Can root-feeders alter the composition of AMF communities? Experimental evidence from the dune grass Ammophila arenaria

Root herbivores and plant mutualists, such as arbuscular mycorrhizal fungi (AMF), have a significant effect on the structure and dynamic of plant communities. Nevertheless, the interactions between the two groups of organisms in natural ecosystems are far from understood. We carried out an inoculation experiment to examine the effect of two root herbivores, Pratylenchus penetrans and P. dunensis (Nematoda), on the composition of the AMF communities associated with two populations of the dune grass Ammophila arenaria. The outcome of the interaction in terms of plant and nematode performance was also analyzed. The total percentage of AMF colonization was not affected by the presence of root-feeders, but they did alter the composition of the AMF communities inside the roots. These changes were dependent on the root-feeder species and the original AMF community: the most severe alterations were observed in the mycorrhizal plants from Wales attacked by P. penetrans. Plant growth was impaired in plants from Wales inoculated with AMF and P. dunensis, which suggests a highly species-specific synergistic interaction with negative consequences for the plant. Root infection by the nematodes was reduced in all mycorrhizal plants when compared to non-mycorrhizal plants. However, a significant reduction of the final number of nematodes was observed only in the mycorrhizal plants from one population.     

Soil microorganisms control plant ectoparasitic nematodes in natural coastal foredunes

Belowground herbivores can exert important controls on the composition of natural plant communities. Until now, relatively few studies have investigated which factors may control the abundance of belowground herbivores. In Dutch coastal foredunes, the root-feeding nematode Tylenchorhynchus ventralis is capable of reducing the performance of the dominant grass Ammophila arenaria (Marram grass). However, field surveys show that populations of this nematode usually are controlled to nondamaging densities, but the control mechanism is unknown. In the present study, we first established that T. ventralis populations are top-down controlled by soil biota. Then, selective removal of soil fauna suggested that soil microorganisms play an important role in controlling T. ventralis. This result was confirmed by an experiment where selective inoculation of microarthropods, nematodes and microbes together with T. ventralis into sterilized dune soil resulted in nematode control when microbes were present. Adding nematodes had some effect, whereas microarthropods did not have a significant effect on T. ventralis. Our results have important implications for the appreciation of herbivore controls in natural soils. Soil food web models assume that herbivorous nematodes are controlled by predaceous invertebrates, whereas many biological control studies focus on managing nematode abundance by soil microorganisms. We propose that soil microorganisms play a more important role than do carnivorous soil invertebrates in the top-down control of herbivorous ectoparasitic nematodes in natural ecosystems. This is opposite to many studies on factors controlling root-feeding insects, which are supposed to be controlled by carnivorous invertebrates, parasitoids, or entomopathogenic nematodes. Our conclusion is that the ectoparasitic nematode T. ventralis is potentially able to limit productivity of the dune grass A. arenaria but that soil organisms, mostly microorganisms, usually prevent the development of growth-reducing population densities.     

Sources of variation in plant responses to belowground insect herbivory: a meta-analysis

Growing interest in belowground herbivory and the remarkable diversity of the accumulated information on this topic inspired us to quantitatively explore the variation in the outcomes of individual studies. We conducted a meta-analysis of 85 experimental studies reporting the effects of root-feeding insect herbivores (36 species) on plants (75 species). On average, belowground herbivory led to a 36.3% loss of root biomass, which was accompanied by a reduction in aboveground growth (-16.3%), photosynthesis (-11.7%) and reproduction (-15.5%). The effects of root herbivory on aboveground plant characteristics were significant in agricultural and biological control studies, but not in studies of natural systems. Experiments conducted in controlled environments yielded larger effects on plants than field experiments, and infestation experiments resulted in more severe effects than removal studies employing natural levels of herbivory. Simulated root herbivory led to greater aboveground growth reductions than similar root loss imposed by insect feeding. External root chewers caused stronger detrimental effects than sap feeders or root borers; specialist herbivores imposed milder adverse effects on plants than generalists. Woody plants suffered from root herbivory more than herbaceous plants, although root loss was similar in these two groups. Evergreen woody plants responded to root herbivory more strongly than deciduous woody plants, and grasses suffered from root herbivory more than herbs. Environmental factors such as drought, poor nutrient supply, among-plant competition, and aboveground herbivory increased the adverse effects of root damage on plants in an additive manner. In general, plant tolerance to root herbivores is lower than tolerance to defoliating aboveground herbivores.     

