Autoclaving is at least as effective as gamma irradiation for biotic clearing and intentional microbial recolonization of soil.

Sterilization is commonly used to remove or reduce the biotic constraints of a soil to allow recolonization by soil-dwelling organisms, with autoclaving and gamma irradiation being the most frequently used approaches. Many studies have characterized sterilization impacts on soil physicochemical properties, with gamma irradiation often described as the preferred approach, despite the lower cost and higher scalability of autoclaving. However, few studies have compared how sterilization techniques impact soil recolonization by microorganisms. Here, we compared how two sterilization approaches (autoclaving; gamma irradiation) and soil washing impacted microbial recolonization of soil from a diverse soil inoculum. Sterilization method had little impact on microbial alpha diversity across recolonized soils. For sterile soil regrowth microcosms, species richness and diversity were significantly reduced by autoclaving relative to gamma irradiation, particularly for fungi. There was no impact of sterilization method on bacterial composition in recolonized soils and minimal impact on fungal composition (P = 0.05). Washing soils had a greater impact on microbial composition than sterilization method, and sterile soil regrowth had negligible impacts on microbial recolonization. These data suggest that sterilization method has no clear impact on microbial recolonization, at least across the soils tested, indicating that soil autoclaving is an appropriate and economical approach for biotically clearing soils.IMPORTANCESterilized soils represent soil-like environments that act as a medium to study microbial colonization dynamics in more "natural" settings relative to artificial culturing environments. Soil sterilization is often carried out by gamma irradiation or autoclaving, which both alter soil properties, but gamma irradiation is thought to be the gentler technique. Gamma irradiation can be cost prohibitive and does not scale well for larger experiments. We sought to examine how soil sterilization technique can impact microbial colonization, and additionally looked at the impact of soil washing which is believed to remove soil toxins that inhibit soil recolonization. We found that both gamma-irradiated and autoclaved soils showed similar colonization patterns when reintroducing microorganisms. Soil washing, relative to sterilization technique, had a greater impact on which microorganisms were able to recolonize the soil. When allowing sterilized soils to regrow (i.e., persisting microorganisms), gamma irradiation performed worse, suggesting that gamma irradiation does not biotically clear soils as well as autoclaving. These data suggest that both sterilization techniques are comparable, and that autoclaving may be more effective at biotically clearing soil.

Transplantation of soil from organic field confers disease suppressive ability to conducive soil.

Organic agriculture is a sustainable method of farming, and confers disease-suppressing abilities to disease-conducive soils via specialized soil microbiomes. This study aimed at transforming a disease-conducive soil from a conventional field into disease-suppressive soil by inoculating soil from an organic field previously established as "disease-suppressive". The effectiveness of the transformed soil was established with the model plant wheat (Triticum aestivum) grown under natural conditions, with regard to its potential in inhibiting fungal phytopathogens, Rhizoctonia solani and Fusarium oxysporum. The conducive soil inoculated with the disease-suppressive soil performed better than the control conducive soil in terms of reduced disease severity in plants, improved soil nutrient content, increased activity of hydrolytic enzymes, and increased abundance of structural and functional microbial markers. The study demonstrates the efficacy of the soil microbiome under long-term organic agriculture in transforming disease-conducive soil into disease-suppressive soils. Such practises are simple and easy to implement, and could greatly improve the sustainability and crop yield in developing countries.

Application of vermicompost and biochar suppresses Fusarium root rot of replanted American ginseng.

Soil sterilization integrated with agronomic measures is an effective method to reduce soilborne replant diseases. However, the effect of vermicompost or biochar application after soil sterilization on soilborne diseases is poorly understood. A pot experiment was conducted in American ginseng to investigate the effects of vermicompost (VF), biochar (BF), and a combination of vermicompost and biochar (VBF) applied after soil sterilization on the incidence of Fusarium root rot using natural recovery (F) as control. After one growing season, the disease index of root rot, the phenolic acids, and the microbial communities of American ginseng rhizosphere soil were analyzed. The disease index of VF, BF, and VBF decreased by 33.32%, 19.03%, and 80.96%, respectively, compared with F. The highest bacterial richness and diversity were observed in the rhizosphere soil of VBF. Besides, VF and VBF significantly increased the relative abundance of beneficial bacteria (Pseudomonas, Lysobacter, and Chryseolinea) in the rhizosphere soil. Higher concentrations of vanillin, one of the phenolic acids in the roots exudates, were recorded in the rhizosphere soils of BF and VBF. The vanillin concentration showed a significant negative correlation with the disease index. To conclude, vermicompost improved the beneficial bacteria of the rhizosphere soil, while biochar regulated the allelopathic effect of the phenolic acids. The study proposes a combined application of biochar and vermicompost to the rhizosphere soil to control Fusarium root rot of replanted American ginseng effectively. KEY POINTS: Vermicompost improves the relative abundance of rhizosphere beneficial bacteria. Biochar inhibits the degradation of phenolic acids by adsorption. The combination of vermicompost and biochar enhances the disease control effect.

