Soil enzyme activity in an old-growth northern hardwood forest: Interactions between soil environment,ectomycorrhizal fungi and plant distribution

Plants and soil microbes produce extracellular enzymes (EE) that catalyze the hydrolysis of nitrogen (N) and phosphorus (P) containing compounds in soil and other enzymes involved in degradation of lignin and cellulose. We explored whether soil enzyme activity involved in carbon (C), N and P cycling were correlated with plant distribution, soil chemical conditions and the identity of fungi colonizing tree roots in an old growth forest remnant. Terminal restriction fragment length polymorphism (TRFLP) was used to determine the presence of root fungi and standard fluorometric analysis was used to determine soil enzyme activities. Soil enzymes were consistently positively correlated with soil C and N, but not CN ratio. Soil P was also correlated with enzyme activity during both June and September sampling. We saw no significant relationships between herbaceous plant cover and enzyme activity in June, but there were significant positive correlations between α-glucosidase and herbaceous plant coverage in September. We also found that some enzymes were significantly correlated with the identity of fungi colonizing tree roots separated from the soil cores. Chitinase and β-glucosidase were positively correlated with the genera Russula and Piloderma while chitinase was negatively correlated with Amanita and Entoloma. In addition, phosphatase was positively correlated with Russula, Meliniomyces and Solenopezia. Our results suggest that enzyme activity in old growth forest soils are affected by a variety of environmental factors, and that herbaceous plants and some root fungi may be associated with sites of elevated or decreased decomposition potential and nutrient cycling.     

Functional relationships between Collembola and plant pathogenic fungi of agricultural soils

The interactions between the collembolan Mesaphorura krausbaueri and four soilborne plant pathogenic fungi all living in the same layer of agricultural soil were studied under laboratory conditions. Mycelia of Gaeumannomyces graminis var. tritici, Fusarium culmorum and Rhizoctonia cerealis were palatable to Collembola and were a food source adequate for reproduction. Hyphae of Bipolaris sorokiniana were toxic and repellent for M. krausbaueri, but its conidia were eaten and proved to be a sufficient diet for reproduction. The four fungi fed had a different impact on the life processes of M. krausbaueri. Depending on the species of fungus, the animals produced significantly different numbers of eggs and had different life spans. In particular, animals fed with G. graminis var. tritici showed early maturation, early burst in egg production, early loss of fertility and early death. The different nutritional values of the four fungi pathogen for cereals might have a different influence on the life strategy of Collembola in the field. Thus, since these fungi usually coexist in agricultural soils, though in different percentages, this influence might also affect the capacity of different fungi to attack the plant and thus modulate the appearance of disease.     

Cause and duration of mustard incorporation effects on soil-borne plant pathogenic fungi

Two fungal plant pathogens, Rhizoctonia solani AG 2-2 and Fusarium oxysporum f.sp. lini, were studied in relation to general responses of soil fungi and bacteria following incorporation of Brassica juncea. Our aim was to understand to what extent the changes in the biological and physicochemical characteristics of the soil could explain the effects on the studied pathogens and diseases, and to determine the temporal nature of the responses. Short-term effects of mustard incorporation (up to 4 months) were investigated in a microcosm experiment, and compared with a treatment where composted plant material was incorporated. In a field experiment, the responses were followed up to 11 months after removal or incorporation of a mustard crop. In general, responses in the variables measured changed more after incorporation of fresh mustard material than after addition of similar amounts of composted plant material (microcosms) or after removal of the mustard crop (field). The soil inoculum potential of R. solani AG 2-2 decreased directly after incorporation of mustard, but increased later to disease levels above those in the untreated soil. Neither of these effects could be explained by changes in the population density of R. solani AG 2-2. Fusarium spp. were less influenced, although an increase in the suppressiveness to Fusarium wilt was observed after mustard incorporation as compared with the treatment where mustard was removed. The microbial responses to mustard incorporation were more pronounced for bacteria than for fungi. After an initial substantial increase, the bacterial density decreased but remained above the levels in the control treatment throughout the experimental periods. The bacterial community structure was modified up to 8 months after mustard incorporation. We conclude that incorporation of fresh mustard influences soil microbial communities, especially the bacteria, and has a potential to control the pathogenic activity of R. solani 2-2 on a short-term perspective. The time dependency in microbial responses is important and should be taken into consideration for the evaluation of the potential of Brassicas to control plant disease on a field scale.     