Invasive plants and their escape from root herbivory: a worldwide comparison of the root-feeding nematode communities of the dune grass <Emphasis Type="Italic">Ammophila arenaria</Emphasis> in natural and introduced ranges

Invasive plants generally have fewer aboveground pathogens and viruses in their introduced range than in their natural range, and they also have fewer pathogens than do similar plant species native to the introduced range. However, although plant abundance is strongly controlled by root herbivores and soil pathogens, there is very little knowledge on how invasive plants escape from belowground enemies. We therefore investigated if the general pattern for aboveground pathogens also applies to root-feeding nematodes and used the natural foredune grass Ammophila arenariaas a model. In the late 1800s, the European A. arenariawas introduced into southeast Australia (Tasmania), New Zealand, South Africa, and the west coast of the USA to be used for sand stabilization. In most of these regions, it has become a threat to native vegetation, because its excessive capacity to stabilize wind-blown sand has changed the geomorphology of coastal dunes. In stable dunes of most introduced regions, A. arenaria is more abundant and persists longer than in stabilized dunes of the natural range. We collected soil and root samples and used additional literature data to quantify the taxon richness of root-feeding nematodes on A.␣arenaria in its natural range and collected samples from the four major regions where it has been introduced. In most introduced regions A. arenaria did not have fewer root-feeding nematode taxa than the average number in its natural range, and native plant species did not have more nematode taxa than the introduced species. However, in the introduced range native plants had more feeding-specialist nematode taxa than A. arenaria and major feeding specialists (the sedentary endoparasitic cyst and root knot nematodes) were not found on A. arenaria in the southern hemisphere. We conclude that invasiveness of A. arenaria correlates with escape from feeding specialist nematodes, so that the pattern of escape from root-feeding nematodes is more alike escape from aboveground insect herbivores than escape from aboveground pathogens and viruses. In the natural range of A. arenaria, the number of specialist-feeding nematode taxa declines towards the margins. Growth experiments are needed to determine the relationship between nematode taxon diversity, abundance, and invasiveness of A. arenaria.     

Above‐ and Belowground Trophic Interactions on Creeping Thistle (Cirsium arvense) in High‐ and Low‐Diversity Plant Communities: Potential for Biotic Resistance?

The capacity of local communities to control introduced plants is called biotic resistance. Biotic resistance has been almost exclusively tested for plant competition and above ground herbivores and pathogens, while neglecting root herbivores and soil pathogens. Here, we present biotic resistance by above- and below ground herbivores in concert, and relate the abundance of the plant enemies to the species diversity of the local plant communities. The study was carried out in a 7-year-old biodiversity field experiment. We used creeping thistle (Cirsium arvense) as a model, and quantified sap-sucking herbivores: above ground aphids, their antagonists, and root-feeding nematodes. As plant diversity treatments, we used field plots sown with high (15 plant species, HSD) or low (4 plant species, LSD) diverse seed mixtures in 1996 and that were not weeded. Creeping thistle became established spontaneously at the start of the experiment. In 2002, in HSD plots, 90 % of the plant community was made up by 11 species, compared to seven species in LSD plots. No differences were found for C. arvense abundance or biomass. Above ground, three aphid species were found on C. arvense-Uroleucon cirsii, Aphis fabae, and Macrosiphum euphorbiae, but the latter was found only in low densities. Significantly more aphid species were found on individual plants in HSD plots. Moreover, in HSD plots, on average 10 % of aphids were parasitized, while no parasitism was observed in LSD plots. In the root zone of C. arvense, significantly more nematodes were found in HSD than in LSD plots, and a significantly higher proportion of those nematodes were plant parasites. The dominant plant parasitic nematode in both treatments was Paratylenchus. We conclude that biotic resistance by natural enemies may be enhanced by plant species diversity, but that above- and below ground sap-sucking herbivores do not necessarily have to respond similarly to the diversity of the surrounding plant community.     