Effect of sterilization on cadmium immobilization and bacterial community in alkaline soil remediated by mercapto-palygorskite.

Cadmium (Cd) pollution in alkaline soil in some areas of northern China has seriously threatened wheat production and human health. However, there are still few effective amendments for alkaline soil, and the mechanism of amendments with a good immobilization effect remains unclear. In this study, soil sterilization experiments were conducted to investigate the effects of soil microorganisms on the immobilization of a novel amendment-mercapto palygorskite (MPAL) in Cd-contaminated alkaline soils. The results showed that the mercapto on the MPAL surface was not affected by autoclaving. Compared with the control, the available Cd concentration in 0.025% MPAL treatments decreased by 18.80-29.23% after 1 d of aging and stabled after 10 d of aging. Importantly, the immobilization of MPAL on Cd in sterilized soil was significantly better than that in natural soil due to the changes in Cd fractions. Compared with MPAL-treated natural soil, exchangeable Cd fraction and carbonate-bound Cd fraction in MPAL-treated sterilized soil decreased by 20.79-27.09% and 20.05-26.45%, while Fe/Mn oxide-bound Cd fraction and organic matter-bound Cd fraction increased by 17.77-22.68% and 18.85-27.32%. Phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) analysis found that the potential functions of the microbial community in normal and sterilized soil were different significantly. Soil sterilization increased the soil pH and decreased the arylsulfatase activity, but did not change the soil zeta potential and available sulfur. The changes in Cd fractions in MPAL-treated sterilized soil may be related to the reduction in the bacterial community and the changes in function microbial, but not to the soil properties. In addition, MPAL application had little effects on the bacterial community, soil pH value, zeta potential, available sulfur, and arylsulfatase. These results showed that the immobilization of MPAL on Cd in alkaline soil was stable and effective, and was not affected by soil sterilization and soil microorganism reduction.

Biogeographic differences in plant-soil biota relationships contribute to the exotic range expansion of Verbascum thapsus.

Exotic plant species can evolve adaptations to environmental conditions in the exotic range. Furthermore, soil biota can foster exotic spread in the absence of negative soil pathogen-plant interactions or because of increased positive soil biota-plant feedbacks in the exotic range. Little is known, however, about the evolutionary dimension of plant-soil biota interactions when comparing native and introduced ranges.To assess the role of soil microbes for rapid evolution in plant invasion, we subjected Verbascum thapsus, a species native to Europe, to a reciprocal transplant experiment with soil and seed material originating from Germany (native) and New Zealand (exotic). Soil samples were treated with biocides to distinguish between effects of soil fungi and bacteria. Seedlings from each of five native and exotic populations were transplanted into soil biota communities originating from all populations and subjected to treatments of soil biota reduction: application of (a) fungicide, (b) biocide, (c) a combination of the two, and (d) control.For most of the investigated traits, native populations showed higher performance than exotic populations; there was no effect of soil biota origin. However, plants developed longer leaves and larger rosettes when treated with their respective home soil communities, indicating that native and exotic plant populations differed in their interaction with soil biota origin. The absence of fungi and bacteria resulted in a higher specific root length, suggesting that V. thapsus may compensate the absence of mutualistic microbes by increasing its root-soil surface contact. Synthesis. Introduced plants can evolve adaptations to soil biota in their new distribution range. This demonstrates the importance of biogeographic differences in plant-soil biota relationships and suggests that future studies addressing evolutionary divergence should account for differential effects of soil biota from the home and exotic range on native and exotic populations of successful plant invaders.

