Groups Of Soil Organisms Research Articles

Mapping Soil Biodiversity in Europe and the Netherlands

Soil is fundamental for the functioning of terrestrial ecosystems, but our knowledge about soil organisms and the habitat they provide (shortly: Soil biodiversity) is poorly developed. For instance, the European Atlas of Soil Biodiversity and the Global Soil Biodiversity Atlas contain maps with rather coarse information on soil biodiversity. This paper presents a methodology to map soil biodiversity with limited data and models. Two issues were addressed. First, the lack of consensus to quantify the soil biodiversity function and second, the limited data to represent large areas. For the later issue, we applied a digital soil mapping (DSM) approach at the scale of the Netherlands and Europe. Data of five groups of soil organisms (earthworms, enchytraeids, micro-arthropods, nematodes, and micro-organisms) in the Netherlands were linked to soil habitat predictors (chemical soil attributes) in a regression analysis. High-resolution maps with soil characteristics were then used together with a model for the soil biodiversity function with equal weights for each group of organisms. To predict soil biodiversity at the scale of Europe, data for soil biological (earthworms and bacteria) and chemical (pH, soil organic matter, and nutrient content) attributes were used in a soil biodiversity model. Differential weights were assigned to the soil attributes after consulting a group of scientists. The issue of reducing uncertainty in soil biodiversity modelling and mapping by the use of data from biological soil attributes is discussed. Considering the importance of soil biodiversity to support the delivery of ecosystem services, the ability to create maps illustrating an aggregate measure of soil biodiversity is a key to future environmental policymaking, optimizing land use, and land management decision support taking into account the loss and gains on soil biodiversity.

Separating effects of soil microorganisms and nematodes on plant community dynamics

AimsSoil food webs include multiple groups of organisms that each can favor or repress particular plant species in a plant community. This study explores how microbes and nematodes alter the temporal dynamics of specific plant species and functional groups (i.e. grasses and forbs) in mixed grassland communities.MethodsWe extracted communities of nematodes and microorganisms from natural grassland soil and inoculated them, separately and in combination, into containers filled with sterilized grassland soil to examine how these groups of soil organisms influence the compositional dynamics of a diverse grassland plant community consisting of 12 species.ResultsAddition of soil microorganisms altered the composition of the plant community by enhancing forb species and promoting evenness, but these effects took time to develop and became significant only 6 months after inoculation. Addition of soil nematodes showed faster effects and reduced plant community evenness via suppressing several subordinate plant species. The nematode community was dominated by root-feeding nematodes that were less abundant when microorganisms were present, indicating a potential inhibitory effect of microorganisms on plant-feeding nematodes.ConclusionsOur results show that soil microorganisms and nematodes may differ in the magnitude and direction of their effects on the compositional dynamics of plant communities in natural grasslands, and that these effects may operate at different timespans. This study highlights the complexity of plant-soil biotic interactions and the importance to explore these interactions at multiple temporal scales.

Fungi participate in driving home-field advantage of litter decomposition in a subtropical forest

Home-field advantage (HFA) hypothesis predicts that plant litter decomposes faster beneath the plant species from which it was derived than beneath other plant species. However, it remains unclear, which groups of soil organisms drive HFA effects across a wide range of litter quality and forest types. We set up a reciprocal transplant decomposition experiment to quantify the HFA effects of broadleaf, coniferous and bamboo litters. Litterbags of different mesh sizes and high-throughput pyrosequencing of microbial rRNA gene were used to test the contribution of different decomposer groups to HFA effect. The recalcitrant broadleaf litter and the labile bamboo litter exhibited HFA. Presence of meso-and macrofauna did not substantially change the HFA effects. Bacterial and fungal community composition on litters were significantly influenced by litter type. Bacterial community composition remained unchanged when the same litter was decomposed in different forest types, whereas fungal community composition on broadleaf and bamboo litters were significantly influenced by incubation site. Our data demonstrate specific association between fungal community composition and faster litter decomposition in the home site, suggesting that fungi probably participate in driving the HFA effect of broadleaf and bamboo litters.

