Effects of defoliation intensity on soil food-web properties in an experimental grassland community

We established a greenhouse experiment based on replicated mini‐ecosystems to evaluate the effects of defoliation intensity on soil food‐web properties in grasslands. Plant communities, composed of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and plantain (Plantago lanceolata) with well‐established root and shoot systems, were subjected to five defoliation intensity treatments: no trimming (defoliation intensity 0, or DI 0), and trimming of all plant material to 35 cm (DI 1), 25 cm (DI 2), 15 cm (DI 3) and 10 cm (DI 4) above soil surface every second week for 14 weeks. Intensification of defoliation reduced shoot production and standing shoot and root mass of plant communities but increased their root to shoot ratio. Soil microbial activity and biomass decreased with intensification of defoliation. Concentrations of NO₃–N in soil steadily increased with intensifying defoliation, whereas NH₄–N concentrations did not vary between treatments. Numbers of microbi‐detritivorous enchytraeids, bacterial‐feeding rotifers and bacterial‐feeding nematodes steadily increased with intensifying defoliation, while the abundance of fungal‐feeding nematodes was significantly enhanced only in DI 3 and DI 4 relative to DI 0. The abundance of herbivorous nematodes per unit soil mass was lower in DI 3 and DI 4 than in DI 0, DI 1 and DI 2, but when calculated per unit root mass, their abundance tended to increase with defoliation intensity. The abundance of omnivorous and predatory nematodes appeared to be highest in the most intensely defoliated systems. The ratio of abundance of fungal‐feeding nematodes to that of bacterial‐feeding nematodes was not significantly affected by defoliation intensity. The results infer that defoliation intensity may significantly alter the structure of soil food webs in grasslands, and that defoliation per se is able to induce patterns observed in grazing studies in the field. The results did not support hypotheses that defoliation per se would cause a shift between the bacterial‐based and fungal‐based energy channels in the decomposer food web, or that herbivore and detritivore densities in soil would be highest under intermediate defoliation. Furthermore, our data for microbes and microbial feeders implies that the effects of defoliation intensity on soil food‐web structure may depend on the duration of defoliation and are therefore likely to be dynamic rather than constant in nature.     

Interactive effects of wildfire and permafrost on microbial communities and soil processes in an Alaskan black spruce forest

Boreal forests contain significant quantities of soil carbon that may be oxidized to CO₂ given future increases in climate warming and wildfire behavior. At the ecosystem scale, decomposition and heterotrophic respiration are strongly controlled by temperature and moisture, but we questioned whether changes in microbial biomass, activity, or community structure induced by fire might also affect these processes. We particularly wanted to understand whether postfire reductions in microbial biomass could affect rates of decomposition. Additionally, we compared the short‐term effects of wildfire to the long‐term effects of climate warming and permafrost decline. We compared soil microbial communities between control and recently burned soils that were located in areas with and without permafrost near Delta Junction, AK. In addition to soil physical variables, we quantified changes in microbial biomass, fungal biomass, fungal community composition, and C cycling processes (phenol oxidase enzyme activity, lignin decomposition, and microbial respiration). Five years following fire, organic surface horizons had lower microbial biomass, fungal biomass, and dissolved organic carbon (DOC) concentrations compared with control soils. Reductions in soil fungi were associated with reductions in phenol oxidase activity and lignin decomposition. Effects of wildfire on microbial biomass and activity in the mineral soil were minor. Microbial community composition was affected by wildfire, but the effect was greater in nonpermafrost soils. Although the presence of permafrost increased soil moisture contents, effects on microbial biomass and activity were limited to mineral soils that showed lower fungal biomass but higher activity compared with soils without permafrost. Fungal abundance and moisture were strong predictors of phenol oxidase enzyme activity in soil. Phenol oxidase enzyme activity, in turn, was linearly related to both ¹³C lignin decomposition and microbial respiration in incubation studies. Taken together, these results indicate that reductions in fungal biomass in postfire soils and lower soil moisture in nonpermafrost soils reduced the potential of soil heterotrophs to decompose soil carbon. Although in the field increased rates of microbial respiration can be observed in postfire soils due to warmer soil conditions, reductions in fungal biomass and activity may limit rates of decomposition.     