Arbuscular Mycorrhizal Fungi Can Compensate for the Loss of Indigenous Microbial Communities to Support the Growth of Liquorice (Glycyrrhiza uralensis Fisch.).

Soil microorganisms play important roles in nutrient mobilization and uptake of mineral nutrition in plants. Agricultural management, such as soil sterilization, can have adverse effects on plant growth because of the elimination of indigenous microorganisms. Arbuscular mycorrhizal (AM) fungi are one of the most important beneficial soil microorganisms for plant growth. However, whether AM fungi can compensate for the loss of indigenous microbial communities to support plant growth and metabolism is largely unknown. In this study, a pot experiment was conducted to investigate the effects of AM fungi on plant growth and secondary metabolism in sterilized and unsterilized soil. We used liquorice (Glycyrrhiza uralensis Fisch.), an important medicinal plant as the host, which was inoculated with the AM fungus Rhizophagus irregularis or not and grown in unsterilized or sterilized soil. Plant photosynthesis traits, plant growth and nutrition level, concentrations of the secondary metabolites, and expression levels of biosynthesis genes were determined. The results showed that soil sterilization decreased plant growth, photosynthesis, and glycyrrhizin and liquiritin accumulation, and moreover, downregulated the expression of related biosynthesis genes. Inoculation with R. irregularis in sterilized soil offset the loss of indigenous microbial communities, resulting in plant growth and glycyrrhizin and liquiritin concentrations similar to those of plants grown in unsterilized soil. Thus, AM fungi could compensate for the loss of indigenous microbial communities by soil sterilization to support plant growth and secondary metabolism.

Effects of soil in larch plantations on the growth of Picea koraiensis and P. crassifolia seedlings.

In addition to soil fertility decline, larch plantation had difficulty in natural regeneration, which challenges the healthy development and sustainable management. A greenhouse experiment was conducted with two kinds of shade-tolerant conifer species (Picea koraiensis and P. crassifolia) to examine the responses of seedling growth of two spruce species on sterilization treatment of larch plantation soils (except spruce biological characteristics), which would provide scientific basis for the transformation, regeneration and multi-storied forest cultivation of larch. The results showed that soil sterilization did not significantly affect the biomass of P. koraiensis and P. crassifolia seedlings. The biomass of P. koraiensis (75.6 and 72.2 mg, respectively) was significantly higher than that of P. crassifolia (55.6 and 60.0 mg, respectively) in both unsterilized and sterilized soils. The root diameter, cortical thickness, stele diameter and stele to root diameter ratio of the first-order roots of P. koraiensis were not affected by soil sterilization. In contrast, the root diameter, stele diameter and stele to root diameter ratio of the first-order roots except cortical thickness of P. crassifolia in sterilized soil (331.30 µm, 143.23 µm and 43.3%, respectively) were significantly higher than those in unsterilized soil (276.50 µm, 99.35 µm and 36.0%), showing a more positive response to sterilized soil. It indicated that P. koraiensis had better adaptability in larch plantation. Because the microbial community function was dominated by ectomycorrhiza which displayed some antagonisms for soil pathogen, seedlings of two spruce species could escape from soil pathogens accumulated in larch plantation and grow normally, with P. koraiensis having more advantages than the P. crassifolia.

Influence of soil γ-irradiation and spiking on sorption of p,p'-DDE and soil organic matter chemistry.

The fate of organic chemicals and their metabolites in soils is often investigated in model matrices having undergone various pre-treatment steps that may qualitatively or quantitatively interfere with the results. Presently, effects associated with soil sterilization by γ-irradiation and soil spiking using an organic solvent were studied in one freshly spiked soil (sterilization prior to contamination) and its field-contaminated (sterilization after contamination) counterpart for the model organic compound 1,1-Dichloro-2,2-bis(4-chlorophenyl)ethene (p,p'-DDE). Changes in the sorption and potential bioavailability of spiked and native p,p'-DDE were measured by supercritical fluid extraction (SFE), XAD-assisted extraction (XAD), and solid-phase microextraction (SPME) and linked to qualitative changes in soil organic matter (SOM) chemistry measured by diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy. Reduced sorption of p,p´-DDE detected with XAD and SPME was associated more clearly with spiking than with sterilization, but SFE showed a negligible impact. Spiking resulted in an increase of the DRIFT-derived hydrophobicity index, but irradiation did not. Spectral peak height ratio descriptors indicated increasing hydrophobicity and hydrophilicity in pristine soil following sterilization, and a greater reduction of hydrophobic over hydrophilic groups as a consequence of spiking. In parallel, reduced sorption of p,p´-DDE upon spiking was observed. Based on the present samples, γ-irradiation appears to alter soil sorptive properties to a lesser extent when compared to common laboratory processes such as spiking with organic solvents.