Integrated management of root-knot nematode, Meloidogyne incognita infestation in tomato and grapevine

An integrated approach with the obligate bacterial parasite, Pasteuria penetrans and nematicides was assessed for the management of the root-knot nematode, Meloidogyne incognita infestation in tomato and grapevine. Seedlings of tomato cv. Co3 were transplanted into pots filled with sterilized soil and inoculated with nematodes (5000 juveniles/pot). The root powder of P. penetrans at 10 mg/pot was applied alone and in combination with carbofuran at 6 mg/pot. Application of P. penetrans along with carbofuran recorded lowest nematode infestation (107 nematodes/200 g soil) compared to control (325 nematodes/200 g soil). The rate of parasitization was 83.1% in the carbofuran and P. penetrans combination treatment as against 61.0% in the P. penetrans treatment only. The plant growth was also higher in the combination treatment compared to all other treatments. A field trial was carried out to assess the efficacy of P. penetrans and nematicides viz., carbofuran and phorate in the management of root-knot nematode, M. incognita infestation of grapevine cv. Muscat Hamburg. A nematode and P. penetrans infested grapevine field was selected and treatments either with carbofuran or phorate at 1 g a.i/vine was given. The observations were recorded at monthly interval. The results showed that the soil nematode population was reduced in nematicide treated plots. Suppression of nematodes was higher under phorate (117 nematodes/200 g soil) than under carbofuran (126.7 nematodes/200 g soil) treatment. The number of juveniles parasitized was also influenced by nematicides and spore load carried/juvenile with phorate being superior and the increase being 17.0 and 29.0% respectively over the control. The results of these experiment confirmed the compatibility of P. penetrans with nematicides and its biological control potential against the root-knot nematode.     

AMF和DSE组合菌剂促生防线虫病效应

本试验旨在探究丛枝菌根真菌（AMF）和暗隔内生真菌（DSE）组合菌剂对南方根结线虫Meloidogyne incognita发育、侵染、黄瓜根结线虫病以及黄瓜生长发育的影响,为进一步探索AMF和DSE协同发挥生理生态效应的作用机制奠定基础。试验对“津优35号”黄瓜接种南方根结线虫、Phoma leveillei（DSE）、Funneliformis mosseae（AMF）、Glomus versiforme（AMF）、Acaulospora laevis（AMF）和/或Scutellospora aurigloba（AMF）。结果表明,供试AMF与DSE 能促进黄瓜生长、提高产量和增强抗病性。AMF+DSE组合处理的菌根和DSE的定殖数量、株高、茎粗、地上部和根系干重、单株产量等显著优于单接种AMF或单接种DSE处理。以F. mosseae + P. leveillei组合抑制南方根结线虫的发育、降低线虫繁殖数量、根内定殖数量、发病率和根结指数的效果最好。     

Interrelated effects of mycorrhiza and free‐living nitrogen fixers cascade up to aboveground herbivores

