Independent effects of arbuscular mycorrhiza and earthworms on plant diversity and newcomer plant establishment

Questions: How do arbuscular mycorrhiza and earthworms affect the structure and diversity of a ruderal plant community? Is the establishment success of newcomer plants enhanced by these soil organisms and their interactions? Methods: We grew a native ruderal plant community composed of different functional groups (grasses, legumes and forbs) in the presence and absence of arbuscular mycorrhizal fungi (AMF) and endogeic earthworms in mesocosms. We introduced seeds of five, mainly exotic, plant species from the same functional groups after a disturbance simulating mowing. The effects of the soil organisms on the native ruderal plant community and seedling establishment of the newcomer plants were assessed. Results: After disturbance, the total above‐ground regrowth of the native plant community was not affected by the soil organisms. However, AMF increased plant diversity and shoot biomass of forbs, but decreased shoot biomass of grasses of the native plant community. Earthworms led to a reduction in total root biomass. Establishment of the introduced newcomer plants increased in the presence of AMF and earthworms. Especially, seedling establishment of the introduced non‐native legume Lupinus polyphyllus and the native forb Plantago lanceolata was promoted in the presence of AMF and earthworms, respectively. The endogeic earthworms gained more weight in the presence of AMF and led to increased extraradical AMF hyphal length in soil. However, earthworms did not seem to modify the effect of AMF on the plant community. Conclusion: The present study shows the importance of mutualistic soil organisms in mediating the establishment success of newcomer plants in a native plant community. Mutualistic soil organisms lead to changes in the structure and diversity of the native plant community and might promote newcomer plants, including exotic species.     

Effects of earthworms and arbuscular mycorrhizal fungi depend on the successional stage of a grassland plant community

Earthworms and arbuscular mycorrhizal fungi (AMF) have profound impacts on plant performance. However, it is largely unknown if and how earthworms and AMF may affect plant succession. We planted mesocosms with an early-mid successional and a mid-late successional grassland plant community and added endogeic earthworms and commercial AMF in a full-factorial way to natural background soil. Earthworms had a positive effect on the total root and shoot biomass of both plant communities, with the effect on the shoots being slightly enhanced by co-inoculation with AMF. Surprisingly, the earthworm effect on the mid successional plant species depended on the successional stage of the plant community. Earthworms had a positive effect on the mid successional plant species when they were growing in the mid-late successional plant community, but no effect when the same plant species were growing in the early-mid successional plant community. Addition of AMF alone tended to reduce the shoot biomass of the early successional plant species, while the addition of earthworms in the presence or absence of AMF increased their shoot biomass. We conclude that the impacts of earthworms on plant species may depend on the successional stage of the plant community, while the effect of AMF addition depends on the successional stage of the plant community and may be changed by the presence of earthworms. Earthworms and AMF addition affect plants and plant communities of different successional stages differently with potential effects on plant succession.     

Effects of earthworms and organic litter distribution on plant performance and aphid reproduction

Human management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of ¹⁵N-labelled grass litter on plants of different functional groups [Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and ¹⁵N uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more ¹⁵N from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores.     

Effects of earthworms on biomass production,nitrogen allocation and nitrogen transfer in wheat-clover intercropping model systems

The effects of earthworms (Lumbricidae) on plant biomass production and N allocation in model intercropping systems of winter wheat and white clover were evaluated in two pot experiments. Wheat and wheat-clover mixtures were grown in a low-organic loam soil, earthworms were added at densities comparable to field population densities and the experiments were terminated 48 and 17 d after earthworm introductions. In both experiments, earthworms significantly increased the biomass and N uptake of wheat while they had generally no effects on clover. As a result, earthworm activity increased the proportion of wheat biomass in the total plant biomass of the mixture. Nitrogen budgets of the experiment lasting 48 d indicated that additional N in the system made available by earthworm activity was primarily taken up by the wheat. Earthworms also affected intra-plant N allocation in wheat which had significantly higher shoot:root N ratios when earthworms were present. When clover was labelled with ¹⁵N in the experiment which lasted 17 d, endogeic earthworms significantly reduced the amounts of ¹⁵N excess transferred from living or decomposing clover roots to accompanying wheat plants. Earthworms assimilated small quantities of ¹⁵N tracer from decomposing clover roots but not from living clover roots. The results of these model experiments suggest that earthworms can affect the balance between intercropped cereals and legumes by altering intra- and inter-plant N allocation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Endogeic earthworms modify soil phosphorus,plant growth and interactions in a legume–cereal intercrop

Background and aims

Intercropping of legumes and cereals appears as an alternative agricultural practice to decrease the use of chemical fertilizers while maintaining high yields. A better understanding of the biotic and abiotic factors determining interactions between plants in such associations is required. Our study aimed to analyse the effect of earthworms on the legume–cereal interactions with a focus on the modifications induced by earthworms on the forms of soil phosphorus (P).