[Soil improvement promoted micro-ecology of farmlands for ginseng cultivation].

This study has revealed the change of the soil micro-ecology of farmlands, which used for ginseng cultivation, brought by comprehensive soil improvement. The process of soil improvement was described as follows： soil was sterilized using trichloronitromethane, and then perilla seeds were planted. After growing up, the perillas were turned over into the field and fermented, then organic fertilizer was added. Rotary tillages were carried out during the intervals. Physical and chemical properties of treated soil were measured, as well as microbial diversity, which was illustrated using 16s high through-put sequencing. The survival rate and growth data of ginseng seedlings were recorded. The analysis showed that after improvement, the soil organic matter content was increased and soil bulk density was decreased, compare to the controls, and the fertility in 0-20 cm of soil layer was increased in the treatment. Additionally, the soil microbial diversity was changed greatly. In detail, alpha diversity of the soil decreased after soil improvement while the beta diversity increased. In order to verify the achievement of soil improvement, ginseng seedlings were planted. Compared to the untreated land blocks, the survival rate of ginseng on improved blocks was increased up to 21.4%, and the ginseng physiological index were all better than the controls. Results showed that comprehensive soil improvements including soil sterilization, green manure planting and organic fertilization application effectively improved the soil micro-ecology in farmlands. This study will pave the way for the future standardization of ginseng cultivation on farmlands.

Seed and Root Endophytic Fungi in a Range Expanding and a Related Plant Species.

Climate change is accelerating the spread of plants and their associated species to new ranges. The differences in range shift capacity of the various types of species may disrupt long-term co-evolved relationships especially those belowground, however, this may be less so for seed-borne endophytic microbes. We collected seeds and soil of the range-expanding Centaurea stoebe and the congeneric Centaurea jacea from three populations growing in Slovenia (native range of both Centaurea species) and the Netherlands (expanded range of C. stoebe, native range of C. jacea). We isolated and identified endophytic fungi directly from seeds, as well as from roots of the plants grown in Slovenian, Dutch or sterilized soil to compare fungal endophyte composition. Furthermore, we investigated whether C. stoebe hosts a reduced community composition of endophytes in the expanded range due to release from plant-species specific fungi while endophyte communities in C. jacea in both ranges are similar. We cultivated 46 unique and phylogenetically diverse endophytes. A majority of the seed endophytes resembled potential pathogens, while most root endophytes were not likely to be pathogenic. Only one endophyte was found in both roots and seeds, but was isolated from different plant species. Unexpectedly, seed endophyte diversity of southern C. stoebe populations was lower than of populations from the north, while the seed endophyte community composition of northern C. stoebe populations was significantly different southern C. stoebe as well as northern and southern C. jacea populations. Root endophyte diversity was considerably lower in C. stoebe than in C. jacea independent of plant and soil origin, but this difference disappeared when plants were grown in sterile soils. We conclude that the community composition of fungal endophytes not only differs between related plant species but also between populations of plants that expand their range compared to their native habitat. Our results suggest that fungal endophytes of two Centaurea species are not able to systemically infect plants. We highlight that endophytes remain poorly studied and further work should investigate the functional importance of endophytes.

Soil improvement promoted micro-ecology of farmlands for ginseng cultivation / 中国中药杂志