Increasing temperature and decreasing specific leaf area amplify centipede predation impact on Collembola

Collembola is an abundant group of soil organisms playing a major role on litter decomposition process and nutrient cycling in forest ecosystems. Habitat structure strongly influences Collembola assemblages as plant litter physical characteristics and quantity provide structural niches and determine the outcome of their interactions with predators. Collembola are also extremely sensitive to environmental conditions such as soil temperature that control their demographic parameters and activity. In this context, increasing temperature with the ongoing climate change can have strong impact on Collembola assemblages and their responses to predation either directly by altering their behaviour or indirectly by altering their habitat structure. We therefore examined how the increase of temperature combined to the decrease of specific leaf area (SLA, a major functional plant trait) of the European common oak (Quercus robur L.) and the presence of a centipede predator (Chilopoda: Lithobiidae) will affect the abundance of Folsomia candida (Collembola: Isotomidae) in a 5-week microcosm experiment. Increasing temperature, decreasing SLA and presence of centipede altered F. candida abundance. We observed a significant temperature × predation interaction suggesting differential effects of increasing temperature on F. candida abundance with and without predator. We also observed a significant SLA × predation interaction highlighting that lower SLA decreases F. candida abundance only in predator presence. Finally, our findings evidenced that increasing temperature and decreasing SLA amplify the negative effect of centipede predation on F. candida abundance, suggesting that both direct and indirect effects of climate change would conjointly strengthen the top-down control of predators on preys.

Soil food web assembly and vegetation development in a glacial chronosequence in Iceland

Worldwide human activities threaten soil quality in terms of the soil’s ability to deliver ecosystem services. This ongoing process of land degradation asks for effective strategies of soil protection. In this context, it is important to understand processes that build up and regenerate soil.The present study investigated how the soil ecosystem, including soil organisms, vegetation and soil ecological processes, develops during the process of soil formation in a chronosequence in a glacier forefield in Iceland. We hypothesised that along successional age we see increases in nutrient content, vegetation cover, and plant species richness linked to increases in soil food webs biomass and complexity.In line with our expectations all measured pools of carbon and nitrogen, and vegetation cover increased with age in the glacial forefield, but plant species richness levelled off after 30 years. Soil organisms generally increased in biomass with successional age, although some of the groups of soil organisms peaked at an intermediate successional stage. In contrast to our expectations, some of the calculated food web complexity metrics such as the number of trophic groups and trophic chain length did not increase linearly, but showed an intermediate peak or even decreased with successional age. However, plant cover and pools of carbon and nitrogen still increased after 120 years. From these results we conclude that soil ecosystem development takes more than a century under Icelandic climatic conditions to fully develop in terms of vegetation succession, food web structure and biogeochemical cycling.

Above‐ and belowground insect herbivores mediate the impact of nitrogen eutrophication on the soil food web in a grassland ecosystem

Insect herbivores are important drivers of ecosystem processes in grasslands, and can mediate the grassland's response to environmental change. For example, recent evidence shows that above‐ and belowground herbivory, individually and in combination, can modify how a plant community responds to nitrogen (N) eutrophication, an important driver of global change. However, knowledge about how such effects extend to the associated soil food web is lacking. In a mesocosm experiment, we investigated how communities of soil nematodes – an abundant and functionally important group of soil organisms – responded to above‐ and belowground insect herbivory at contrasting N levels. We found that the strongest influence of above‐ and belowground herbivory on the nematode community appeared at elevated N. The abundance of root‐feeding nematodes increased when either above‐ or belowground insect herbivores were present at elevated N, but when applied together the two herbivore types cancelled out one another's effect. Additionally, at elevated N aboveground herbivory increased the abundance of fungal‐feeders relative to bacterial‐feeders, which indicates changes in decomposition pathways induced by N and herbivory. Belowground herbivory increased the abundance of omnivorous nematodes. The shifts in both the herbivorous and detrital parts of the soil food web demonstrate that above‐ and belowground herbivory does not only mediate the response of the plant community to N eutrophication, but in extension also the soil food web sustained by the plant community. We conclude that feedbacks between effects of above‐ and belowground herbivory mediate the response of the grassland ecosystem to N eutrophication.