Methods

In a glasshouse experiment we investigated the effect of an endogeic earthworm (Allolobophora chlorotica) on the plant biomass and on N and P acquisition by durum wheat (Triticum turgidum durum L.) and chickpea (Cicer arietinum L.) either grown alone or intercropped. The modifications of the different organic and inorganic P forms in the bulk soil were measured.

Results

There was no overyielding of the intercrop in the absence of earthworms. Earthworms had a strong influence on biomass and resource allocation between roots and shoots whereas no modification was observed in terms of total biomass production and P acquisition. Earthworms changed the interaction between the intercropped species mainly by reducing the competition for nutrients. Facilitation (positive plant–plant interactions) was only observed for the root biomass and P acquisition in the presence of earthworms. Earthworms decreased the amount of organic P extracted with NaOH (Po NaOH), while they increased the water soluble inorganic P (Pi H₂O) content.

Conclusions

In this experiment, earthworms could be seen as “troubleshooter” in plant–plant interaction as they reduced the competition between the intercropped species. Our study brings new insights into how earthworms affect plant growth and the P cycle.     

Earthworm effects on plant growth do not necessarily decrease with soil fertility

Earthworms are known to generally increase plant growth. However, because plant-earthworm interactions are potentially mediated by soil characteristics the response of plants to earthworms should depend on the soil type. In a greenhouse microcosm experiment, the responsiveness of plants (Veronica persica, Trifolium dubium and Poa annua) to two earthworm species (in combination or not) belonging to different functional groups (Aporrectodea. caliginosa an endogeic species, Lumbricus terrestris an anecic species) was measured in term of biomass accumulation. This responsiveness was compared in two soils (nutrient rich and nutrient poor) and two mineral fertilization treatments (with and without). The main significant effects on plant growth were due to the anecic earthworm species. L. terrestris increased the shoot biomass and the total biomass of T. dubium only in the rich soil. It increased also the total biomass of P. annua without mineral fertilization but had the opposite effect with fertilization. Mineral fertilization, in the presence of L. terrestris, also reduced the total biomass of V. persica. L. terrestris did not only affect plant growth. In P. annua and V. persica A. caliginosa and L. terrestris also affected the shoot/root ratio and this effect depended on soil type. Finally, few significant interactions were found between the anecic and the endogeic earthworms and these interactions did not depend on the soil type. A general idea would be that earthworms mostly increase plant growth through the enhancement of mineralization and that earthworm effects should decrease in nutrient-rich soils or with mineral fertilization. However, our results show that this view does not hold and that other mechanisms are influential.     

Interaction between Earthworms and Arbuscular Mycorrhizal Fungi in Plants: A Review

Different kinds of soil animals and microorganisms inhabit the plant rhizosphere, which function closely to plant roots. Of them, arbuscular mycorrhizal fungi (AMF) and earthworms play a critical role in sustaining the soil-plant health. Earthworms and AMF belong to the soil community and are soil beneficial organisms at different trophic levels. Both of them improve soil fertility and structural development, collectively promoting plant growth and nutrient acquisition capacity. Earthworm activities redistribute mycorrhizal fungi spores and give diversified effects on root mycorrhizal fungal colonization. Dual inoculation with both earthworms and AMF strongly magnifies the response on plant growth through increased soil enzyme activities and changes in soil nutrient availability, collectively mitigating the negative effects of heavy metal pollution in plants and soils. This thus enhances phytoremediation and plant disease resistance. This review simply outlines the effects of earthworms and AMF on the soil-plant relationship. The effects of earthworms on root AMF colonization and activities are also analyzed. This paper also summarizes the interaction between earthworms and AMF on plants along with suggested future research.     