This study has revealed the change of the soil micro-ecology of farmlands, which used for ginseng cultivation, brought by comprehensive soil improvement. The process of soil improvement was described as follows： soil was sterilized using trichloronitromethane, and then perilla seeds were planted. After growing up, the perillas were turned over into the field and fermented, then organic fertilizer was added. Rotary tillages were carried out during the intervals. Physical and chemical properties of treated soil were measured, as well as microbial diversity, which was illustrated using 16s high through-put sequencing. The survival rate and growth data of ginseng seedlings were recorded. The analysis showed that after improvement, the soil organic matter content was increased and soil bulk density was decreased, compare to the controls, and the fertility in 0-20 cm of soil layer was increased in the treatment. Additionally, the soil microbial diversity was changed greatly. In detail, alpha diversity of the soil decreased after soil improvement while the beta diversity increased. In order to verify the achievement of soil improvement, ginseng seedlings were planted. Compared to the untreated land blocks, the survival rate of ginseng on improved blocks was increased up to 21.4%, and the ginseng physiological index were all better than the controls. Results showed that comprehensive soil improvements including soil sterilization, green manure planting and organic fertilization application effectively improved the soil micro-ecology in farmlands. This study will pave the way for the future standardization of ginseng cultivation on farmlands.

Effects of γ-irradiation of original and organic matter-amended soils on the sorption of triclosan and diuron from aqueous solutions.

Soil γ-irradiation is a well-known method of inhibiting microbial activity in studies of the soil sorption of organic compounds. However, few studies have addressed the possible effect of γ-irradiation on the sorptive ability of soils enriched with different types of organic matter (OM). The objective of this study was to probe the effect of soil γ-irradiation on organic compound-soil interactions in two different situations representing adding OM to soils through land disposal of (a) OM-rich sewage sludge-originating biosolids and (b) olive mill wastewater (OMW). Both situations describe frequent environmental and agricultural scenarios. Comparisons of aqueous sorption on cobalt-60 γ-irradiated and non-irradiated soil sorbents were carried out for (a) triclosan (in a series of three soils and their lab-incubated mixtures with three different types of biosolids), and (b) the pesticide diuron (in two different untreated and OMW-affected soils). In each case, sodium azide was used as a biocide. Soil γ-irradiation affected the sorption of organic compounds by a factor generally not exceeding 2-3. Specifically, for triclosan, the sorbed concentration ratio between irradiated and non-irradiated soils when averaged over all the soil samples was 0.94. No significant effects of γ-irradiation on soil organic carbon or total nitrogen contents were observed. The effect of γ-irradiation on a soil sorbent may be less important when a rough estimate of a soil sorption coefficient of an organic compound is needed. However, it may need to be taken into account in mechanistic sorption studies, specifically, when the shape of sorption isotherms is of interest.

Impact of an invasive nitrogen-fixing tree on arbuscular mycorrhizal fungi and the development of native species.

Arbuscular mycorrhizal fungi (AMF) are obligate soil biotrophs that establish intimate relationships with 80 % of terrestrial plant families. Arbuscular mycorrhizal fungi obtain carbon from host plants and contribute to the acquisition of mineral nutrients, mainly phosphorus. The presence of invasive plants has been identified as a soil disturbance factor, often conditioning the structure and function of soil microorganisms. Despite the investigation of many aspects related to the invasion ofAcacia dealbata, the effect produced on the structure of AMF communities has never been assessed. We hypothesize thatA. dealbatamodifies the structure of AMF community, influencing the establishment and growth of plants that are dependent on these mutualisms. To validate our hypothesis, we carried out denaturing gradient gel electrophoresis (DGGE) analysis and also grew plants ofPlantago lanceolatain pots using roots of native shrublands or fromA. dealbata, as inoculum of AMF. Cluster analyses from DGGE indicated an alteration in the structure of AMF communities in invaded soils. After 15 weeks, we found that plants grown in pots containing native roots presented higher stem and root growth and also produced higher biomass in comparison with plants grown withA. dealbatainoculum. Furthermore, plants that presented the highest biomass and growth exhibited the maximum mycorrhizal colonization and phosphorus content. Moreover, fluorescence measurements indicated that plants grown withA. dealbatainoculum even presented higher photosynthetic damage. Our results indicate that the presence of the invaderA. dealbatamodify the composition of the arbuscular fungal community, conditioning the establishment of native plants.

Atrazine degradation by fungal co-culture enzyme extracts under different soil conditions.