Interactive effects of plant-available soil silicon and herbivory on competition between two grass species

Background and Aims

The herbivore defence system of true grasses (Poaceae) is predominantly based on silicon that is taken up from the soil and deposited in the leaves in the form of abrasive phytoliths. Silicon uptake mechanisms can be both passive and active, with the latter suggesting that there is an energetic cost to silicon uptake. This study assessed the effects of plant-available soil silicon and herbivory on the competitive interactions between the grasses Poa annua, a species that has previously been reported to accumulate only small amounts of silicon, and Lolium perenne, a high silicon accumulator.

Methods

Plants were grown in mono- and mixed cultures under greenhouse conditions. Plant-available soil silicon levels were manipulated by adding silicon to the soil in the form of sodium silicate. Subsets of mixed culture pots were exposed to above-ground herbivory by desert locusts (Schistocerca gregaria).

Key Results

In the absence of herbivory, silicon addition increased biomass of P. annua but decreased biomass of L. perenne. Silicon addition increased foliar silicon concentrations of both grass species >4-fold. Under low soil-silicon availability the herbivores removed more leaf biomass from L. perenne than from P. annua, whereas under high silicon availability the reverse was true. Consequently, herbivory shifted the competitive balance between the two grass species, with the outcome depending on the availability of soil silicon.

Conclusions

It is concluded that a complex interplay between herbivore abundance, growth–defence trade-offs and the availability of soil silicon in the grasses'' local environment affects the outcome of inter-specific competition, and so has the potential to impact on plant community structure.     

Flooding effects on plants recovering from defoliation in Paspalum dilatatum and Lotus tenuis

Background and Aims

Flooding and grazing are major disturbances that simultaneously affect plant performance in many humid grassland ecosystems. The effects of flooding on plant recovery from defoliation were studied in two species: the grass Paspalum dilatatum, regrowing primarily from current assimilation; and the legume, Lotus tenuis, which can use crown reserves during regrowth.

Methods

Plants of both species were subjected to intense defoliation in combination with 15 d of flooding at 6 cm water depth. Plant recovery was evaluated during a subsequent 30-d growth period under well-watered conditions. Plant responses in tissue porosity, height, tiller or shoot number and biomass of the different organs were assessed.

Key Results

Flooding increased porosity in both P. dilatatum and L. tenuis, as expected in flood-tolerant species. In P. dilatatum, defoliation of flooded plants induced a reduction in plant height, thus encouraging the prostrated-growth response typical of defoliated plants rather than the restoration of contact with atmospheric oxygen, and most tillers remained submerged until the end of the flooding period. In contrast, in L. tenuis, plant height was not reduced when defoliated and flooded, a high proportion of shoots being presented emerging above water (72 %). In consequence, flooding plus defoliation did not depress plant recovery from defoliation in the legume species, which showed high sprouting and use of crown biomass during regrowth, whereas in the grass species it negatively affected plant recovery, achieving 32 % lower biomass than plants subjected to flooding or defoliation as single treatments.

Conclusions

The interactive effect of flooding and defoliation determines a reduction in the regrowth of P. dilatatum that was not detected in L. tenuis. In the legume, the use of crown reserves seems to be a key factor in plant recovery from defoliation under flooding conditions.Key words: Allocation, defoliation, flooding, Lotus tenuis, Paspalum dilatatum, submergence     