Arbuscular mycorrhizal fungi enhance P uptake and alter plant morphology in the invasive plant Microstegium vimineum

Invasive plant species can interact with native soil microbes in ways that change how they use nutrients and allocate biomass. To examine whether Microstegium vimineum form symbiotic associations with arbuscular mycorrhizal fungi (AMF) and whether AMF mediate nutrient acquisition and growth of the plant, we conducted a field survey in Raleigh, NC and Hangzhou, China and two experiments in growth chambers. This is the first report that M. vimineum is mycorrhizal, with colonization rates of 47 and 21 % in its native and invaded range, respectively. In the growth chamber, addition of an AMF inoculum mixture significantly promoted M. vimineum biomass accumulation in both field and sterilized soils, particularly after 64 days of growth. Arbuscular mycorrhizal fungi also increased plant phosphorous (P) uptake but did not consistently affect total plant nitrogen (N) acquisition, leading to decreases in plant N:P ratios. More interestingly, AMF significantly altered plant morphology, increasing the number of stolons and aerial roots per individual (59 and 723 %), aerial roots per gram aboveground biomass (374 %) and aerial roots per stolon (404 %). Our results suggest that mycorrhizal enhancement of plant growth by stimulating tillering may serve as another mechanism by which M. vimineum can quickly take over new territory. Future studies on invasive plant-microbial interactions are needed to understand the mechanisms through which microbes contribute to the competitive ability of invasive plants.     

Earthworms and eco-consequences: Considerations to soil biological indicators and plant function: A review

Earthworms have been well reported to have a beneficial effect on soil microbes, soil microbial biomass (SMB), fungal community, soil structure, water retention and plant growth in different terrestrial ecosystems. However, the interactions between environmental stressors and various species of earthworms and the subsequent effect on soil microbes, organic matter, soil structure and plant growth are still uncertain. The purpose of this analysis was to test 1- the impact of environmental stressors on earthworm behaviour. 2- the effect of various earthworms on soil microbes, plant growth, soil structure and the carbon cycle. We noted that less fatal temperatures are generally unknown, but higher fatal temperatures range from 25 to 48 °C. Earthworms have a role to play, depending on the nature of organic residues, in both the formation and degradation of soil aggregates. Improvements in microbial biomass and plant growth have been established according to temperature, soil toxicity, soil type, earthworms abundance, organic residues types and field conditions. We observed that although the summer temperature in the arid area was approximately (°C 48), it was found that a particular type of earthworm (Namalycastis indica) was responsible for improving soil characteristics.While a great deal of analysis has been carried out on the role of earthworms within the soil ecology, such a review identifies important knowledge gaps, particularly in the determination of the impacts of earthworm species on the soil structure, microbial biomass and plant productivity, in particular since most papers focused on European species and overlooked the role of earthworms in the arid landscape. Further research is recommended to compare the impacts of different earthworms species on soil microbes and plant growth in various soil types, earthworm abundance, field conditions, organic residues locations, inorganic fertilizers, pesticides, fertile or non-fertile soils and diverse conditions of drought and moisture.     

Protozoa,Nematoda and Lumbricidae in the rhizosphere of Hordelymus europeaus (Poaceae): faunal interactions,response of microorganisms and effects on plant growth