On the controls of abundance for soil‐dwelling organisms on the Tibetan Plateau

AbstractAfter decades of research, we are starting to understand more about why the number of species varies from place to place on the planet. However, little is known about spatial variation in abundance, especially for soil‐dwelling organisms. In this study, we aimed to disentangle the relative influences of climatic factors, soil properties, and plant diversity on the abundance of soil‐dwelling invertebrates (i.e., nematodes and soil arthropods) at 48 alpine grassland sites on the Tibetan Plateau. We found that the abundance of these two groups of soil organisms was negatively correlated with soil pH and temperature seasonality, and was positively correlated with soil organic carbon (SOC), mean annual precipitation, and plant species richness; there was no effect of mean annual temperature or seasonality in precipitation on the abundance of nematodes or soil‐dwelling arthropods. When we considered only the nematodes, we found that soil pH, mean annual precipitation, temperature seasonality, and SOC were the best predictors of abundance. However, plant species richness was the best predictor of the abundance of soil‐dwelling arthropods. Different orders within the arthropods responded differently to the suite of factors we examined. Taken together, our results suggest that increases in temperature alone might not alter the abundances of soil organisms in these alpine grasslands. Instead, altered precipitation regimes and increases in intra‐annual variation in temperature, changes in plant community diversity, and the resulting changes in soil characteristics (e.g., pH and organic carbon) could reshape soil communities in the Tibetan grassland ecosystems, and likely elsewhere on the planet.

Native and exotic foundation grasses differ in traits and responses to belowground tri-trophic interactions

A plant’s growth and fitness are influenced by species interactions, including those belowground. In primary successional systems, belowground organisms are known to have particularly important control over plant growth. Exotic plant invasions in these and other habitats may in part be explained by altered associations with belowground organisms compared to native plants. We investigated the growth responses of two foundation grasses on Great Lakes sand dunes, the native grass Ammophila breviligulata and the exotic grass Leymus arenarius, to two groups of soil organisms with important roles in dune succession: arbuscular mycorrhizal fungi (AMF) and plant-parasitic nematodes (PPN). We manipulated the presence/absence of two generalist belowground species known to occur in Great Lakes dunes, Rhizophagus intraradices (AMF) and Pratylenchus penetrans (PPN) in a factorial greenhouse experiment and assessed the biomass production and root architectural traits of the plants. There were clear differences in growth and above- and belowground architecture between Ammophila and Leymus, with Leymus plants being bigger, taller, and having longer roots than Ammophila. Inoculation with Rhizophagus increased above- and belowground biomass production by ~32% for both plant species. Inoculation with Pratylenchus decreased aboveground biomass production by ~36% for both plant species. However belowground, the exotic Leymus was significantly more resistant to PPN than the native Ammophila, and gained more benefits from AMF in belowground tri-trophic interactions than Ammophila. Overall, our results indicate that differences in plant architecture coupled with altered belowground interactions with AMF and PPN have the potential to promote exotic plant invasion.

The bacterial-fungal energy channel concept challenged by enormous functional versatility of soil protists

Protists (=protozoa) are commonly treated as bacterivores that control the bacterial energy channel in soil food webs. This ecologist’s perspective is, however, challenged by taxonomic studies showing that a range of protists feed on fungi, other protists and even nematodes. Recently, it was revealed that obligate and facultative mycophagous protists are common soil inhabitants, while others are facultative nematophagous. Furthermore, protists act as parasites and pathogens of plants and animals. This neglected functional diversity of protists, that is similarly prevalent for other groups of soil organisms, reveals that current food web model models are oversimplified.Facultative feeding of various protist taxa on bacteria and fungi, the source of both major energy channels, strongly implies that a clear split of the energy channels at lower trophic levels does not exist and that more complex energy flows prevail in soil food webs. Future efforts should therefore target ecological functioning of protists and other groups of soil organisms, on a species-specific level, to create more meaningful functional units that then need incorporation in modified soil food web models. Such efforts will help disentangling the structure, diversity and resulting functioning of complex soil systems, including energy flows through the soil food web.

Possibilities of using the German Federal States' permanent soil monitoring program for the monitoring of potential effects of genetically modified organisms (GMO).