The effects of the spatial pattern of defoliation on regrowth of a tussock grass

Summary The spatial pattern of foliage removal from a tussock grass can influence regrowth through effects on daily carbon gain (CER_d). This field study examined the extent to which tussock photosynthetic responses to different defoliation patterns were associated with changes in whole-canopy attributes (e.g., foliage age structure, canopy light microclimate). During the spring growing season, 60% of the green foliage area was removed from individual Agropyron desertorum tussocks with scissors in different spatial patterns. These patterns represented extremes of defoliation patterns that might be inflicted by natural herbivores. Tussock photosynthesis (per unit foliage area) at high light (2000 mol photons m^–2 s^–1 between 400 and 700 nm; P₂₀₀₀) increased following clipping with all defoliation patterns. The increases in P₂₀₀₀ were greater when leaves were removed from low in the tussock (older leaves) than if leaves high in the canopy (younger leaves) were removed. These relative changes of P₂₀₀₀ among clipping patterns paralleled the responses of CER_d and regrowth from an earlier study. Furthermore, the changes in P₂₀₀₀ corresponded with increases in the proportion of foliage within the tussocks that was directly illuminated at midday. The greater photosynthesis of tussocks after lower-leaf removal was directly related to a higher proportion of younger foliage and a smaller fraction of foliage shaded within the tussock. In a dense canopy, such as these grass tussocks, the influence of defoliation on whole-canopy attributes may be of primary importance to whole-plant photosynthetic responses.     

Interactive effects of predators and a pesticide on aquatic communities

Chemical contaminants are ubiquitous in nature and a major goal of ecologists has been to understand and predict their impacts on natural communities. While direct toxic effects can be garnered from single‐species laboratory studies, the full suite of possible effects can only be observed when organisms are embedded within a community. In this study, we manipulated the concentration of malathion (a broad‐spectrum insecticide) to determine the impacts on aquatic communities containing phytoplankton, periphyton, and 27 species of animals (16 zooplankton, 5 snails, 3 tadpoles and 3 predatory insects). Using relatively low concentrations (0.13 to 0.46 mg l^?1), we found important direct (and interactive) effects of predators and malathion on the food web as well as a number of apparent density‐ and trait‐mediated indirect effects. Malathion initiated an indirect effect by decreasing zooplankton diversity and abundance, which propagated an increase in phytoplankton, a decrease in periphyton, and a subsequent decrease in the growth of leopard frog tadpoles. There also was an apparent trait‐mediated indirect effect whereby increased amounts of the pesticide reduced predation rates on amphibians without affecting the survival of the primary amphibian predator (larval Anax dragonflies). In contrast, snail survival and growth was unaffected by the pesticide but there were strong, species‐specific effects from their primary predator (adult Belostoma water bugs). This is one of few studies to examine the impacts of malathion on aquatic communities across a range of concentrations, despite the fact that it is currently the most commonly applied insecticide in the United States, it is applied around the world, and it can be legally directly sprayed over aquatic habitats to control the mosquitoes that carry malaria and West Nile virus. Our results suggest several mechanisms by which a wide variety of pesticides with similar modes of action might impact aquatic communities.     

Interactive regulation of grass yield and chemical properties by defoliation,a salivary chemical,and inorganic nutrition

Summary Clones of 2 C₄ grass species, Sprobolus ioclados and S. pyramidalis, were obtained from more and less heavily grazed grasslands, respectively, in Tanzania's Serengeti National Park. Plants were grown in a factorial experiment to determine the effects of severe defoliation, nutrient limitation, and a salivary chemical (thiamine) on plant growth, nitrogen content, and non-structural carbohydrate content. The experimental design included: (1) species; (2) clipping, with plants either unclipped or clipped weekly to a height of 5 cm; (3) 0.2 ml of distilled water of 0.2 ml of 10 ppb aqueous thiamine sprayed on plants from an atomizer after clipping and identical treatments at the same time to unclipped plants; (4) phosphorus (P) at 0.2 or 1 mM; (5) nitrogen (N) at 3 or 15 mM. Clipping was the major variable affecting plant growth. Total and litter yields were reduced to half and residual plant yield was reduced to 30% of the values for unclipped plants. Clipping interacted strongly with other variables since they commonly had minor effects on clipped plants and major effects on unclipped plants. Exceptions to this generalization were generally due to better performance by S. ioclados under clipping. Compared to lower treatment levels, higher treatment levels promoted total yield of unclipped plants by 52% for N, 43% for thiamine, and 33% for P. In general, thiamine had greater effects than P but lesser effects than N. Thiamine promoted yield and modified the chemical balance of plants by promoting carbohydrate (CHO) concentrations and reducing N concentrations. N and P deficiencies promoted CHO accumulation. Clipping promoted the N of leaves and crowns and reduced the N levels in roots. Leaf blade water and N contents were positively correlated with very little scatter. The slope of the line was different for S. ioclados and S. pyramidalis. Leaf blade water and CHO contents were negatively related but there was more scatter and the species could not be distinguished. The species from more heavily grazed grasslands was conspicuously more sensitive to thiamine application. The results indicated that leaf treatment with thiamine, the only likely source of which in natural grasslands is saliva deposited by feeding herbivores, can have major effects on plant yield and metabolic balances at very low application levels. But under defoliation levels as severe as those imposed in this experiment, which reduced above ground plant biomass to a fourth of the level produced by unclipped plants, growth was so strongly limited by defoliation that neither thiamine nor inorganic nutrients affected plant yield residual from clipping. Therefore, whether chemicals such as thiamine that may be introduced onto grass foliage by grazing ungulates and other herbivores will influence the growth of grazed plants will depend upon the grazing intensity associated with the transfer.     