Interactions among protozoa (mixed cultures of ciliates, flagellates and naked amoebae), bacteria-feeding nematodes (Pellioditis pellio Schneider) and the endogeic earthworm species Aporrectodea caliginosa (Savigny) were investigated in experimental chambers with soil from a beechwood (Fagus sylvatica L.) on limestone. Experimental chambers were planted with the grass Hordelymus europeaus L. (Poaceae) and three compartments separated by 45-m mesh were established: rhizosphere, intermediate and non-rhizosphere. The experiment lasted for 16 weeks and the following parameters were measured at the end of the experiment: shoot and root mass of H. europaeus, carbon and nitrogen content in shoots and roots, density of ciliates, amoebae, flagellates and nematodes, microbial biomass (SIR), basal respiration, streptomycin sensitive respiration, ammonium and nitrate contents, phosphate content of soil compartments. In addition, leaching of nutrients (nitrogen and phosphorus) and leachate pH were measured at regular intervals in leachate obtained from suction cups in the experimental chambers. Protozoa stimulated the recovery of nitrifying bacteria following defaunation (by chloroform fumigation) and increased nitrogen losses as nitrate in leachate. In contrast, protozoa and nematodes reduced leaching of phosphate, an effect ascribed to stimulation of microbial growth early in the experiment. Earthworms strongly increased the amount of extractable mineral nitrogen whereas it was strongly reduced by protozoa and nematodes. Both protozoa and nematodes reduced the stimulatory effect of earthworms on nitrogen mineralization. Microbial biomass, basal respiration, and numbers of protozoa and nematodes increased in the vicinity of the root. Protozoa generally caused a decrease in microbial biomass whereas nematodes and earthworms reduced microbial biomass only in the absence of protozoa. None of the animals studied significantly affected basal respiration, but specific respiration of microorganisms (O₂ consumption per unit biomass) was generally higher in animal treatments. The stimulatory effect of nematodes and earthworms, however, occurred only in the absence of protozoa. The sensitivity of respiration to streptomycin suggested that protozoa selectively grazed on bacterial biomass but the bacterial/fungal ratio appeared to be unaffected by grazing of P. pellio. Earthworms reduced root biomass of H. europaeus, although shoot biomass remained unaffected, and concentrations of nitrogen in shoots and particularly in roots were strongly increased by earthworms. Both nematodes and protozoa increased plant biomass, particularly that of roots. This increase in plant biomass was accompanied by a marked decrease in nitrogen concentrations in roots and to a lesser extent in shoots. Generally, the effects of protozoa on plant growth considerably exceeded those of nematodes. It is concluded that nematodes and protozoa stimulated plant growth by non-nutritional effects, whereas the effects of earthworms were caused by an increase in nutrient supply to H. europaeus.     

Arbuscular mycorrhizas in phosphate-polluted soil: interrelations between root colonization and nitrogen

To investigate whether arbuscular mycorrhizal fungi (AMF) – abundant in a phosphate-polluted but nitrogen-poor field site – improve plant N nutrition, we carried out a two-factorial experiment, including N fertilization and fungicide treatment. Percentage of root length colonized (% RLC) by AMF and tissue element concentrations were determined for four resident plant species. Furthermore, soil nutrient levels and N effects on aboveground biomass of individual species were measured. Nitrogen fertilization lowered % RLC by AMF of Artemisia vulgaris L., Picris hieracioides L. and Poa compressa L., but not of Bromus japonicus Thunb. This – together with positive N addition effects on N status, N:P-ratio and aboveground biomass of most species – suggested that plants are mycorrhizal because of N deficiency. Fungicide treatment, which reduced % RLC in all species, resulted in lower N concentrations in A. vulgaris and P. hieracioides, a higher N concentration in P. compressa, and did not consistently affect N status of B. japonicus. Evidently, AMF had an influence on the N nutrition of plants in this P-rich soil; however – potentially due to differences in their mycorrhizal responsiveness – not all species seemed to benefit from a mycorrhiza-mediated N uptake and accordingly, N distribution.     

Colonization of Tanacetum vulgare by aphids is reduced by earthworms

Soil organisms affect plant growth and chemistry and have subsequent effects on aboveground herbivore performance. However, whether herbivores discriminate between plants exposed to different soil organisms when colonizing their host plants is largely unexplored. In a greenhouse study, Tanacetum vulgare L. (Asteraceae) growing in a ruderal plant community in the presence and absence of arbuscular mycorrhizal fungi (AMF) and earthworms [Aporrectodea spp. (Haplotaxida: Lumbricidae)] was colonized by aphids [Myzus persicae Sulzer (Hemiptera: Aphididae)]. The aphids preferred to colonize plants without earthworms in the soil, and the numbers of aphids remained lower on the plants with earthworms, irrespective of the presence of AMF. Although the N, C, and P concentrations of the shoots were not affected by the soil organisms, AMF increased total aboveground biomass, total N, C, and P content, and photosynthetic activity (measured as electron transport rate) in the leaves under high light intensity. These results suggest that earthworms affect chemical cues that are used by aphids to judge host quality prior to feeding. Discrimination between plants with and without exposure to earthworms by aboveground herbivores is a novel aspect of plant‐mediated interactions between below‐ and aboveground organisms.     