BackgroundIn the Directive 2001/18/EC on the deliberate release of genetically modified organisms (GMO) into the environment, a monitoring of potential risks is prescribed after their deliberate release or placing on the market. Experience and data of already existing monitoring networks should be included. The present paper summarizes the major findings of a project funded by the Federal Agency for Nature Conservation (Nutzungsmöglichkeiten der Boden—Dauerbeobachtung der Länder für das Monitoring der Umweltwirkungen gentechnisch veränderter Pflanzen. BfN Skripten, Bonn-Bad Godesberg 369, 2014). The full report in german language can be accessed on http://www.bfn.de and is available as Additional file 1. The aim of the project was to check if it is possible to use the German permanent soil monitoring program (PSM) for the monitoring of GMO. Soil organism communities are highly diverse and relevant with respect to the sustainability of soil functions. They are exposed to GMO material directly by feeding or indirectly through food chain interactions. Other impacts are possible due to their close association to soil particles.ResultsThe PSM program can be considered as representative with regard to different soil types and ecoregions in Germany, but not for all habitat types relevant for soil organisms. Nevertheless, it is suitable as a basic grid for monitoring the potential effects of GMO on soil invertebrates.ConclusionsPSM sites should be used to derive reference values, i.e. range of abundance and presence of different relevant species of soil organisms. Based on these references, it is possible to derive threshold values to define the limit of acceptable change or impact. Therefore, a minimum set of sites and minimum set of standardized methods are needed, i.e. characterization of each site, sampling of selected soil organism groups, adequate adaptation of methods for the purpose of monitoring of potential effects of GMO. Finally, and probably most demanding, it is needed to develop a harmonized evaluation concept.Electronic supplementary materialThe online version of this article (doi:10.1186/s12302-015-0057-2) contains supplementary material, which is available to authorized users.

Stable isotope composition (δ(13)C and δ(15)N values) of slime molds: placing bacterivorous soil protozoans in the food web context.

Data on the bulk stable isotope composition of soil bacteria and bacterivorous soil animals are required to estimate the nutrient and energy fluxes via bacterial channels within detrital food webs. We measured the isotopic composition of slime molds (Myxogastria, Amoebozoa), a group of soil protozoans forming macroscopic spore-bearing fruiting bodies. An analysis of largely bacterivorous slime molds can provide information on the bulk stable isotope composition of soil bacteria. Fruiting bodies of slime molds were collected in a monsoon tropical forest of Cat Tien National Park, Vietnam, and analyzed by continuous-flow isotope ratio mass spectrometry. Prior to stable isotope analysis, carbonates were removed from a subset of samples by acidification. To estimate the trophic position of slime molds, their δ(13) C and δ(15) N values were compared with those of plant debris, soil, microbial destructors (litter-decomposing, humus-decomposing, and ectomycorrhizal fungi) and members of higher trophic levels (oribatid mites, termites, predatory macroinvertebrates). Eight species of slime molds represented by at least three independent samples were 3-6‰ enriched in (13) C and (15) N relative to plant litter. A small but significant difference in the δ(13) C and δ(15) N values suggests that different species of myxomycetes can differ in feeding behavior. The slime molds were enriched in (15) N compared with litter-decomposing fungi, and depleted in (15) N compared with mycorrhizal or humus-decomposing fungi. Slime mold sporocarps and plasmodia largely overlapped with oribatid mites in the isotopic bi-plot, but were depleted in (15) N compared with predatory invertebrates and humiphagous termites. A comparison with reference groups of soil organisms suggests strong trophic links of slime molds to saprotrophic microorganisms which decompose plant litter, but not to humus-decomposing microorganisms or to mycorrhizal fungi. Under the assumption that slime molds are primarily feeding on bacteria, the isotopic similarity of slime molds and mycophagous soil animals indicates that saprotrophic soil bacteria and fungi are similar in bulk isotopic composition.

Independent role of belowground organisms and plant cultivar diversity in legume-grass communities

Plant diversity and different groups of belowground organisms have been shown to influence the characteristics of plant communities and their associated aboveground organisms; however, little is known about the interactions and combined effects of these biotic factors. Furthermore, while plant species richness has received a lot of attention, intraspecific diversity has only recently come into the focus of community ecology. The objective of this study was to determine whether the addition of beneficial soil organisms (arbuscular mycorrhizal fungi (AMF) and anecic earthworms (Lumbricus terrestris L.) and different levels of plant cultivar diversity interactively influence productivity, weed establishment, leaf damage by herbivores and pathogens, and the diversity of aboveground herbivores in legume-grass communities. Our results indicated that the addition of AMF increased aboveground plant productivity and tended to decrease the diversity of aboveground herbivores. Earthworms attenuated the effect of AMF addition on AMF root colonization, but had no effect on plant productivity or herbivore diversity. Weed biomass was significantly lower in plant communities with high cultivar diversity compared to low cultivar diversity. We did not find interactive effects of the two soil organism groups and plant cultivar diversity. Our results demonstrate the independent roles and additive positive effects of AMF and plant cultivar diversity on functions such as productivity and resistance against weeds in the field. We suggest that AMF and plant cultivar diversity manipulations may be applied more frequently in agriculture management programs that aim for sustainable yield enhancement and biocontrol of herbivores and weeds.

