which the mesofauna was extracted again. The organisms were assorted into higher taxonomic groups and Acari and Collembola were respectively assorted into order/sub-order/cohort and family. Collembolans were still classified according to morphological traits and used as a case-study of trait based risk assessment (TERA; Baird et al., 2008) of pesticides.The exposure to insecticide contamination caused the impoverishment of the taxonomic diversity in both communities. Significant shifts in the microarthropod community structure in the different carbofuran treatments were found for both soils, although effects were more pronounced in the assay performed with the soil from Brazil. Collembolans were the most affected group with a strong decline in their abundance. A dose–response relationship was observed, showing a consistent decline on the relative abundance of Isotomidae, closely followed by an increase of Entomobryidae. Contrastingly, Acari (especially Oribatida) tended to increase their numbers with higher concentrations.Trait based analysis of Collembola data suggested that a shift in the functional composition of the communities occurred due to carbofuran soil contamination and that species adapted to deeper soil layers were more vulnerable to insecticide toxicity.     

Responses of soil microarthropods to inorganic and organic fertilizers in a poplar plantation in a coastal area of eastern China

Soil microarthropod community is an essential functional unit of soil food webs. Fertilizers can induce an alteration of quantity and quality of food for soil fauna and trigger profound changes in soil faunal communities. We initiated this study to examine the influence of organic and inorganic fertilizers on soil microarthropods in poplar plantations (Populus deltoides) in a coastal region of northern Jiangsu, eastern China. We established a control and four fertilizer application treatments: low and high levels of organic fertilizers, low and high levels of inorganic fertilizers. Organic fertilizer amendments increased both soil organic carbon (C) and total nitrogen (N), whereas inorganic fertilizer had a positive significant effect on soil total N. The application of both inorganic and organic fertilizers resulted in significantly reduced soil pH. We found that both inorganic and organic fertilizers increased the abundance of all soil microarthropods, bacterivorous Acari, and hemiedaphic and epedaphic Collembola, but had no influence on the total taxonomic richness, Shannon diversity index and DG diversity index of the microarthropod community. The abundance of soil microarthropods was positively correlated with soil C and N, and negatively with pH. Our results indicate that changes in the quality and quantity of soil organic matter and other immediate chemical properties after fertilizer application can increase the abundance of soil microarthropods, but have a limited influence on their diversity in the coastal alkaline soils of eastern China.     

Fungal diversity in set-aide agricultural soil investigated using terminal-restriction fragment length polymorphism

As part of the restoration of biodiversity on former agricultural land there has been focused on methods to enhance the rate of transition from agricultural land towards natural grasslands or forest ecosystems. Management practices such as sowing seed mixtures and inoculating soil of later successional stages have been used. The aim of this study was to determine the effects of a managed plant community on the diversity of soil fungi in a newly abandoned agricultural land. A field site was set up consisting of 20 plots where the plant diversity was managed by either sowing 15 plant species, or natural colonization was allowed to occur. The plant mixture contained five species each of grasses, legumes and forbs that all were expected to occur at the site. A subset of the plots (five from each treatment) was inoculated with soil cores from a late successional stage. The plant community composition was subject to a principal component analysis based on the coverage of each species. Five years after abandonment, soil samples were taken from the plots, DNA was extracted and the ITS region of the rDNA gene was amplified using fluorescently labelled fungal specific primers (ITS 1F/ITS 4). The PCR products were digested using HinfI and TaqI and sequenced. Results from both restriction enzymes were combined and a principal component analysis performed on the presence/absence of fragments. Also the fungal diversity expressed as number of restriction fragments were analysed. There was significantly higher fungal species richness in the experimental plots compared to the forest and field soils, but no differences between sown and naturally colonized plots. The different plant treatments did not influence the below ground fungal community composition. Soil water content on the other hand had an impact on the fungal community composition.     