Earthworms,Collembola and residue management change wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) and herbivore pest performance (Aphidina: <Emphasis Type="Italic">Rhophalosiphum padi</Emphasis>)

Management practices of arable systems determine the distribution of soil organic matter thereby changing decomposer animal activity and their impact on nutrient mineralization, plant growth and plant-herbivore interactions. Decomposer-mediated changes in plant growth and insect pest performance were investigated in wheat-aphid model systems in the greenhouse. Three types of litter distribution were established: litter patch at the soil surface (simulating mulching), litter patch deeper in soil (simulating ploughing) and litter homogeneously mixed into soil (simulating disk cultivation). The litter was labelled with (15)N to follow the mineralization and uptake of nutrients by the plants. Earthworms (Aporrectodea caliginosa) and Collembola (Protaphorura armata) were included as representatives of major functional groups of decomposers. Wheat (Triticum aestivum) was planted and aphids (Rhophalosiphum padi) were introduced to leaves as one of the most important pests. Earthworms, Collembola and litter distribution affected plant growth, N acquisition and aphid development in an interactive way. Earthworms and Collembola increased biomass of seeds, shoots and roots of wheat. Increased plant growth by earthworms and Collembola was mainly due to increased transfer of N from soil (rather than litter) into plants. Despite increasing plant growth, earthworms reduced aphid reproduction. Aphid reproduction was not correlated closely with plant N concentrations, but rather with the concentration of litter N in wheat. Unexpectedly, both Collembola and earthworms predominantly affected the mobilization of N from soil organic matter, and by altering the distribution of litter earthworms reduced infestation of crops by aphids via reducing plant capture of litter N, in particular if the litter was concentrated deeper in soil. The results suggest that management practices stimulating a continuous moderate increase in nutrient mobilization from soil organic matter rather than nutrient flushes from decomposing fresh organic matter result in maximum plant growth with minimum plant pest infestation.     

两种代表性蚯蚓对设施菜地土壤微生物群落结构及理化性质的影响

不同生态型蚯蚓的取食偏好和生境有所差异,因此蚯蚓的生态型差异可能关乎其对土壤性质的不同影响;有关不同生态型蚯蚓对土壤性质尤其是微生物学性质影响的研究有助于了解蚯蚓生态功能的作用机制。在野外调控试验的第4年采集土壤,研究了牛粪混施和表施处理下内层种威廉腔环蚓(Metaphire guillelmi)和表层种赤子爱胜蚓(Eisenia foetida)对设施菜地土壤微生物群落结构和主要理化性质的影响。结果表明,土壤微生物群落结构同时受到蚯蚓种类和牛粪施用方式的影响。牛粪表施时,两种蚯蚓均显著降低了菌根真菌、真菌生物量和原生动物生物量(P0.05);牛粪混施时,不同蚯蚓的影响有所差异,威廉腔环蚓明显增加了菌根真菌、真菌生物量和放线菌生物量,而赤子爱胜蚓的作用不明显。此外,两种蚯蚓均提高了土壤孔隙度、团聚体稳定性和土壤p H、矿质氮以及微生物生物量碳氮水平,但提高幅度取决于蚯蚓种类和牛粪施用方式。冗余分析表明蚯蚓影响下土壤微生物群落结构的变化与团聚体稳定性、p H、速效磷、矿质氮呈正相关,而与土壤容重呈负相关。     

Arbuscular mycorrhizal fungi and water availability affect biomass and C:N:P ecological stoichiometry in alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis> L.) during regrowth

Ecological stoichiometry has been widely studied in terrestrial ecosystems, but these studies have been limited in terms of symbiotic association between alfalfa and arbuscular mycorrhizal fungi (AMF), especially during regrowth. To evaluate the effect of AMF on the regrowth and C:N:P stoichiometry of alfalfa (Medicago sativa L.) under well-watered and drought conditions, alfalfa plants inoculated with AMF (Rhizophagus irregularis, M), nitrogen-fixing bacteria (Sinorhizobium, R), both nitrogen-fixing bacteria and AMF or no inoculations (CK) were evaluated in a pot experiment under controlled conditions. The biomass and organic carbon (C), nitrogen (N) and phosphorus (P) nutritional status of plant leaves and roots were measured under two water treatments during regrowth. Water deficit reduced the accumulation of dry matter and the concentrations of C and N in leaves and P in roots but increased the concentrations of P in leaves and C and N in roots of alfalfa during regrowth. Compared to CK plants, inoculation significantly improved the regrowth biomass and the concentrations of C, N and P in the leaves and roots and especially increased P levels when the plant were inoculated with AMF. However, this effect of microbes on alfalfa regrowth was dependent on the soil water status. Drought reduced the C:N and C:P in the leaves and the C:N in roots, while N:P and C:P increased in the roots. Inoculation of AMF decreased the C:P and N:P in the leaves and the C:N and C:P in the roots, whereas it increased the C:N under water stress. These results indicate that AMF play a significant role in regrowth and C:N:P ecological stoichiometry after defoliation by influencing C assimilation, N and P uptake and that the responses in the leaves and the roots are opposite.     