Aboveground plant performance is strongly influenced by belowground microorganisms, some of which are pathogenic and have negative effects, while others, such as nitrogen‐fixing bacteria and arbuscular mycorrhizal fungi, usually have positive effects. Recent research revealed that belowground interactions between plants and functionally distinct groups of microorganisms cascade up to aboveground plant associates such as herbivores and their natural enemies. However, while functionally distinct belowground microorganisms commonly co‐occur in the rhizosphere, their combined effects, and relative contributions, respectively, on performance of aboveground plant‐associated organisms are virtually unexplored. Here, we scrutinized and disentangled the effects of free‐living nitrogen‐fixing (diazotrophic) bacteria Azotobacter chroococcum (DB) and arbuscular mycorrhizal fungi Glomus mosseae (AMF) on host plant choice and reproduction of the herbivorous two‐spotted spider mite Tetranychus urticae on common bean plants Phaseolus vulgaris. Additionally, we assessed plant growth, and AMF and DB occurrence and density as affected by each other. Both AMF alone and DB alone increased spider mite reproduction to similar levels, as compared to the control, and exerted additive effects under co‐occurrence. These effects were similarly apparent in host plant choice, that is, the mites preferred leaves from plants with both AMF and DB to plants with AMF or DB to plants grown without AMF and DB. DB, which also act as AMF helper bacteria, enhanced root colonization by AMF, whereas AMF did not affect DB abundance. AMF but not DB increased growth of reproductive plant tissue and seed production, respectively. Both AMF and DB increased the biomass of vegetative aboveground plant tissue. Our study breaks new ground in multitrophic belowground–aboveground research by providing first insights into the fitness implications of plant‐mediated interactions between interrelated belowground fungi–bacteria and aboveground herbivores.     

Biological control potential of the obligate parasite Pasteuria penetransagainst the root-knot nematode, Meloidogyne incognita infestation in Brinjal

The efficacy of the obligate bacterial parasite, Pasteuria penetrans against the rootknot nematode, Meloidogyne incognita infestation was assessed in brinjal. The seedling pans with sterilized soil were inoculated with nematodes and root powder of P. penetrans were applied at different dosages viz., 0 x 10(6), 0.5 x 10(6) spores and 1 x 10(6) spores/pan. Seeds of brinjal cv Co2 were sown in the pans and seedlings were allowed to grow. The seedlings were transplanted to microplots containing sterilized soil. Observations on nematode infestation and plant growth were recorded at seedling, flowering, and fruiting stages. Nematode infestation was significantly reduced by P. penetrans treatment. There was 22, 75 and 86% reduction in nematode population of soil over control at seedling, flowering and fruiting stages, respectively, at higher spore density (1 x 10(6)). Egg mass production was decreased by 63, 78 and 89% over control at 35 (seedling), 100 (flowering) and 160 (fruiting) days after sowing respectively, at 1 x 10(6) spores treated soil. The parasitizing ability of P. penetrans increased with the age of the crop. At higher spore density the percentage of parasitization was increased from 52.0 (35 days after sowing) to 90.0 (160 days after sowing) %. At these stages of the crop, the spore load per juvenile also increased at the higher dose. The P. penetrans application enhanced the plant growth. The weight of the shoot was increased by 17.6% whereas root weight by 41.0% over the control at fruiting stage. The experimental results revealed the potential use of P. penetrans as biological control agent of M. incognita. Application of P. penetrans spores in the nursery is a good strategy since the mass multiplication is quite difficult.     

Phylogenetic analysis of Pasteuria penetrans by use of multiple genetic loci

Pasteuria penetrans is a gram-positive, endospore-forming eubacterium that apparently is a member of the Bacillus-Clostridium clade. It is an obligate parasite of root knot nematodes (Meloidogyne spp.) and preferentially grows on the developing ovaries, inhibiting reproduction. Root knot nematodes are devastating root pests of economically important crop plants and are difficult to control. Consequently, P. penetrans has long been recognized as a potential biocontrol agent for root knot nematodes, but the fastidious life cycle and the obligate nature of parasitism have inhibited progress on mass culture and deployment. We are currently sequencing the genome of the Pasteuria bacterium and have performed amino acid level analyses of 33 bacterial species (including P. penetrans) using concatenation of 40 housekeeping genes, with and without insertions/deletions (indels) removed, and using each gene individually. By application of maximum-likelihood, maximum-parsimony, and Bayesian methods to the resulting data sets, P. penetrans was found to cluster tightly, with a high level of confidence, in the Bacillus class of the gram-positive, low-G+C-content eubacteria. Strikingly, our analyses identified P. penetrans as ancestral to Bacillus spp. Additionally, all analyses revealed that P. penetrans is surprisingly more closely related to the saprophytic extremophile Bacillus haladurans and Bacillus subtilis than to the pathogenic species Bacillus anthracis and Bacillus cereus. Collectively, these findings strongly imply that P. penetrans is an ancient member of the Bacillus group. We suggest that P. penetrans may have evolved from an ancient symbiotic bacterial associate of nematodes, possibly as the root knot nematode evolved to be a highly specialized parasite of plants.     