Do Collembola affect the competitive relationships among soil-borne plant pathogenic fungi?

The feeding preference of the collembolan Protaphorura armata in the presence of Fusarium culmorum and Gaeumannomyces graminis var. tritici, two soil-borne fungi pathogenic for winter cereals, was studied in a simplified experimental system including wheat seedlings. Analysis of gut content of all animals from microcosms containing inoculum of both fungi showed that F. culmorum was clearly preferred but that G. graminis var. tritici was also fed. At microscopic examination the majority of F. culmorum conidia present in the gut lacked cytoplasmic content, and only few conidial cells were intact. The feeding preference of P. armata favoured G. graminis var. tritici over F. culmorum in the competition for infection sites on wheat plants; in fact, the former resulted the prevalent cause of plant disease.

The viability of fungal propagules after passage through the gut of P. armata was also studied. No colonies of G. graminis var. tritici and only a few colonies of F. culmorum developed from faecal pellets set on agar medium. Fungal propagules dispersed by springtails were not sufficient to induce disease, as demonstrated by introducing animals, previously fed on fungal cultures separately, into microcosms containing a sterile substrate where wheat kernels were seeded.     

Interactions between mycorrhizal fungi and Collembola: effects on root structure of competing plant species

Mycorrhizal fungi influence plant nutrition and therefore likely modify competition between plants. By affecting mycorrhiza formation and nutrient availability of plants, Collembola may influence competitive interactions of plant roots. We investigated the effect of Collembola (Protaphorura fimata Gisin), a mycorrhizal fungus (Glomus intraradices Schenck and Smith), and their interaction on plant growth and root structure of two plant species, Lolium perenne L. (perennial ryegrass) and Trifolium repens L. (white clover). In a laboratory experiment, two individuals of each plant species were grown either in monoculture or in competition to the respective other plant species. Overall, L. perenne built up more biomass than T. repens. The clover competed poorly with grass, whereas the L. perenne grew less in presence of conspecifics. In particular, presence of conspecifics in the grass and presence of grass in clover reduced shoot and root biomass, root length, number of root tips, and root volume. Collembola reduced shoot biomass in L. perenne, enhanced root length and number of root tips, but reduced root diameter and volume. The effects of Collembola on T. repens were less pronounced, but Collembola enhanced root length and number of root tips. In contrast to our hypothesis, changes in plant biomass and root structure in the presence of Collembola were not associated with a reduction in mycorrhizal formation. Presumably, Collembola affected root structure via changes in the amount of nutrients available and their spatial distribution.     

Fitting plants to soil through mycorrhizal fungi: Mycorrhiza effects on plant growth and soil organic matter

Summary Vesicular-arbuscular mycorrhizal (VAM) fungi affect diverse aspects of plant form and function. Since mycorrhiza-mediated changes in host-plant responses to root colonization by different VAM fungi vary widely, it is important to assess each endophyte for each specific effect it can elicit from its host as part of the screening process for effectiveness. Three species of VAM fungi and a mixture of species were compared with non-VAM controls for their effects on soil organic matter contents and on nutrition and morphology in two varieties (native and hybrid) of corn (Zea mays L.) and one of sunflower (Helianthus annuus L.) in P-sufficient and N-deficient soil in pot cultures. Differences in soil organic matter due to the fungal applications were highly significant with all host plants. Native corn responded more to VAM colonization than the hybrid did; differences in treatments were significant in leaf area, plant biomass, and root: shoot ratio in the former, but not in the latter. Responses in the sunflower were similar to those in the native corn. Significant VAM treatment-related differences in shoot N and P contents were not reflected in shoot biomass, which was invariant. Correlations between plant or soil parameters and the intensity of VAM colonization were found only in soil organic matter with the native corn, in specific leaf area in the hybrid corn, and in plant biomass in the sunflower. The presence of the different endophytes and not the intensity of colonization apparently elicited different host responses.     