Do endogeic earthworms change plant competition? A microcosm study

Plants compete for limited resources. Although nutrient availability for plants is affected by resource distribution and soil organisms, surprisingly few studies investigate their combined effects on plant growth and competition. Effects of endogeic earthworms (Aporrectodea jassyensis), root-knot nematodes (Meloidogyne incognita) and the spatial distribution of ¹⁵N labelled grass litter on the competition between a grass (Lolium perenne), a forb (Plantago lanceolata) and a legume (Trifolium repens) were investigated in the greenhouse. Earthworms promoted N uptake and growth of L. perenne. Contrastingly, shoot biomass and N uptake of T. repens decreased in the presence of earthworms. P. lanceolata was not affected by the earthworms. We suggest that earthworms enhanced the competitive ability of L. perenne against T. repens. Nematodes increased the proportion of litter N in each of the plant species. Litter distribution (homogeneous vs. patch) did not affect the biomass of any plant species. However, P. lanceolata took up more ¹⁵N, when the litter was homogeneously mixed into the soil. The results suggest that endogeic earthworms may affect plant competition by promoting individual plant species. More studies including decomposers are necessary to understand their role in determining plant community structure.     

Spatial soil heterogeneity has a greater effect on symbiotic arbuscular mycorrhizal fungal communities and plant growth than genetic modification with Bacillus thuringiensis toxin genes

Maize, genetically modified with the insect toxin genes of Bacillus thuringiensis (Bt), is widely cultivated, yet its impacts on soil organisms are poorly understood. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with plant roots and may be uniquely sensitive to genetic changes within a plant host. In this field study, the effects of nine different lines of Bt maize and their corresponding non‐Bt parental isolines were evaluated on AMF colonization and community diversity in plant roots. Plants were harvested 60 days after sowing, and data were collected on plant growth and per cent AMF colonization of roots. AMF community composition in roots was assessed using 454 pyrosequencing of the 28S rRNA genes, and spatial variation in mycorrhizal communities within replicated experimental field plots was examined. Growth responses, per cent AMF colonization of roots and AMF community diversity in roots did not differ between Bt and non‐Bt maize, but root and shoot biomass and per cent colonization by arbuscules varied by maize cultivar. Plot identity had the most significant effect on plant growth, AMF colonization and AMF community composition in roots, indicating spatial heterogeneity in the field. Mycorrhizal fungal communities in maize roots were autocorrelated within approximately 1 m, but at greater distances, AMF community composition of roots differed between plants. Our findings indicate that spatial variation and heterogeneity in the field has a greater effect on the structure of AMF communities than host plant cultivar or modification by Bt toxin genes.     