Decomposers and root feeders interactively affect plant defence in <Emphasis Type="Italic">Sinapis alba</Emphasis>

Aboveground herbivory is well known to change plant growth and defence. In contrast, effects of soil organisms, acting alone or in concert, on allocation patterns are less well understood. We investigated separate and combined effects of the endogeic earthworm species Aporrectodea caliginosa and the root feeding nematode species Pratylenchus penetrans and Meloidogyne incognita on plant responses including growth and defence metabolite concentrations in leaves of white mustard, Sinapis alba. Soil biota had a strong impact on plant traits, with the intensity varying due to species combinations. Nematode infestation reduced shoot biomass and nitrogen concentration but only in the absence of earthworms. Earthworms likely counteracted the negative effects of nematodes. Infestation with the migratory lesion-nematode P. penetrans combined with earthworms led to increased root length. Earthworm biomass increased in the presence of this species, indicating that these nematodes increased the food resources of earthworms—presumably dead and decaying roots. Nitrogen-based defence compounds, i.e. glucosinolates, did not correlate with nitrogen levels. In the presence of earthworms, concentrations of aromatic glucosinolates in leaves were significantly increased. In contrast, infection with P. penetrans strongly decreased concentrations of glucosinolates (up to 81%). Infestation with the sedentary nematode M. incognita induced aromatic glucosinolates by more than 50% but only when earthworms were also present. Myrosinase activities, glucosinolate-hydrolysing enzymes, were unaffected by nematodes but reduced in the presence of earthworms. Our results document that root-feeding nematodes elicit systemic plant responses in defence metabolites, with the responses varying drastically with nematode species of different functional groups. Furthermore, systemic plant responses are also altered by decomposer animals, such as earthworms, challenging the assumption that induction of plant responses including defence traits is restricted to herbivores. Soil animals even interact and modulate the individual effects on plant growth and plant defence, thereby likely also influencing shoot herbivore attack.     

Effects of co-inoculation with arbuscular mycorrhizal fungi and rhizobia on soybean growth as related to root architecture and availability of N and P

Soybean plants can form tripartite symbiotic associations with rhizobia and arbuscular mycorrhizal (AM) fungi, but little is known about effects of co-inoculation with rhizobia and AM fungi on plant growth, or their relationships to root architecture as well as nitrogen (N) and phosphorus (P) availability. In the present study, two soybean genotypes contrasting in root architecture were grown in a field experiment to evaluate relationships among soybean root architecture, AMF colonization, and nodulation under natural conditions. Additionally, a soil pot experiment in greenhouse was conducted to investigate the effects of co-inoculation with rhizobia and AM fungi on soybean growth, and uptake of N and P. Our results indicated that there was a complementary relationship between root architecture and AMF colonization in the field. The deep root soybean genotype had greater AMF colonization at low P, but better nodulation with high P supply than the shallow root genotype. A synergistic relationship dependent on N and P status exists between rhizobia and AM fungi on soybean growth. Co-inoculation with rhizobia and AM fungi significantly increased soybean growth under low P and/or low N conditions as indicated by increased shoot dry weight, along with plant N and P content. There were no significant effects of inoculation under adequate N and P conditions. Furthermore, the effects of co-inoculation were related to root architecture. The deep root genotype, HN112, benefited more from co-inoculation than the shallow root genotype, HN89. Our results elucidate new insights into the relationship between rhizobia, AM fungi, and plant growth under limitation of multiple nutrients, and thereby provides a theoretical basis for application of co-inoculation in field-grown soybean.     