内盖夫荒漠中小型土壤节肢动物群落的时空变化

Desert ecosystems are characterized by sparse vegetation that affects both abiotic parameters and soil biota along the soil profile.This study was conducted in 2010–2011 in a loess plain in the northern Negev Desert highlands, Israel, to test two main hypotheses:1) the abundance and diversity of microarthropods would vary seasonally in the top 30-cm soil layer, but would be relatively stable at soil depths between 30 and 50 cm and 2) soil microarthropods would be more abundant in soils under shrubs with large litter accumulations than under shrubs with less litter or bare soil. Soil samples were collected each season from the 0–50 cm profile at10-cm intervals under the canopies of Hammada scoparia and Zygophyllum dumosum and from the bare interspaces between them.Soil moisture and soil organic carbon in the top 30-cm layers varied seasonally, but there was little variation in the soil layers deeper than 30 cm. Soil mites were most abundant in the top 30-cm soil layer in autumn and winter, with the highest number of families found in winter. There were no differences in soil microarthropod abundance attributable to the presence or absence of shrubs of either species. The microarthropod communities of the microhabitats studied consisted of Acari, Psocoptera, and Collembola. The Acari were mostly identified to the family level and were dominated by Oribatida(55%) and Prostigmata(41%) in all seasons and microhabitats, while the psocopterans were most abundant in summer. These results are opposite to those obtained in other studies in similar xeric environments. Moreover, our findings were not in line with our hypothesis that a better microhabitat played a major role in microarthropod community composition, diversity, and density.     

A reciprocal soil exchange experiment highlights tree root colonization effects on soil nitrification

For forest ecosystems, the relationship between root biomass, root growth and soil nitrification is still debated. Following repeated findings of significant differences in soil nitrification beneath comparable stands at the Breuil experimental site, a reciprocal soil exchange experiment combining high (H)‐ and low (L)‐nitrifying stands was conducted to highlight the effect of tree root colonization on the control of nitrification. Soil percent nitrification and fine root biomass were measured in undisturbed and in transplanted soil cores after 16 and 28 months. In undisturbed soils, the fine root biomass varied by tree species and explained only 14% of the variation in percent nitrification. In transplanted soil cores, percent nitrification converged, at different rates, towards values close to those measured in the undisturbed soil at the receptor stands. On the one hand, percent nitrification increased rapidly in soil cores from L transferred to H, while soil core colonization by roots remained low during the study period. Soil cores might have been colonized by active nitrifiers from their new environment, or/and the activity of the nitrifiers originally present was less suppressed by root activity in their new environment. On the other hand, percent nitrification decreased progressively in soil cores from H transferred to L as root colonization increased. This suggests that root colonization reduced nitrifier population and activity. Our findings suggest that the often‐reported influence of forest species on soil nitrification is probably multifactorial but the tree root colonization contributes.     

Effects of soil compaction on N-mineralization and microbial-C and -N. II. Laboratory simulation

Soil compaction can affect the turnover of C and N (e.g. by changing soil aeration or by changing microbial community structure). In order to study this in greater detail, a laboratory experiment simulating total soil porosities representative of field conditions in cropped and pasture soils was set up. Soils were silty clay loams (Typic Endoaquepts) from a site that had been cropped with cereals continuously for 28 years, a permanent pasture and a site that had been cropped with maize continuously for 10 years. Soils from the three sites were compacted into cores to different total porosities (corresponding bulk densities ranging from 0.88 to 1.30 Mg m⁻³). The soil cores were equilibrated to different matric potentials (ranging from −1 to −100 kPa), yielding values for the fraction of air-filled pores of < 0.01 to 0.53 m³ m⁻³, and then incubated at 25°C for 21 days. C-mineralization was on average 15, 33 and 21 μg C g⁻¹ day⁻¹ for soils from the cropped, pasture and maize sites, respectively, and was positively correlated with soil water contents. Net N-mineralization showed a similar pattern only for well-aerated, high total porosity cores (corresponding bulk density 0.88 Mg m⁻³) from the pasture soil. Denitrification at < 0.20 m³ m⁻³ for the fraction of air-filled pores may have caused the low N-mineralization rates observed in treatments with high water content or low porosity. Microbial biomass estimates decreased significantly with increasing water contents if measured by fumigation-extraction, but were not significantly affected by water content if estimated by the substrate-induced respiration method. The degree of soil compaction did not affect the microbial biomass estimates significantly but did affect microbial activity indirectly by altering aeration status.     