A meta-analysis of soil biodiversity impacts on the carbon cycle

Abstract. Loss of biodiversity impacts ecosystem functions, such as carbon (C) cycling. Soils are the largest terrestrial C reservoir, containing more C globally than the biotic and atmospheric pools together. As such, soil C cycling, and the processes controlling it, has the potential to affect atmospheric CO2 concentrations and subsequent climate change. Despite the growing evidence of links between plant diversity and soil C cycling, there is a dearth of information on whether similar relationships exist between soil biodiversity and C cycling. This knowledge gap occurs even though there has been increased recognition that soil communities display high levels of both taxonomic and functional diversity and are key drivers of fluxes of C between the atmosphere and terrestrial ecosystems. Here, we used meta-analysis and regression analysis to quantitatively assess how soil biodiversity affects soil C cycling pools and processes (i.e., soil C respiration, litter decomposition, and plant biomass). We compared the response of process variables to changes in diversity both within and across groups of soil organisms that differed in body size, a grouping that typically correlates with ecological function. When studies that manipulated both within- and across-body size group diversity were included in the meta-analysis, loss of diversity significantly reduced soil C respiration (−27.5%) and plant tissue decomposition (−18%) but did not affect above- or belowground plant biomass. The loss of within-group diversity significantly reduced soil C respiration, while loss of across-group diversity did not. Decomposition was negatively affected both by loss of within-group and across-group diversity. Furthermore, loss of microbial diversity strongly reduced soil C respiration (−41%). In contrast, plant tissue decomposition was negatively affected by loss of soil faunal diversity but was unaffected by loss of microbial diversity. Taken together, our findings show that loss of soil biodiversity strongly impacts on soil C cycling processes, and highlight the importance of diversity across groups of organisms (e.g., primary consumers and secondary decomposers) for maintaining full functionality of C cycle processes. However, our understanding of the complex relationships between soil biodiversity and C cycling processes is currently limited by the sheer number of methodological concerns associated with these studies, which can greatly overestimate or underestimate the impact of soil biodiversity on soil C cycling, challenging extrapolation to natural field settings. Future studies should attempt to further elucidate the relative importance of taxonomic diversity (species numbers) versus functional diversity.

An ecosystem approach to assess soil quality in organically and conventionally managed farms in Iceland and Austria

Abstract. Intensive agricultural production can be an important driver for the loss of long-term soil quality. For this reason, the European Critical Zone Observatory (CZO) network adopted four pairs of agricultural CZO sites that differ in their management: conventional or organic. The CZO sites include two pairs of grassland farms in Iceland and two pairs of arable farms in Austria. Conventional fields differed from the organic fields in the use of artificial fertilisers and pesticides. Soils of these eight farms were analysed in terms of their physical, chemical, and biological properties, including soil aggregate size distribution, soil organic matter contents, abundance of soil microbes and soil fauna, and taxonomic diversity of soil microarthropods. In Icelandic grasslands, organically farmed soils had larger mean weight diameters of soil aggregates than the conventional farms, while there were no differences on the Austrian farms. Organic farming did not systematically influence organic matter contents or composition, nor soil carbon and nitrogen contents. Also, soil food web structures, in terms of presence of trophic groups of soil organisms, were highly similar among all farms, indicating a low sensitivity of trophic structure to land use or climate. However, soil organism biomass, especially of bacteria and nematodes, was consistently higher on organic farms than on conventional farms. Within the microarthropods, taxonomic diversity was systematically higher in the organic farms compared to the conventional farms. This difference was found across countries and farm, crop, and soil types. The results do not show systematic differences in physical and chemical properties between organic and conventional farms, but confirm that organic farming can enhance soil biomass and that microarthropod diversity is a sensitive and consistent indicator for land management.