Interactions between above- and belowground insect herbivores as mediated by the plant defense system

Plants are frequently attacked by both above- and belowground arthropod herbivores. Nevertheless, studies rarely consider root and shoot herbivory in conjunction. Here we provide evidence that the root-feeding insect Agriotes lineatus reduces the performance of the foliage feeding insect Spodoptera exigua on cotton plants. In a bioassay, S. exigua larvae were allowed to feed on either undamaged plants, or on plants that had previously been exposed to root herbivory, foliar herbivory, or a combination of both. Previous root herbivory reduced the relative growth rates as well as the food consumption of S. exigua by more than 50% in comparison to larvae feeding on the undamaged controls. We found no effects in the opposite direction, as aboveground herbivory by S. exigua did not affect the relative growth rates of root-feeding A. lineatus . Remarkably, neither did the treatment with foliar herbivory affect the food consumption and relative growth rate of S. exigua in the bioassay. However, this treatment did result in a significant change in the distribution of S. exigua feeding. Plants that had been pre-exposed to foliar herbivory suffered significantly less damage on their young terminal leaves. While plant growth and foliar nitrogen levels were not affected by any of the treatments, we did find significant differences between treatments with respect to the level and distribution of plant defensive chemicals (terpenoids). Exposure to root herbivores resulted in an increase in terpenoid levels in both roots as well as in mature and immature foliage. Foliar damage, on the other hand, resulted in high terpenoid levels in young, terminal leaves only. Our results show that root-feeding herbivores may change the level and distribution of plant defenses aboveground. Our data suggest that the reported interactions between below- and aboveground insect herbivores are mediated by induced changes in plant secondary chemistry.     

Interaction of Vesicular-arbuscular Mycorrhizal Fungi and Phosphorus with Meloidogyne incognita on Tomato

The influence of two vesicular-arbuscular mycorrhizal fungi and phosphorus (P) nutrition on penetration, development, and reproduction by Meloidogyne incognita on Walter tomato was studied in the greenhouse. Inoculation with either Gigaspora margarita or Glomus mosseae 2 wk prior to nematode inoculation did not alter infection by M. incognita compared with nonmycorrhizal plants, regardless of soil P level (either 3 μg [low P] or 30 μg [high P] available P/g soil). At a given soil P level, nematode penetration and reproduction did not differ in mycorrhizal and nonmycorrhizal plants. However, plants grown in high P soil had greater root weights, increased nematode penetration and egg production per plant, and decreased colonization by mycorrhizal fungi, compared with plants grown in low P soil. The number of eggs per female nematode on mycorrhizal and nonmycorrhizal plants was not influenced by P treatment. Tomato plants with split root systems grown in double-compartment containers which had either low P soil in both sides or high P in one side and low P in the other, were inoculated at transplanting with G. margarita and 2 wk later one-half of the split root system of each plant was inoculated with M. incognita larvae. Although the mycoorhizal fungus increased the inorganic P content of the root to a level comparable to that in plants grown in high P soil, nematode penetration and reproduction were not altered. In a third series of experiments, the rate of nematode development was not influenced by either the presence of G. margarita or high soil P, compared with control plants grown in low P soil. These data indicate that supplemental P (30 μ/g soil) alters root-knot nematode infection of tomato more than G. mosseae and G. margarita.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

Soil mycorrhizal and nematode diversity vary in response to bioenergy crop identity and fertilization