Effects of grazing on soil physical properties and soil erodibility in semiarid grassland of the Northern Loess Plateau (China)

Grazing animals provide a livelihood for farmers, but they may also produce adverse environmental effects. We investigated whether grazing leads to deterioration of soil physical properties that subsequently increases topsoil erodibility. We sampled three sites (an ungrazed grassland, a continuously grazed grassland, and a track trampled by stock) on the northern Loess Plateau of China. The bulk density, water content, proportion of stable aggregates, infiltration rate, and resistance to scouring were determined for each soil sample. The results showed that the track had the highest soil bulk density and the lowest soil water content, proportion of stable aggregates, infiltration rate, and ability to resist scouring. The ungrazed plots had the best results for these parameters, in terms of reduced erosion. Soil bulk density and the proportion of stable aggregates differed significantly with depth beneath the track. However, the effect of depth on water content, infiltration rate, and the soil resistance to scouring was not significant at any sampling site. The ability of the soil to resist scouring was negatively correlated with the soil's bulk density and positively correlated with the soil's water content, infiltration rate, and proportion of stable aggregates. Thus, soil physical properties played an important role in determining soil erodibility. Grazing and trampling by livestock therefore appear to cause deterioration of soil physical properties and to increase soil erodibility.     

Landscape variation in microarthropod response to calcium addition in a northern hardwood forest ecosystem

Loss of base cations from soils of the northeastern US may adversely influence forest health. A watershed-level calcium (as wollastonite, CaSiO₃) addition experiment was initiated at the Hubbard Brook Experimental Forest, NH, USA, in fall 1999 to examine responses of forest ecosystem structure and function to restoration of exchangeable Ca that was lost over previous decades. We quantified the response of soil microarthropods to Ca addition as part of this larger study, and we also quantified microarthropod response to smaller scale (1×2 m plots) wollastonite additions within one site in an untreated reference watershed. We observed a small increase in pH in the surface organic horizon, accompanied by a decline in microarthropod abundance per gram organic matter three years following wollastonite application. However, responses varied among microarthropod groups, across years, and among study plots that were distributed along an elevation gradient in treatment and reference watersheds. Collembola declined in the treatment relative to the reference watershed in the second year but recovered in the third year following wollastonite application, whereas oribatid, prostigmatid, and mesostigmatid mites all declined in the treatment relative to the reference watershed by the third year following treatment. The largest treatment responses were observed in two base-poor hardwood stands, whereas at a slightly less poor hardwood site we found the least abundant and least variable microarthropod populations, and no response was observed to either the watershed or the plot-level treatments. Our results demonstrate that soil microarthropods are sensitive to relatively small changes in soil Ca, but that landscape position must be considered in testing responses of these soil biota to soil base status.     

Ecological risks of the use of sewage sludge as fertilizer in soil restoration: effects on the soil microarthropod populations

Experimental plots were laid out in a limestone quarry in Girona (Catalonia, Spain) to test the effects of sewage sludge on the soil microarthropod populations. Two different doses of sludge (7·5 per cent and 15 per cent) were applied to fertilize soil that was used to restore a quarry after opencast mining. Mean annual arthropod density increased when sludge was applied, but the 15 per cent dose caused an impoverishment of the community structure and a decrease of the soil oribatid diversity. Julidae (Diplopoda) and large predators of the mite family Parasitidae (Mesostigmata) were the most depressed taxa, whereas some groups depending on the availability or quality of organic matter (such as the immature Coleoptera or the Uropodidae), or on water availability (such as Collembola) were stimulated. Among the oribatids, Punctoribates sp. was the most depressed, whereas the more common little Oppias were significantly favoured. The application of sludge at a dose of 15 per cent must be avoided in order to preserve the soil biodiversity. Copyright © 1999 John Wiley & Sons, Ltd.     

Cotton lint yield variability in a heterogeneous soil at a landscape scale

Landscape variability associated with topographic features affects the spatial pattern of soil water and N redistribution, and thus N uptake and crop yield. A landscape-scale study was conducted in a center pivot irrigated field on the southern High Plains of Texas in 1999 to assess soil water, soil NO₃-N, cotton (Gossypium hirsutum L.) lint yield, and N uptake variability in the landscape, and to determine the spatial correlation between these landscape variables using a state-space approach. The treatments were irrigation at 50 and 75% cotton potential evapotranspiration (ET). Neutron access tubes were placed at a 15-m interval along a 710 m (50% ET) and 820 m (75% ET) transect across the field. Soil NO₃-N in early spring was autocorrelated at a distance varying between 60 and 80 m. Measured soil volumetric water content (WC), total N uptake, and lint yield were generally higher on lower landscape positions. Cotton lint yield was significantly correlated to soil WC (r=0.76), soil NO₃-N (r=0.35), and site elevation (r=−0.54). Differences of site elevation between local neighboring points explained the soil water, NO₃-N and lint yield variability at the micro-scale level in the landscape. Soil WC, cotton lint yield, N uptake, and clay content were crosscorrelated with site elevation across a lag distance of ±30–40 m. The state-space analysis showed that cotton lint yield was positively weighted on soil WC availability and negatively weighted on site elevation. Cotton lint yield state-space models give insights on the association of soil physical and chemical properties, lint yield, and landscape processes, and have the potential to improve water and N management at the landscape-scale.     