This investigation was undertaken to determine the atrazine degradation by fungal enzyme extracts (FEEs) in a clay-loam soil microcosm contaminated at field application rate (5 µg g(-1)) and to study the influence of different soil microcosm conditions, including the effect of soil sterilization, water holding capacity, soil pH and type of FEEs used in atrazine degradation through a 2(4) factorial experimental design. The Trametes maxima-Paecilomyces carneus co-culture extract contained more laccase activity and hydrogen peroxide (H2O2) content (laccase = 18956.0 U mg protein(-1), H2O2 = 6.2 mg L(-1)) than the T. maxima monoculture extract (laccase = 12866.7 U mg protein(-1), H2O2 = 4.0 mg L(-1)). Both extracts were able to degrade atrazine at 100%; however, the T. maxima monoculture extract (0.32 h) achieved a lower half-degradation time than its co-culture with P. carneus (1.2 h). The FEE type (p = 0.03) and soil pH (p = 0.01) significantly affected atrazine degradation. The best degradation rate was achieved by the T. maxima monoculture extract in an acid soil (pH = 4.86). This study demonstrated that both the monoculture extracts of the native strain T. maxima and its co-culture with P. carneus can efficiently and quickly degrade atrazine in clay-loam soils.

[Effects of improving measures on soil micro-ecology and survival rate of ginseng in farmlands].

This study analysed the changes of the soil micro-ecology in the process of soil sterilization, green manure returning farmlands and fertilization. The methods of soil improvement was initially built which ensured the successful proceed of ginseng cultivation in farmlands. The soil chemical properties were analysed, the diversity and composition of bacterial community after soil sterilization, sterilization+green manure returning farmlands and sterilization+green manure returning farmlands+fertilization. The results exhibited that measures of soil improvement decreased the pH, increased soil fertility, declined the diversity of bacterial community and changed the composition of soil bacterial community. The comprehensive measures of sterilization+green manure returning farmlands+fertilization decreased the ginseng death rate compared to the control. Our data indicated that soil micro-ecological environment was changed by the treatments of soil sterilization, sterilization+green manure returning farmlands and sterilization+green manure returning farmlands+fertilization, and comprehensive measures improved the survival rate and guaranteed the development of ginseng cultivation in farmlands.

Effects of improving measures on soil micro-ecology and survival rate of ginseng in farmlands / 中国中药杂志

This study analysed the changes of the soil micro-ecology in the process of soil sterilization, green manure returning farmlands and fertilization. The methods of soil improvement was initially built which ensured the successful proceed of ginseng cultivation in farmlands. The soil chemical properties were analysed, the diversity and composition of bacterial community after soil sterilization, sterilization+green manure returning farmlands and sterilization+green manure returning farmlands+fertilization. The results exhibited that measures of soil improvement decreased the pH, increased soil fertility, declined the diversity of bacterial community and changed the composition of soil bacterial community. The comprehensive measures of sterilization+green manure returning farmlands+fertilization decreased the ginseng death rate compared to the control. Our data indicated that soil micro-ecological environment was changed by the treatments of soil sterilization, sterilization+green manure returning farmlands and sterilization+green manure returning farmlands+fertilization, and comprehensive measures improved the survival rate and guaranteed the development of ginseng cultivation in farmlands.

Quantifying the effects of soil temperature, moisture and sterilization on elemental mercury formation in boreal soils.

Soils are a source of elemental mercury (Hg(0)) to the atmosphere, however the effects of soil temperature and moisture on Hg(0) formation is not well defined. This research quantifies the effect of varying soil temperature (278-303 K), moisture (15-80% water filled pore space (WFPS)) and sterilization on the kinetics of Hg(0) formation in forested soils of Nova Scotia, Canada. Both, the logarithm of cumulative mass of Hg(0) formed in soils and the reduction rate constants (k values) increased with temperature and moisture respectively. Sterilizing soils significantly (p < 0.05, n = 10) decreased the percent of total Hg reduced to Hg(0). We describe the fundamentals of Hg(0) formation in soils and our results highlight two key processes: (i) a fast abiotic process that peaks at 45% WFPS and depletes a small pool of Hg(0) and; (ii) a slower, rate limiting biotic process that generates a large pool of reducible Hg(II).

Interactions with soil biota shift from negative to positive when a tree species is moved outside its native range.