The mandate by the Energy Independence and Security Act of 2007 to increase renewable fuel production in the USA has resulted in extensive research into the sustainability of perennial bioenergy crops such as switchgrass (Panicum virgatum) and miscanthus (Miscanthus× giganteus). Perennial grassland crops have been shown to support greater aboveground biodiversity and ecosystem function than annual crops. However, management considerations, such as what crop to plant or whether to use fertilizer, may alter belowground diversity and ecosystem functioning associated with these grasslands as well. In this study, we compared crop type (switchgrass or miscanthus) and nitrogen fertilization effects on arbuscular mycorrhizal fungal (AMF) and soil nematode abundance, activity, and diversity in a long‐term experiment. We quantified AMF root colonization, AMF extra‐radical hyphal length, soil glomalin concentrations, AMF richness and diversity, plant‐parasitic nematode abundance, and nematode family richness and diversity in each treatment. Mycorrhizal activity and diversity were higher with switchgrass than with miscanthus, leading to higher potential soil carbon contributions via increased hyphal growth and glomalin production. Plant‐parasitic nematode (PPN) abundance was 2.3 ×  higher in miscanthus plots compared to switchgrass, mostly due to increases in dagger nematodes (Xiphinema). The higher PPN abundance in miscanthus may be a consequence of lower AMF in this species, as AMF can provide protection against PPN through a variety of mechanisms. Nitrogen fertilization had minor negative effects on AMF and nematode diversity associated with these crops. Overall, we found that crop type and fertilizer application associated with perennial bioenergy cropping systems can have detectable effects on the diversity and composition of soil communities, which may have important consequences for the ecosystem services provided by these systems.     

Climate affects predator control of an herbivore outbreak

Herbivore outbreaks and the accompanying devastation of plant biomass can have enormous ecological effects. Climate directly affects such outbreaks through plant stress or alterations in herbivore life-history traits. Large-scale variation in climate can indirectly affect outbreaks through trophic interactions, but the magnitude of such effects is unknown. On the California coast, rainfall in years during and immediately previous to mass lupine mortality was two-thirds that of years without such mortality. However, neither mature lupines nor their root-feeding herbivores are directly affected by annual variation in rainfall. By increasing soil moisture to levels characteristic of summers following El Ni?o/Southern Oscillation (ENSO) events, we increased persistence of a predator (the entomopathogenic nematode Heterorhabditis marelatus). This led to suppression of an outbreak of the herbivorous moth Hepialus californicus, indirectly protecting bush lupine (Lupinus arboreus). Our results are consistent with the marine-oriented Menge-Sutherland hypothesis (Menge and Sutherland 1987) that abiotic stress has greater effects on higher than on lower trophic levels. The mechanisms producing these results differ from those proposed by Menge-Sutherland, however, highlighting differences between trophic processes in underground and terrestrial/marine food webs. Our evidence suggests that herbivore outbreaks and mass lupine mortality are indirectly affected by ENSO's facilitation of top-down control in this food web.     

Effects of intraspecific variation in white cabbage (Brassica oleracea var. capitata) on soil organisms

Intraspecific variation in plants can affect soil organisms. However, little is known about whether the magnitude of the effect depends on the degree of interaction with the roots. We analyzed effects of plant intraspecific variation on root herbivores and other soil organisms that interact directly with living plant roots, as well as on decomposer organisms that interact more indirectly with roots. We used four different white cabbage (Brassica oleracea var. capitata) cultivars exhibiting a high degree of intraspecific variation in root glucosinolate profiles. Intraspecific variation affected root-feeding nematodes, whereas decomposer organisms such as earthworms and Collembola were not affected. Root-feeding nematodes were most abundant in one of the cultivars, Badger Shipper, which lacked the glucosinolate gluconasturtiin. The effect of the intraspecific variation in glucosinolate composition may have been restricted to root-feeding nematodes due to the rapid degradation of glucosinolates and their breakdown products in the soil. Additionally, the low biomass of root-feeding nematodes, relative to other soil organisms, limits the possibility to affect higher trophic level organisms. Our results show that variation in root chemistry predominantly affects belowground herbivores and that these effects do not extend into the soil food web.