Interactive effects of root endophytes and arbuscular mycorrhizal fungi on an experimental plant community

Plant-soil microbial interactions have moved into focus as an important mechanism for understanding plant coexistence and composition of communities. Both arbuscular mycorrhizal (AM) as well as other root endophytic fungi co-occur in plant roots, and therefore have the potential to influence relative abundances of plant species in local assemblages. However, no study has experimentally examined how these key root endosymbiont groups might interact and affect plant community composition. Here, using an assemblage of five plant species in mesocosms in a fully factorial experiment, we added an assemblage of AM fungi and/or a mixture of root endophytic fungal isolates, all obtained from the same grassland field site. The results demonstrate that the AM fungi and root endophytes interact to affect plant community composition by changing relative species abundance, and consequently aboveground productivity. Our study highlights the need to explicitly consider interactions of root-inhabiting fungal groups in studies of plant assemblages.     

Interactive effects of mechanical stress,sand burial and defoliation on growth and mechanical properties in Cynanchum komarovii

In drylands, wind, sand burial and grazing are three important factors affecting growth and mechanical properties of plants, but their interactive effects have not yet been investigated. Plants of the semi‐shrub Cynanchum komarovii, common in semi‐arid parts of NE Asia, were subjected to brushing, burial and defoliation. We measured biomass allocation and relative increment rates of dry mass (RGR_m), height (RGR_h) and basal diameter (RGR_d). We also measured the stem mechanical properties, Young’s modulus (E), second moment of area (I), flexural stiffness (EI) and breaking stress (σ_b), and scaled these traits to the whole‐plant level to determine the maximum lateral force (F_lateral) and the buckling safety factor (BSF). Brushing increased RGR_m; neither burial nor defoliation independently affected RGR_m, but together they reduced it. Among buried plants, brushing positively affected stem rigidity and strength through increasing RGR_d, E, I and EI, and at whole plant level this resulted in a larger BSF and F_lateral. However, among unburied plants this pattern was not observed. Our results thus show that effects of mechanical stress and grazing on plants can be strongly modified by burial, and these interactions should be taken into account when considering adaptive significance of plant mechanical traits in drylands.     