Production-ecological modelling explains the difference between potential soil N mineralisation and actual herbage N uptake

We studied two different grassland fertiliser management regimes on sand and peat soils: above-ground application of a combination of organic N-rich slurry manure and solid cattle manure (SCM) vs. slit-injected, mineral N-rich slurry manure, whether or not supplemented with chemical fertiliser (non-SCM). Measurements of field N mineralisation as estimated from herbage N uptake in unfertilised plots were compared with (i) potential N mineralisation as determined from a standard laboratory soil incubation, (ii) the contribution of groups of soil organisms to N mineralisation based on production-ecological model calculations, and (iii) N mineralisation calculated according to the Dutch fertilisation recommendation for grasslands. Density and biomass of soil biota (bacteria, fungi, enchytraeids, microarthropods and earthworms) as well as net plant N-uptake were higher in the SCM input grasslands compared to the non-SCM input grasslands. The currently used method in Dutch fertilisation recommendations underestimated actual soil N supply capacity by, on average, 102kg Nha−1 (202 vs. 304kgha−1=34%). The summed production-ecological model estimate for N mineralisation by bacteria, fungi, protozoa, and enchytraeids was 87–120% of the measured potential soil N mineralisation. Adding the modelled N mineralisation by earthworms to potential soil N mineralisation explained 98–107% of the measured herbage N uptake from soil. For all grasslands and soil biota groups together, the model estimated 105% of the measured net herbage N uptake from soil. Soil biota production-ecological modelling is a powerful tool to understand and predict N uptake in grassland, reflecting the effects of previous manure management and soil type. The results show that combining production ecological modelling to predict N supply with existing soil N tests using aerobic incubation methods, can add to a scientifically based improvement of the N fertilisation recommendations for production grasslands.

Deep 16S rRNA pyrosequencing reveals a bacterial community associated with Banana Fusarium Wilt disease suppression induced by bio-organic fertilizer application.

Our previous work demonstrated that application of a bio-organic fertilizer (BIO) to a banana mono-culture orchard with serious Fusarium wilt disease effectively decreased the number of soil Fusarium sp. and controlled the soil-borne disease. Because bacteria are an abundant and diverse group of soil organisms that responds to soil health, deep 16 S rRNA pyrosequencing was employed to characterize the composition of the bacterial community to investigate how it responded to BIO or the application of other common composts and to explore the potential correlation between bacterial community, BIO application and Fusarium wilt disease suppression. After basal quality control, 137,646 sequences and 9,388 operational taxonomic units (OTUs) were obtained from the 15 soil samples. Proteobacteria, Acidobacteria, Bacteroidetes, Gemmatimonadetes and Actinobacteria were the most frequent phyla and comprised up to 75.3% of the total sequences. Compared to the other soil samples, BIO-treated soil revealed higher abundances of Gemmatimonadetes and Acidobacteria, while Bacteroidetes were found in lower abundance. Meanwhile, on genus level, higher abundances compared to other treatments were observed for Gemmatimonas and Gp4. Correlation and redundancy analysis showed that the abundance of Gemmatimonas and Sphingomonas and the soil total nitrogen and ammonium nitrogen content were higher after BIO application, and they were all positively correlated with disease suppression. Cumulatively, the reduced Fusarium wilt disease incidence that was seen after BIO was applied for 1-year might be attributed to the general suppression based on a shift within the bacteria soil community, including specific enrichment of Gemmatimonas and Sphingomonas.

Interactive effects of lambda‐cyhalothrin, soil moisture, and temperature onFolsomia candidaandSinella curviseta(Collembola)

The authors investigated whether and how 2 environmental factors could influence the toxicity of a pyrethroid to 2 representatives of an important group of soil organisms. The impacts of different temperatures (20 °C and 26 °C) and soil moisture levels (30%, 50%, and 70% of water holding capacity) were investigated in combination with the insecticide λ-cyhalothrin on the reproduction success of Folsomia candida and Sinella curviseta in a full factorial design. Testing was based on the standard collembolan reproduction test (Organisation for Economic Co-operation and Development, guideline 232) following an effect concentration design. The results showed an effect of environmental and chemical factors on the number of juveniles of these animals. Particularly in dry soil, the reproduction of both species was reduced, while higher soil moisture levels influenced the number of juveniles positively compared with the middle soil moisture level. In general, however, higher soil moisture led to increased sensitivity to λ-cyhalothrin. In both organisms, temperature affected the toxicity of the pesticide but in different directions: high temperature led to higher toxicity in F. candida but to lower toxicity in S. curviseta.