Interactions in microbial use of soluble plant components in soil

Glucose, glutamic acid, alanine, valine, and albumin were used as model substances to represent readily available soluble plant components. Their mineralisation in soil was monitored by making hourly measurements of total CO₂ evolution and periodic measurements of ¹⁴CO₂ during a 5-day incubation period. When glucose and the amino acids were combined in dual substrates the two components were mineralised simultaneously. In each combination the component with the shortest lag phase or highest specific growth rate () was mineralised preferentially. Glucose accelerated the mineralisation of the amino acids that had longer lag phases than glucose (i.e., alamine and valine). During the first 8 h, glucose mineralisation was reduced in the dual substrates compared with its mineralisation as a single substrate. The smaller the difference between the lag phase of glucose and the amino acid, the greater was the reduction in the glucose mineralisation rate. During the exponential phase, the effects on glucose mineralisation were dependent on the nature and concentration of the amino acid. The interactions observed between glucose and amino acid decomposition indicated that these substances were mineralised, at least partly, by the same microorganisms. In contrast, glucose and the soluble protein albumin were decomposed independently of each other.     

海带渣菌肥作为土壤添加剂对土壤抑制病原真菌活性影响的研究

试验以海带渣固体发酵微生物菌肥作为土壤添加剂为研究对象,以土壤浸出液抑制植物病原真菌寄生曲霉的活性来表示土壤自身抑制植物病原真菌的活性。试验通过Tip-culture定量检测方法对不同处理组的不同时期土壤浸出液抑制病原真菌寄生曲霉活性进行定量的测定。试验表明, 海带渣固体发酵微生物菌肥能显著提高土壤抑制植物病原真菌活性的能力。花生盆栽试验结果表明, 海带渣固体发酵微生物菌肥能极显著地提高土壤抑制病原真菌寄生曲霉的能力,其抑制活性对比未添加的CK对照组的抑菌活性提高了37.67%。海带渣固体发酵微生物菌肥作为土壤添加剂提高了土壤抑制病原真菌活性的能力,可用于特异抑菌土壤的培育。     

Improved soil structure and citrus growth after inoculation with three arbuscular mycorrhizal fungi under drought stress

In a controlled potted experiment, citrus (Poncirus trifoliata) seedlings were inoculated with three species of arbuscular mycorrhizal (AM) fungi, Glomus mosseae, G. versiforme or G. diaphanum. Two soil-water levels (ample water, −0.10 MPa; drought stress, −0.44 MPa) were applied to the pots 4 months after transplantation. Eighty days after water treatments, the soils and the citrus seedlings were well colonized by the three AM fungi. Mycorrhizal fungus inoculation improved plant biomass regardless of soil-water status but decreased the concentrations of hot water-extractable and hydrolyzable carbohydrates of soils. Mycorrhizal soils exhibited higher Bradford-reactive soil protein concentrations than non-mycorrhizal soils. Mycorrhizas enhanced >2 mm, 1–2 mm and >0.25 mm water-stable aggregate fractions but reduced 0.25–0.5 mm water-stable aggregates. Peroxidase activity was higher in AM than in non-AM soils whether drought stressed or not, whereas catalase activity was lower in AM than non-AM soils. Drought stress and AM fungus inoculation did not affect polyphenol oxidase activity of soils. A positive correlation between the Bradford-reactive soil protein concentrations, soil hyphal length densities, and water-stable aggregates (only >2 mm, 1–2 mm and >0.25 mm) suggests beneficial effects of the AM symbiosis on soil structure. It concluded that AM fungus colonization enhanced plant growth under drought stress indirectly through affecting the soil moisture retention via glomalin's effect on soil water-stable aggregates, although direct mineral nutritional effects could not be excluded.     