Interactive effects of soil fertility and herbivory on Brassica nigra

Soil nutrient availability may affect both the amount of damage that plants receive from herbivores and the ability of plants to recover from herbivory, but these two factors are rarely considered together. In the experiment reported here, I examined how soil fertility influenced both the degree of defoliation and compensation for herbivory for Brassica nigra plants damaged by Pieris rapae caterpillars. Realistic levels of defoliation were obtained by placing caterpillars on potted host plants early in the life cycle and allowing them to feed until just before pupation on the designated plant. Percent defoliation was more than twice as great at low soil fertility compared to high (48.2% and 21.0%, respectively), even though plants grown at high soil fertility lost a greater absolute amount of leaf area (38.2 cm² and 22.1 cm², respectively). At both low and high soil fertility, total seed number and mean mass per seed of damaged plants were equivalent to those of undamaged plants. Thus soil fertility did not influence plant compensation in terms of maternal fitness. However, the pathways used to achieve compensation in seed production were different at low and high soil fertility. At low soil fertility, relative leaf growth rates (area added per inital area per day) of damaged plants were drastically reduced over the second week of caterpillar feeding. Damaged plants recovered the leaf area lost to herbivory in the two weeks following insect removal by increasing leaf relative growth rates above the levels seen for undamaged plants, but the replacement of leaf tissue lost to herbivory came at the expense of stem biomass. At high soil fertility, relative leaf growth rates of damaged plants were similar to those of undamaged plants both over the second week of caterpillar feeding and following caterpillar removal, and stem biomass was not affected by herbivory. These results suggest that higher levels of soil nutrients increased the ability of plants to stay ahead of their herbivores as they were being eaten. Because damaged plants at high soil fertility were able to maintain leaf growth rates to a greater extent than damaged plants at low soil fertility, they did not fall as far behind undamaged plants over the period of insect feeding and did not have as much catching up to do after feeding ended to compensate for herbivory.     

Interactive effects of diversity, nutrients and elevated CO2 on experimental plant communities

The effects of species richness and elevated CO₂ on community productivity under altered nutrient levels were studied in experimental herbaceous communities composed of species from the Midwestern United States annual community, which consists of three functional groups C₃, C₄ and N‐fixers. Aboveground and belowground biomass were measured at flowering stage and at the end of the experiment when fruits of most plants were ripe. At the low nutrient level, species richness did not have a significant effect on community productivity. However, at the high nutrient level, the community biomass decreased with decreasing species richness at both ambient and elevated CO₂ in the first harvest, and at elevated CO₂ in the second harvest. At low nutrient level, CO₂ slightly increased community biomass at medium and high species richness. At high nutrient level, CO₂ significantly increased community biomass in all species‐richness treatments in the first harvest, but a significant response was observed only in the high richness treatment in the second harvest. At the functional group level, biomass of C₃ responded positively to CO₂, and C₄ responded very negatively to CO₂. The N‐fixers responded positively to CO₂ at low and medium species richness, but negatively at high species richness, showing a CO₂×richness interaction. CO₂ increased species evenness in the communities, depending on nutrient level. Species varied in the responses of light‐saturated net photosynthesis (P_max) to elevated CO₂, even within functional groups. Our findings suggest that (1) the relationship between productivity and species diversity was dependent on nutrient levels. (2) Species diversity enhances responses of communities to elevated CO₂. (3) Harvest time can affect the results of diversity‐productivity experiments. (4) Responses of C₃, C₄, and N‐fixers to elevated CO₂ in communities did not follow the prediction based on functional groups or plants grown individually, rather it depended on species richness.     

Linking above-ground and below-ground effects in autotrophic microcosms: effects of shading and defoliation on plant and soil properties

Although factors affecting plant growth and plant carbon/nutrient balance – e.g., light availability and defoliation by herbivores – may also propagate changes in below‐ground food webs, few studies have aimed at linking the above‐ground and below‐ground effects. We established a 29‐week laboratory experiment (～one growing season) using autotrophic microcosms to study the effects of light and defoliation on plant growth, plant carbon/nutrient balance, soil inorganic N content, and microbial activity and biomass in soil. Each microcosm contained three substrate layers – mineral soil, humus and plant litter – and one Nothofagus solandri var. cliffortioides seedling. The experiment constituted of the presence or absence of two treatments in a full factorial design: shading (50% decrease in light) and artificial defoliation (approximately 50% decrease in leaf area in the beginning of the growing season). At the end of the experiment a range of above‐ground and below‐ground properties were measured. The shading treatment reduced root and shoot mass, root/shoot ratio and leaf production of the seedlings, while the defoliation treatment significantly decreased leaf mass only. Leaf C and N content were not affected by either treatment. Shading increased NO ₃–N concentration and decreased microbial biomass in humus, while defoliation did not significantly affect inorganic N or microbes in humus. The results show that plant responses to above‐ground treatments have effects which propagate below ground, and that rather straightforward mechanisms may link above‐ground and below‐ground effects. The shading treatment, which reduced overall seedling growth and thus below‐ground N use and C allocation, also led to changes in humus N content and microbial biomass, whereas defoliation, which did not affect overall growth, did not influence these below‐ground properties. The study also shows the carbon/nutrient balance of N. solandri var. cliffortioides seedlings to be highly invariant to both shading and defoliation.     

