Modifications of degradation-resistant soil organic matter by soil saprobic microfungi

Modifications of humic (HA) and fulvic (FA) acids in their solutions and in sterile soil by microfungal species and two well-known HA degraders were studied by measurement of total oxidizable carbon (OC), absorbances, enzyme activities and CO₂ release. The effect of glucose on FA and HA, and also minerals on FA utilization was also observed. Microfungi affected HA more than FA. Common microfungal species decolorized HA and decreased their molecular size (evaluated in terms of A₄/A₆ ratio). Some of them decreased aromaticity of HA and FA as the only carbon sources. They did not affect OC, although released CO₂ from FA. Under higher availability of mineral nutrients, the FA aromaticity increased and FA decolorization decreased. The molecular size of HA decreased in the presence of glucose. In the FA medium complemented by minerals, the known basidiomycete HA degrader, Trametes versicolor, decreased the amount of aromatic compounds in contrast to microfungal species Alternaria alternata, Clonostachys rosea, Exophiala cf. salmonis, Fusarium coeruleum, F. redolens, Penicillium canescens, Phoma sp. and another basidiomycete Phanerochaete chrysosporium. No microfungal species exhibited lignin peroxidase activity. On the other hand, activities of manganese peroxidase (MnP) were recorded for all species incubated in FA. Carbon dioxide produced from soil inoculated by microfungi negatively correlated with the decolorization, aromaticity and OC of/in FA reisolated from the soil. The results support the hypothesis that soil microfungi can attack both HA and FA and can represent an important factor in their transformations in arable soils. The enzyme involved in FA modifications is probably fungal MnP. We enriched a group of known HA and FA degraders and showed some abilities of a few frequent soil microfungal species. This can be one of the first but important step towards learning the functioning of carbon release from the big reservoir represented by humic substances in arable soils.     

Diversity of soil basidiomycete communities associated with Quercus suber L. in Portuguese montados

The montado is an agro-silvo pastoral system characterized by an open oak formation combined with shrubs or a rotation of cultures/pastures/fallow. A key, but frequently overlooked, component of these systems is the fungal community associated with the dominant oaks. We present a study of the basidiomycete community in montado areas of Quercus suber L. from Central-Western Portugal, based on fruiting body assessment and PCR-DGGE of soil mycelium. Soil available phosphorus and organic matter content were also determined. The most frequent fruiting body found in the cork oak montados belonged to Lactarius, Clitocybe, Russula and Cortinarius species. Lactarius chrysorrheus was the most widely distributed species. Soil management practices cause a temporal increase in soil available phosphorus, and harrowing and fertilizer application also result in an increase in soil organic matter content. Mechanical clearing with recent soil disturbance had a negative effect on the richness and diversity of the basidiomycete community assessed using fruiting body. The molecular analysis of the basidiomycete community separated the sites with recent shrub-removal from the others sites. Our results show that soil management techniques that avoid disturbance of the top-soil layers are the best way to preserve the structure and diversity of the soil fungal community in the montado.     

Nitrogen and water addition regulate soil fungal diversity and co-occurrence networks

Purpose

A field experiment was conducted to assess the role of nitrogen (N) and water addition in shaping soil fungal communities and co-occurrence networks in temperate grassland, northern China.

Materials and methods

We measured soil fungal and plant community compositions, and also soil properties including available N, phosphorus, potassium concentrations, soil pH, and soil moisture. Soil fungal co-occurrence networks were constructed using a random matrix theory–based network inference approach.

Results and discussion

Plant species richness was decreased by N addition but increased by water addition, whereas fungal richness was decreased by N addition. The fungal community composition was significantly changed by both N addition and water addition. Soil fungal α diversity and β diversity were explained by a combination of variations in plant species richness and plant functional composition, and also by changes in soil pH via the soil acidification pathway induced by N and water addition. The fungal co-occurrence networks were more complex and clustered under water addition than that in ambient precipitation.

Conclusions

Our results suggested that plant functional composition, plant species richness, and soil acidification should be incorporated into ecosystem models for predicting soil fungal communities under future climate changes in terrestrial ecosystems.