Studies evaluating plant-soil biota interactions in both native and introduced plant ranges are rare, and thus far have lacked robust experimental designs to account for several potential confounding factors. Here, we investigated the effects of soil biota on growth of Pinus contorta, which has been introduced from Canada to Sweden. Using Swedish and Canadian soils, we conducted two glasshouse experiments. The first experiment utilized unsterilized soil from each country, with a full-factorial cross of soil origin, tree provenance, and fertilizer addition. The second experiment utilized gamma-irradiated sterile soil from each country, with a full-factorial cross of soil origin, soil biota inoculation treatments, tree provenance, and fertilizer addition. The first experiment showed higher seedling growth on Swedish soil relative to Canadian soil. The second experiment showed this effect was due to differences in soil biotic communities between the two countries, and occurred independently of all other experimental factors. Our results provide strong evidence that plant interactions with soil biota can shift from negative to positive following introduction to a new region, and are relevant for understanding the success of some exotic forest plantations, and invasive and range-expanding native species.

Biotic soil factors affecting the growth and development of Ammophila arenaria.

To study the origin of replant disease of Ammophila arenaria (L.) Link the growth and development in sand originating from the rhizosphere of a natural Ammophila vegetation was compared with the growth in sand from the sea-floor. In a greenhouse experiment, the growth of Ammophila seedlings in rhizosphere sand, when compared with that in sea sand, was significantly reduced. As sterilization by means of gamma-irradiation increased the biomass production of Ammophila seedlings significantly, it was concluded that the rhizosphere sand contained biotic factors that were harmful to Ammophila. In rhizosphere sand the roots of Ammophila were brown and poorly developed, and the specific uptake rates of N, P and K were reduced. The shoot weight proportion of the total plant dry matter was hardly influenced. In an outdoor experiment with Ammophila seedlings and cuttings, using both sands, the mortality was high and the plants were feeble in rhizosphere sand whereas plants in sea sand grew vigorously. It seems plausible that the plants in rhizophere sand were dessicated because the root system was shallow and badly developed. In the greenhouse experiments, Ammophila cuttings were less sensitive to the inhibiting factors in the rhizosphere than seedlings. This was confirmed in the outdoor experiment. Calammophila baltica (Fluegge ex Schrader) Brand, however, was hardly affected by the harmful biotic factors in the greenhouse. These results are discussed with reference to the ecology of Ammophila. It is assumed that the catching of fresh windblown sand provides Ammophila with a way to escape from harmful biotic soil factors, and it was concluded that degeneration of Ammophila is caused mainly by self-intolerance due to these biotic soil factors.

Effects of microwave treatment of soil on growth of birch (Betula pendula) seedlings and infection of them by ectomycorrhizal fungi.

Seedlings of birch (Betula pendula Roth.) were grown for 12 weeks in small volumes of field soils heated in a microwave (MW) oven for different times and then either supplemented or not with inocula of ectomycorrhizal fungi and, in one case, with fresh agricultural soil. Growth of roots and shoots was assessed, and the incidence of different mycorrhizal types, distinguished by colour and morphology, was recorded. Bacterial and fungal populations were determined by dilution plating after MW treatment of soils. In four soils, shoot and root growth were markedly enhanced by MW treatments that raised the soil temperature to 60°C or more, irrespective of the exposure times (20-50 s) needed for this. Shoot dry weights were increased 1·9- to 2·9-fold by MW treatment of two soils from young (11-16 years) birch stands, compared with 8·0- and 11·4-fold for two soils from old birchwoods. But MW treatment of a colliery spoil reduced shoot growth 5-fold. The growth responses were not related to the size of seedlings in untreated soils. The responses coincided with substantial reductions in the total populations of bacteria and fungi. Hebeloma subsaponaceum Karsten developed similar numbers of mycorrhizas from mycelial inoculum added to untreated and MW treated soil. But Lactarius pubescent (Fr. ex Krombh.) Fr. developed significantly more mycorrhizas from inoculum added to MW-treated soil than to untreated soil. This difference between the fungi accords with their classification as, respectively, early stage and late stage in reported mycorrhizal successions on birch. Some mycorrhizal types developed on seedlings from resident inoculum in soil. Their incidence was reduced by different amounts following MW treatment, suggesting differences in heat-tolerance of their propagules. Hebeloma sacchariolens Quel., which forms sclerotium-like bodies and which probably developed from resident inoculum, was unaffected by MW treatment of soil. Some mycorrhizal fungi, notably Thelephora terrestris (Ehrh.) Fr., developed in place of others in soils treated for long times (80-180 s), and they probably developed from airborne spores.