植物残体对青藏高原高寒草甸土壤、微生物和胞外酶C∶N∶P化学计量特征的影响

植物残体是引起土壤、微生物和胞外酶C∶N∶P改变的关键因素,但是其作用机理尚不明确。本研究以青藏高原东缘高寒草甸为对象,通过测定土壤、微生物生物量和胞外酶活性等指标,探究移除地上植物或根系及植物残体添加对土壤、微生物和胞外酶C∶N∶P的影响。结果表明: 与无人为扰动草甸相比,移除地上植物显著降低了土壤C∶N(变幅为-23.7%,下同)、C∶P(-14.7%)、微生物生物生物量C∶P、N∶P,显著提高了微生物生物量C∶N、胞外酶C∶N∶P。与移除地上植物相比,移除地上植物和根系显著降低了土壤C∶N(-11.6%)、C∶P(-24.0%)、N∶P(-23.3%)和微生物生物量C∶N,显著提高了微生物生物量N∶P和胞外酶N∶P;移除地上植物后添加植物残体显著提高了微生物生物量C∶N、C∶P和胞外酶C∶N,显著降低了胞外酶N∶P。与移除地上植物和根系相比,移除地上植物和根系后添加植物残体显著降低了土壤C∶N(-16.4%)、微生物生物量C∶P、N∶P和胞外酶N∶P,显著提高了胞外酶C∶N。综上可知,去除植物显著影响土壤、微生物和胞外酶的C∶N∶P,微生物生物量和胞外酶C∶N∶P对植物残体的响应更为敏感。有无根系是添加植物残体时土壤、微生物和胞外酶的生态化学计量稳定性强弱的关键所在。添加植物残体的措施适用于植物根系尚且完好的草甸,有利于高寒草甸土壤碳固存,对没有根系的草甸土壤可能不适用,会增加土壤CO₂排放。     

Arbuscular mycorrhizal fungi differ in affecting the flowering of a host plant under two soil phosphorus conditions

Aims Studies have showed that arbuscular mycorrhizal fungi (AMF) can greatly promote the growth of host plants, but how AMF affect flowering phenology of host plants is not well known. Here, we conducted a pot experiment to test whether life cycle and flowering phenology traits of host plant Medicago truncatula Gaertn can be altered by AMF under low and high soil phosphorus (P) levels.Methods The experiment was conducted in a greenhouse at Zhejiang University in China (120°19′E, 30°26′N) and had a completely randomized design with two factors: AMF treatments and soil P levels. Six AMF species (Acaulospora scrobiculata, As; Gigaspora margarita, Gma; Funneliformis geosporum, Fg; Rhizophagus intraradices, Ri; Funneliformis mosseae, Fmo and Glomus tortuosum, Gt.) were used, and two soil P levels (24.0 and 5.7 mg kg^-1 Olsen-soluble P) were designed. The six AMF species were separately inoculated or in a mixture (Mix), and a non-AMF control (NAMF) was included. When plants began to flower, the number of flowers in each pot was recorded daily. During fruit ripening, the number of mature fruits was also recorded daily. After ~4 months, the biomass, biomass P content and AMF colonization of host plant were measured. Correlation between root colonization and first flowering time, or P content and first flowering time was analyzed.Important findings Under the low P level, first flowering time negatively correlated with root colonization and biomass P. Only host plants with AMF species As, Fg, Ri, or Mix were able to complete their life cycle within 112 days after sowing. And treatment with AMF species Fg, Gt, or As resulted in two periods of rapid flower production while other fungi treatments resulted in only one within 112 days after sowing. The cumulative number of flowers produced and biomass P content were highest with species Fg. Host biomass allocation significantly differed depending on the species of AMF. Under both soil P levels, the host plant tended to allocate more biomass to fruits in the Mix treatment than in the other treatments. These results indicated that the effects of AMF on host flowering phenology and biomass allocation differed depending on AMF species and soil P levels.     

Minerals in the rhizosphere: overlooked mediators of soil nitrogen availability to plants and microbes

Non-native earthworms are a continued source of environmental change in the northeastern United States that may affect trace metals in the plant-soil system, with largely unknown effects. We assessed earthworm impacts on exchangeable and strong acid extractable (total) concentrations and pools of Al, Fe, Cu, Zn, Mo, Pb in non-point source polluted, forest soil horizons (Organic, A, and B) and foliar metals concentrations in young (<?3 years) Acer saccharum and Polystichum acrostichoides at four proximal forests in the Finger Lakes Region of New York. We observed decreasing total trace metal Organic horizon pools and increasing total trace metal A horizon concentrations as a function of increasing earthworm biomass. Earthworms had limited effects on exchangeable concentrations in A and B horizons and total metal concentrations in the B horizon. Foliar trace metal concentrations in Acer were better explained by earthworm biomass than soil concentrations but foliar concentrations for Polystichum were poorly predicted by both earthworm biomass and soil metal concentrations. Our results suggest that earthworms can affect trace metal uptake by some plants, but not by increasing soil trace metal exchangeability or from changing soil properties (pH, %SOM, or cation exchange capacity). Instead, non-native earthworms may indirectly alter understory plant uptake of trace metals.