Interactive effects of soil nutrient heterogeneity and belowground herbivory on the growth of plants with different root foraging traits

Aims

Plants with precise root foraging patterns can proliferate roots preferentially in nutrient-rich soil patches. When nutrients are distributed heterogeneously, this trait is often competitively advantageous in pot experiments but not field experiments. We hypothesized that this difference is due to belowground herbivory under field conditions.

Methods

We performed pot experiments using seedlings of Lolium perenne (a more precise root foraging species) and Plantago lanceolata (a less precise root foraging species). The experiment had a two-way factorial randomized block design, with nutrient distribution pattern (homogeneous or heterogeneous) and belowground herbivore (present or absent) as the two factors. Each pot contained one seedling of each species.

Results

With no herbivore present, plant biomass was smaller in the heterogeneous nutrient treatment than in the homogeneous treatment in P. lanceolata, but not in L. perenne. Under homogeneous nutrient distribution, plant biomass was lower in both species with a herbivore present than with no herbivore. Under heterogeneous nutrient distribution, biomass reduction due to herbivory occurred only in L. perenne.

Conclusions

Roots of the precise root foraging species were grazed more under the heterogeneous nutrient distribution, suggesting that the herbivore more efficiently foraged for roots in nutrient-rich soil patches.     

Short-term effects of amoxicillin on bacterial communities in manured soil

Antibiotic-resistant bacteria, nutrients and antibiotics that enter the soil by means of manure may enhance the proportion of bacteria displaying antibiotic resistance among soil bacteria and may affect bacterial community structure and function. To investigate the effect of manure and amoxicillin added to manure on soil bacterial communities, microcosm experiments were performed with two soil types and the following treatments: (1) nontreated, (2) manure-treated, (3) treated with manure supplemented with 10 mg amoxicillin kg(-1) soil and (4) treated with manure supplemented with 100 mg amoxicillin kg(-1) soil, with four replicates per treatment. Manure significantly increased the total CFU count and the amoxicillin-resistant CFU count of both soil types. However, only the soil with a history of manure treatment showed a significant increase in the relative number of amoxicillin-resistant bacteria as a result of amoxicillin amendment. The majority of plasmids exogenously isolated from soil originated from soil treated with amoxicillin-supplemented manure. All 16 characterized plasmids carried the bla-TEM gene, and 10 of them belonged to the IncN group. The bla-TEM gene was detected in DNA directly extracted from soil by dot-blot hybridization of PCR amplicons and showed an increased abundance in soil samples treated with manure. Molecular fingerprint analysis of 16S rRNA gene fragments amplified from soil DNA revealed significant effects of manure and amoxicillin on the bacterial community of both soils.     

Predation and priority effects in experimental zooplankton communities

During colonization of new habitats, the sequence of arrival among species is in many cases determined by chance. Priority effects imply that differences in arrival time may lead to long-lasting differences in species dominance. To evaluate the importance of priority effects, we performed a community assembly experiment, manipulating the inoculation order of three large cladoceran zooplankton species. The inoculation treatments were crossed with a predation treatment to test whether the presence of a predator (larvae of the phantom midge Chaoborus ) results in changes in the strength of species sorting and priority effects. Our results clearly demonstrate priority effects, but also that the presence of a predator impacts both community assembly and the strength of priority effects. In the predation-free treatments, communities were dominated by either Daphnia magna or Daphnia obtusa , depending on the species arrival sequence. Whenever D. obtusa was inoculated after D. magna , the species displayed negative growth. In the presence of Chaoborus predation, the communities were generally dominated by the third species, Simocephalus vetulus . Here, the growth of D. magna was negative when the species was inoculated as second or third. Overall, our results underscore the importance of both priority effects and species sorting during community assembly.     