Nitrogen Addition Regulates Soil Nematode Community Composition through Ammonium Suppression

Nitrogen (N) enrichment resulting from anthropogenic activities has greatly changed the composition and functioning of soil communities. Nematodes are one of the most abundant and diverse groups of soil organisms, and they occupy key trophic positions in the soil detritus food web. Nematodes have therefore been proposed as useful indicators for shifts in soil ecosystem functioning under N enrichment. Here, we monitored temporal dynamics of the soil nematode community using a multi-level N addition experiment in an Inner Mongolia grassland. Measurements were made three years after the start of the experiment. We used structural equation modeling (SEM) to explore the mechanisms regulating nematode responses to N enrichment. Across the N enrichment gradient, significant reductions in total nematode abundance, diversity (H' and taxonomic richness), maturity index (MI), and the abundance of root herbivores, fungivores and omnivores-predators were found in August. Root herbivores recovered in September, contributing to the temporal variation of total nematode abundance across the N gradient. Bacterivores showed a hump-shaped relationship with N addition rate, both in August and September. Ammonium concentration was negatively correlated with the abundance of total and herbivorous nematodes in August, but not in September. Ammonium suppression explained 61% of the variation in nematode richness and 43% of the variation in nematode trophic group composition. Ammonium toxicity may occur when herbivorous nematodes feed on root fluid, providing a possible explanation for the negative relationship between herbivorous nematodes and ammonium concentration in August. We found a significantly positive relationship between fungivores and fungal phospholipid fatty acids (PLFA), suggesting bottom-up control of fungivores. No such relationship was found between bacterivorous nematodes and bacterial PLFA. Our findings contribute to the understanding of effects of N enrichment in semiarid grassland on soil nematode trophic groups, and the cascading effects in the detrital soil food web.

Complexity and composition of pasture swards affect plant productivity and soil organisms

McElroy, M. S., Papadopoulos, Y. A. and Adl, M. S. 2012. Complexity and composition of pasture swards affect plant productivity and soil organisms. Can. J. Plant Sci. 92: 687–697. The relationships between ecosystem diversity, productivity, and stability is a central theme in current ecological research; the links between above-ground and below-ground ecosystems, as well as their effects on ecosystem services, are becoming more understood. While plant communities differ in primary productivity, and in the communities of soil organisms they support, it is unclear whether these differences are attributable mainly to plant community diversity or to the dominant plant species. This study evaluated the effect of these two factors on plant productivity, and abundance of soil microorganisms and functional diversity, in an establishing pasture using sward complexity (plant species present) and sward composition (identity of species) as treatments in a design using the step-wise addition of grass species. While sward complexity affected plant productivity, showing higher productivity in plots of higher diversity, abundance and functional diversity of soil organism groups were generally not consistently affected by sward complexity or composition. Sward composition did influence soil community composition; there was a close correlation between microbial catabolic activity and sward composition. This study shows that grassland plants have a limited effect on the size and diversity of soil communities while they are being established. This result may have consequences for soil ecosystem services.

Indigenous arbuscular mycorrhizal fungal assemblages protect grassland host plants from pathogens.

Plant roots can establish associations with neutral, beneficial and pathogenic groups of soil organisms. Although it has been recognized from the study of individual isolates that these associations are individually important for plant growth, little is known about interactions of whole assemblages of beneficial and pathogenic microorganisms associating with plants.We investigated the influence of an interaction between local arbuscular mycorrhizal (AM) fungal and pathogenic/saprobic microbial assemblages on the growth of two different plant species from semi-arid grasslands in NE Germany (Mallnow near Berlin). In a greenhouse experiment each plant species was grown for six months in either sterile soil or in sterile soil with one of three different treatments: 1) an AM fungal spore fraction isolated from field soil from Mallnow; 2) a soil pathogen/saprobe fraction consisting of a microbial community prepared with field soil from Mallnow and; 3) the combined AM fungal and pathogen/saprobe fractions. While both plant species grew significantly larger in the presence of AM fungi, they responded negatively to the pathogen/saprobe treatment. For both plant species, we found evidence of pathogen protection effects provided by the AM fungal assemblages. These results indicate that interactions between assemblages of beneficial and pathogenic microorganisms can influence the growth of host plants, but that the magnitude of these effects is plant species-specific.