Interactions of prion proteins with soil

Prions, are proteinaceous particles recognized as the agents of a class of neurodegenerative disorders, called transmissible spongiform encephalopathies (TSE), or prion diseases. Epidemiological data suggest that TSE-contaminated environments may serve as source of infectivity, but there is no information about adsorption of prions onto soil. We carried out experiments by mixing, healthy, or scrapie-infected hamster brains homogenates with three types of soil suspended in different buffers: (i) two saline buffers, i.e., phosphate buffer solution (PBS) and CaCl₂ solution; (ii) a mix of nondenaturing detergents, i.e., Triton X-100 and sodium deoxycholate (DOC) solution; (iii) a non-ionic detergent, i.e., lauryl maltoside; (iv) two anionic detergents, i.e., Sarkosyl or sodium dodecyl sulphate (SDS); and (v) a chaotropic agent, i.e., urea. The unbound prion proteins were detected in the supernatants (after centrifugation of soil suspension) by Western blotting. Results clearly demonstrate that both the no infectious (PrP^C) and infectious (PrP^Sc) forms are adsorbed by all soils. Only 1% sodium dodecylsulphate (SDS) partially impeded the association of PrP^C, but not that of PrP^Sc with the sandy loam soil. Agents with different interacting properties towards hydrophilic and/or hydrophobic domains failed to extract PrP^Sc from sediments of soil-brain homogenate mixtures. The strong interaction of PrP^Sc with soil favors the accumulation of prions in soils, especially if amended with prion-containing organic fertilizers and/or whenever TSE-affected animal carcasses, placenta, and excreta in general are buried or laid at the soil surface.     

Arbuscular mycorrhizal fungi reduce decomposition of woody plant litter while increasing soil aggregation

The decomposition of plant organic matter and the stability of soil aggregates are important components of soil carbon cycling, and the relationship between decomposition rate and arbuscular mycorrhizal fungi (AMF) has recently received considerable attention. The interaction of AMF with their associated microorganisms and the consequences for litter decomposition and soil aggregation still remain fairly unclear. In a laboratory pot experiment we simultaneously tested the single and combined effects of one AMF species (Rhizophagus irregularis) and a natural non-AMF microbial community on the decomposition of small wooden sticks and on soil aggregation. To disentangle effects of hyphae and roots we placed mesh bags as root exclusion compartments in the soil. The decomposition of the wooden sticks in this compartment was significantly reduced in the presence of AMF, but not with the non-AMF microbial community only, compared to the control, while aggregation was increased in all treatments compared to the control. We suggest that AMF directly (via localized nutrient removal or altered moisture conditions) or indirectly (by providing an alternative carbon source) inhibited the activity of decomposers, leading to different levels of plant litter degradation under our experimental settings. Reduced decomposition of woody litter in presence of AMF can be important for nutrient cycling in AMF-dominated forests and in the case of woody plants and perennials that develop lignified roots in grasslands.     

Distribution with depth of protozoa,bacteria and fungi in soil profiles from three Danish forest sites

The numbers and biomass of protozoa, bacteria and fungi were measured at various depths (1.5–122.5 cm) in the unsaturated zone of three contrasting pristine Danish forest site profiles: a dry beech (Fagus silvatica) forest on mor, a wet peaty spruce (Picea abies)/birch (Betula pubescens) forest and a dry spruce (P. abies) forest on mor. All sites were situated on a Weichel moraine. Except for a bacterial peak at 42.5 cm in the peat profile, the general tendency was a decrease in biomass with increasing depth for all groups examined. Protozoa decreased more rapidly with increasing depth than the other two groups of organisms examined. An evaluation of the bacterial–protozoan relationship by a simple mathematical model indicated that the subsurface protozoan populations are active and not accidental percolated cysts. The low protozoan numbers found in shallow subsurface sites contrast markedly with the results from contaminated sites where much larger protozoan populations have been reported even at considerable depths. Consequently, the results suggest that protozoa are good indicators of organic pollution in subsurface soils; however, more work involving the comparison of polluted and unpolluted soils is needed to confirm this suggestion.     

Interactions of assemblages of mycorrhizal fungi with two Florida wetland plants

Arbuscular mycorrhizal (AM) fungi are known to exist in wetlands, but little is known about their function in these environments. We conducted greenhouse experiments to study the effects of AM fungal assemblages—collected from different vegetation communities in a Florida wetland-under free-drained and flooded conditions, and at three phosphorus (P) levels on growth and P nutrition of Typha latifolia L. and Panicum hemitomon Schult. We also studied the effects of flooding on the spread of extraradical hyphae from P. hemitomon roots. For both plants no AM fungal assemblage had a consistent effect on plant growth and P nutrition. For T. latifolia, flooding nearly eliminated AM fungal colonization and, in the free-drained treatments, P amendment suppressed colonization. Furthermore, colonization by some mycorrhizal assemblages increased shoot- and root-P concentrations, but there were no significant plant growth responses. For P. hemitomon, the mycorrhizal association was suppressed by flooding and P amendment but, among the fungal assemblages, there were differences in root colonization. Mycorrhizal colonization improved some plant-growth and P-nutrition parameters at lower P levels relative to nonmycorrhizal controls, but generally conferred no benefit or was detrimental at higher P levels. Extraradical hyphae of most assemblages were restricted by flooding to 2.5 cm, though differences among AM fungal assemblages occurred with a maximum observed extension of 16.5 cm. We conclude that the impact of the mycorrhizal association on these wetland plants was a function of the complex interactions among the AM fungal assemblages, plant species, water condition, and P level. Future studies should focus on understanding the species composition of the assemblages, and potential adaptation to wetland conditions among these fungal species.     