Functional and molecular responses of soil microbial communities under differing soil management practices

The effects of soil management on some microbiological properties and soil bacterial community structure were evaluated. Two field sites with the same soil type, located on the same geographic area adjacent to one other, have received different soil management practices and cultivation. One site has been subjected for 20 years to intensive horticulture under conventional tillage and irrigation with low quality salt-rich water; the second field site has been uncultivated for a long period and was turned to organic farming practices over the last 5 years and is currently cultivated with fruit orchard. Total bacterial counts, microbial ATP, microbial community metabolic (BIOLOG^®) profiles, and DNA fingerprinting by PCR-DGGE were determined. Two-way ANOVA revealed that total bacterial counts were not significantly (P>0.3) affected by the two different management practices; ATP content was consistently and significantly (P<0.001) lower in salt-water irrigated soil than in organic soil at the three sampling times. The cluster analysis of community level physiological profiles indicated that microbial communities were much more uniform in organic soil than in irrigated one, suggesting that salt-water irrigation could have affected the size of the microbial population, its metabolic activities, as well as its composition. Molecular patterns fitted the BIOLOG^® profile diversity. In particular, at any sampling time, PCR-DGGE patterns of bacterial DNA, extracted by an indirect method, significantly discriminated irrigated from organic soil samples. The PCR-DGGE patterns of total soil DNA, extracted by a direct method, showed a moderate to significant variation among irrigated and organic soil samples. Biochemical, microbiological and molecular data contributed to evidence a significantly different response of indigenous microflora to soil management by using saline water or organic farming.     

Plant biomass, soil water content and soil N:P ratio regulating soil microbial functional diversity in a temperate steppe: A regional scale study

Soil microorganisms are influenced by various abiotic and biotic factors at the field plot scale. Little is known, however, about the factors that determine soil microbial community functional diversity at a larger spatial scale. Here we conducted a regional scale study to assess the driving forces governing soil microbial community functional diversity in a temperate steppe of Hulunbeir, Inner Mongolia, northern China. Redundancy analysis and regression analysis were used to examine the relationships between soil microbial community properties and environmental variables. The results showed that the functional diversity of soil microbial communities was correlated with aboveground plant biomass, root biomass, soil water content and soil N: P ratio, suggesting that plant biomass, soil water availability and soil N availability were major determinants of soil microbial community functional diversity. Since plant biomass can indicate resource availability, which is mainly constrained by soil water availability and N availability in temperate steppes, we consider that soil microbial community functional diversity was mainly controlled by resource availability in temperate steppes at a regional scale.     

Above- and belowground carbon inputs affect seasonal variations of soil microbial biomass in a subtropical monsoon forest of southwest China

Soil microbial activity drives carbon and nutrient cycling in terrestrial ecosystems. Soil microbial biomass is commonly limited by environmental factors and soil carbon availability. We employed plant litter removal, root trenching and stem-girdling treatments to examine the effects of environmental factors, above- and belowground carbon inputs on soil microbial C in a subtropical monsoon forest in southwest China. During the experimental period from July 2006 through April 2007, 2 years after initiation of the treatments, microbial biomass C in the humus layer did not vary with seasonal changes in soil temperature or water content. Mineral soil microbial C decreased throughout the experimental period and varied with soil temperature and water content. Litter removal reduced mineral soil microbial C by 19.0% in the ungirdled plots, but only 4.0% in girdled plots. Root trenching, stem girdling and their interactions influenced microbial C in humus layer. Neither root trenching nor girdling significantly influenced mineral soil microbial C. Mineral soil microbial C correlated with following-month plant litterfall in control plots, but these correlations were not observed in root-trenching plots or girdling plots. Our results suggest that belowground carbon retranslocated from shoots and present in soil organic matter, rather than aboveground fresh plant litter inputs, determines seasonal fluctuation of mineral soil microbial biomass.