Unpredicted impacts of insect endosymbionts on interactions between soil organisms,plants and aphids

Ecologically significant symbiotic associations are frequently studied in isolation, but such studies of two-way interactions cannot always predict the responses of organisms in a community setting. To explore this issue, we adopt a community approach to examine the role of plant–microbial and insect–microbial symbioses in modulating a plant–herbivore interaction. Potato plants were grown under glass in controlled conditions and subjected to feeding from the potato aphid Macrosiphum euphorbiae. By comparing plant growth in sterile, uncultivated and cultivated soils and the performance of M. euphorbiae clones with and without the facultative endosymbiont Hamiltonella defensa, we provide evidence for complex indirect interactions between insect– and plant–microbial systems. Plant biomass responded positively to the live soil treatments, on average increasing by 15% relative to sterile soil, while aphid feeding produced shifts (increases in stem biomass and reductions in stolon biomass) in plant resource allocation irrespective of soil treatment. Aphid fecundity also responded to soil treatment with aphids on sterile soil exhibiting higher fecundities than those in the uncultivated treatment. The relative allocation of biomass to roots was reduced in the presence of aphids harbouring H. defensa compared with plants inoculated with H. defensa-free aphids and aphid-free control plants. This study provides evidence for the potential of plant and insect symbionts to shift the dynamics of plant–herbivore interactions.     

Plant and soil responses to defoliation: a comparative study of grass species with contrasting life history strategies

The overall aim of this study was to test for inter-species variation in plant and soil responses to defoliation among a broad range of temperate grass species and life-history strategies. We used a microcosm experiment where a range of grass species differing in life history traits were subjected to different intensities of defoliation, and a range of aboveground and belowground plant and soil responses were measured. All plant attributes, including accumulated shoot biomass, root biomass and root length, showed a strong negative response to defoliation, although plant species exhibited subtle differences in the way that they responded to increased severity of defoliation. Defoliation also exerted a strong influence on soil properties, decreasing soil microbial carbon (C) and the soil microbial C:nitrogen (N) ratio, and increasing inorganic N availability and potential N mineralisation across all species. Despite the wide range in life history strategies, plant species did not differ in their influence on most of the soil variables, except for the rate of nitrate mineralisation, which was lowest under plant species that displayed the least relative detrimental responses to defoliation. Collectively, our results suggest that plant and soil responses to defoliation are reasonably consistent across a broad range of grass species, with only subtle inter-specific differences among species.     

Indirect effects of polycyclic aromatic hydrocarbon contamination on microbial communities in legume and grass rhizospheres

Background and aims

Biodegradation of polycyclic aromatic hydrocarbons (PAHs) is accelerated in the presence of plants, due to the stimulation of rhizosphere microbes by plant exudates (nonspecific enhancement). However, plants may also recruit specific microbial groups in response to PAH stress (specific enhancement). In this study, plant effects on the development of rhizosphere microbial communities in heterogeneously contaminated soils were assessed for three grasses (ryegrass, red fescue and Yorkshire fog) and four legumes (white clover, chickpea, subterranean clover and red lentil).

Methods

Plants were cultivated using a split-root model with their roots divided between two independent pots containing either uncontaminated soil or PAH-contaminated soil (pyrene or phenanthrene). Microbial community development in the two halves of the rhizosphere was assessed by T-RFLP (bacterial and fungal community) or DGGE (bacterial community), and by 16S rRNA gene tag-pyrosequencing.

Results

In legume rhizospheres, the microbial community structure in the uncontaminated part of the split-root model was significantly influenced by the presence of PAH-contamination in the other part of the root system (indirect effect), but this effect was not seen for grasses. In the contaminated rhizospheres, Verrucomicrobia and Actinobacteria showed increased populations, and there was a dramatic increase in Denitratisoma numbers, suggesting that this genus may be important in rhizoremediation processes.

Conclusion

Our results show that Trifolium and other legumes respond to PAH-contamination stress in a systemic manner, to influence the microbial diversity in their rhizospheres.