Variable impacts of enchytraeid worms and ectomycorrhizal fungi on plant growth in raw humus soil treated with wood ash

An increasing amount of evidence shows the context dependent nature of various biotic interactions across terrestrial and aquatic ecosystems. We established a laboratory experiment to study whether the effects of Cognettia sphagnetorum (Enchytraeidae) and ectomycorrhizal fungi on Scots pine (Pinus sylvestris) seedling growth are influenced by wood ash application. Acidic coniferous forest soil was treated with wood ash at 5000 kg ha⁻¹ or left as ash-free control and inoculated with soil saprotrophic microbes and nematodes. The microcosms were destructively sampled 26 and 51 weeks after initiation of the experiment. We measured enchytraeid and pine seedling biomass, abundance of nematodes and leaching of NH₄⁺-N and NO₃⁻-N at both samplings, and root length and N concentration of pine needles at the end of the experiment. On average enchytraeids and mycorrhizal fungi enhanced pine biomass production in the ash-free control soils, however, their impact was most pronounced when these organisms were alone in the systems. In fact, mycorrhizas tended to have a negative impact on the seedlings in the presence of enchytraeids. Wood ash had a clear negative impact on enchytraeid populations. Wood ash decreased pine growth when enchytraeids and EM-fungi were alone in the systems, but when together they apparently compensated for the negative effects of wood ash on the seedlings. It is concluded that interactions between soil fauna, mycorrhizal fungi and plants are context dependent, thereby rendering predictions of the outcome of species interactions in soil food webs a demanding task.     

The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility

Beneficial plant-microbe interactions in the rhizosphere are primary determinants of plant health and soil fertility. Arbuscular mycorrhizas are the most important microbial symbioses for the majority of plants and, under conditions of P-limitation, influence plant community development, nutrient uptake, water relations and above-ground productivity. They also act as bioprotectants against pathogens and toxic stresses. This review discusses the mechanism by which these benefits are conferred through abiotic and biotic interactions in the rhizosphere. Attention is paid to the conservation of biodiversity in arbuscular mycorrhizal fungi (AMF). Examples are provided in which the ecology of AMF has been taken into account and has had an impact in landscape regeneration, horticulture, alleviation of desertification and in the bioremediation of contaminated soils. It is vital that soil scientists and agriculturalists pay due attention to the management of AMF in any schemes to increase, restore or maintain soil fertility.     

Recovery and dynamics of decomposing plant residue in soil: an evaluation of three fractionation methods

Our goals in this study were to track the incorporation of plant residue into soil organic matter (SOM) and test the effectiveness of different fractionation methods to evaluate this transformation. We incubated soil amended with ¹³C‐labelled barley (Hordeum vulgare L.) residue and used three fractionation methods based on size (> 250, 53–250, 5–53 and < 5 µm) and density (< 1.7 g cm^?3, i.e. light fraction (LF)) and determined its quantity and the rate of C loss or gain or both in these fractions as decomposition progressed. One method was based on size only, another involved density separation followed by size fractionation and a third separated organic matter fractions by size first and then by density. There were significant quantitative differences between the methods for the amount of residue in the fractions, but there was no effect of fractionation method on the rate of change in the residue that comprised the fractions. The density method did not appear to identify all of the most recently added (i.e. least decomposed) residue in the LF or that there was a redistribution of SOM among the fractions. The amount of residue C and the C:N ratio of the residue in the two smallest fractions increased early during the incubation (0–2 months), but subsequently decreased towards the end. The initially small C:N ratio in the clay fraction probably reflects the accumulation of microbial by‐products from the rapid decomposition of water‐soluble compounds. The subsequent increase and decrease in both residue C and C:N ratio reflects the balance of the accumulation of sorbed water‐soluble compounds and dense plant residue fragments and their mineralization over time. We conclude that clay is a sink for residue C (i.e. microbial metabolites) early during decomposition, and that there is a transfer among fractions and mineralization of residue C as decomposition proceeds. These findings indicate that the clay fraction contains a dynamic pool of C that can cycle within short time‐scales.     

Assessing biological activity of agricultural biostimulants: Bioassays for plant growth regulators in three soil additives

Abstract

Laboratory bioassays were used to investigate plant growth‐regulating effects of three different experimental soil additives, designated EXP95, W91, and Z96. A yeast growth test was used as a general assay of bioactivity, responses to soil additives were compared to those of known plant growth regulators [indoleacetic acid (IAA), gibberellic acid (GA₃), and kinetin]. A corn coleoptile elongation test was used to assay for auxin‐like activity and a dwarf pea bioassay was used for gibberellin‐like activity. The three soil additives were tested at five solution concentrations ranging from 1 to 10,000 ppm (by volume). All three soil additives stimulated yeast growth, depending on the concentration of the test solutions. However, all three soil additives inhibited plant growth in the two plant bioassays. Although this study clearly demonstrated that the three soil additives had significant biological activity at very low concentrations, there was little evidence for auxin‐like or gibberellin‐like activity.     

Response of soybean cultivars toward inoculation with three arbuscular mycorrhizal fungi and Bradyrhizobium japonicum in the alluvial soil

The aim of this study was to assess the comparative efficacy of three arbuscular mycorrhizal fungi (AMF) combined with cultivar specific Bradyrhizobium japonicum (CSBJ) in soybean under greenhouse conditions. Soybean seeds of four cultivars namely JS 335, JS 71-05, NRC 2 and NRC 7 were inoculated with three AM fungi (Glomus intraradices, Acaulospora tuberculata and Gigaspora gigantea) and CSBJ isolates, individually or in combination, and were grown in pots using autoclaved alluvial soil of a non-legume cultivated field of Ajmer (Rajasthan). Assessment of the data on nodulation, plant growth and seed yield revealed that amongst the single inoculations of three AMF, G. intraradices produced the largest increases in the parameters studied followed by A. tuberculata and G. gigantea indicating that plant acted selectively on AMF symbiosis. The dual inoculation with AMF + CSBJ further improved these parameters demonstrating synergism between the two microsymbionts. Among all the dual treatments, G. intraradices + B. japonicum brought about the largest increases in the studied characteristics particularly in seed weight per plant that increased up to 115.19%, which suggested that a strong selective synergistic relationship existed between AMF and B. japonicum. The cv. JS 335 exhibited maximum positive response towards inoculation. The variations in efficacy of different treatments with different soybean cultivars indicate the specificity of the inoculation response. These results provide a basis for selection of an appropriate combination of specific AMF and Bradyrhizobium which could further be utilized for verifying the symbiotic effectiveness and competitive ability of microsymbionts under field conditions of Ajmer region.     

Inoculation of seeds and soil with basidiospores of mycorrhizal fungi

It was demonstrated that basidiospores of the fungus Rhizopoyon luteolus, mycorrhizal for Pinus radiata, could be used successfully as seed inoculum after freeze-drying and storage for 3 months at 22°C, provided the inoculum level was increased 100-fold. Spore inoculum applied to seed could be held dry for at least 2 days before planting provided inoculum was increased 10-fold. On sowing freshly inoculated seed to sterile soil, 3 × 10³ basidiospores/seed were adequate for infection but maximum mycorrhizal infection occurred with 3 × 10⁴ spores/seed.A dose-response curve was obtained for mycorrhizal infection when basidiospores were applied to soil. As few as 100 spores/290 cm³ pot were sufficient for mycorrhizal infection although infection increased with greater spore dose to a maximum of 10⁵ spores/pot. Plant growth response was related to intensity of infection. It is suggested that the percentage germination of basidiospores in the rhizosphere may be considerably greater than those reported in studies with synthetic medium. A rhizosphere effect on germination of basidiospores was demonstrated and a method developed to facilitate studies of spore germination in the rhizosphere.