     

Calcium oxalate accumulation and soil weathering in mats of the hypogeous fungus Hysterangium crassum

Fungal mats of Hysterangium crassum (Tul. and Tul.) Fischer occupied a mean of 9.6% of the upper 10 cm of soil developed under a 40–65 yr old stand of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in Oregon. This hypogeous basidiomycete exudes large amounts of oxalic acid, some of which precipitates with Ca in microscopic crystals of calcium oxalate, resulting in a mean CaC₂O₄ content of 82g m^?2 for the entire soil. Soil oxalate concentration was significantly greater within fungal mats (P < 0.01) and soil pH was significantly lower (P < 0.01) than in soil adjacent to mats. The quantity of Ca present as CaC₂O₄ is 0.5 the amount of exchangeable Ca in the soil and exceeds the mass of Ca lost annually in runoff. Scanning electron micrographs show intense chemical weathering, attributable to oxalate attack, in the immediate vicinity of hyphae. X-ray diffraction patterns of clay indicate bulk weathering is more intense within the fungal mats than in adjacent uncolonized soil.     

Species richness of ectomycorrhizal hyphal necromass increases soil CO2 efflux under laboratory conditions

The ectomycorrhizal mycelium is a large component of boreal and temperate forest soil microbial biomass and the resulting necromass is likely to be an important source of nutrients for saprotrophic microorganisms. Here we test the effects of species richness of ectomycorrhizal mycelial biomass on short-term CO₂ efflux by amending forest soil with necromass from 8 fungal species added separately and in mixtures of 2, 4 and 8 species. All additions of necromass rapidly increased soil CO₂ efflux compared to unamended controls but CO₂ efflux increased significantly with species richness. Efflux of CO₂ did not correlate with the carbon (C) or nitrogen (N) contents or the C:N ratio of the added necromass. The study demonstrates that species diversity of dead ectomycorrhizal fungal hyphae can have important consequences for soil CO₂ efflux, and suggests decomposition of hyphae is regulated by specific constituents of the nutrient pools in the necromass rather than the total quantities added.     

Short term response of soil microinvertebrates to application of entomopathogenic nematode-infected insects in two tillage systems

Entomopathogenic nematodes (EPN) in the families Steinernematidae and Heterorhabditidae occur naturally in the soil and are produced commercially for the management of soil-dwelling pests. EPN infected cadavers also represent a potential resource for other soil organisms. We examined the short-term (24 h) response in abundance, diversity and community composition of localized soil microinvertebrates to the presence of EPN-infected insect cadavers in no-till and conventional-till maize. We hypothesized that the response of soil microinvertebrates to the EPN-infected cadavers would vary by soil management practices and EPN species. We expected to observe greater numbers and diversity of arthropods in no-till compared with conventional-till soil, and in the vicinity of steinernematid-infected insect cadavers compare to what would be found in the vicinity of heterorhabditid-infected cadavers. 45,606 invertebrates were collected and identified to 134 morphotaxa. Tillage regime accounted for the majority of the variation observed (84.6%), whereas nematode treatment accounted for 7.5%. Taxonomic richness of invertebrates was greater in treatments with Steinernema carpocapsae-infected cadavers than with Heterorhabditis bacteriophora-infected cadavers. Some invertebrates increased in abundance where EPN were applied whereas others decreased, regardless of tillage practice. Applications of Galleria cadavers infected with steinernematids elicited positive responses from two mite taxa, Galumnidae and Scheloribates spp., while negative responses were elicited from three mite (Histiostomatidae, Scheloribates spp., Eupodes spp.), taxa and Entomobryidae (Collembola) in response to applications of Heterorhabditis-infected cadavers.     

Arbuscular mycorrhizal fungal community structure and diversity in response to 3-year conservation tillage management in a sandy loam soil in North China

Purpose

Modern agricultural science has greatly reduced the use of tillage. Monitoring conservation versus conventional tillage effects on soil microbes could improve our understanding of soil biochemical processes and thus help us to develop sound management strategies. The objective of this study was to investigate the effects of conservation tillage on the spore community structure and the diversity of soil arbuscular mycorrhizal (AM) fungi and to find out the main factors that influence these parameters.

Materials and methods

A long-term field experiment established in a sandy loam soil in Northern China has received continuous tillage management treatments for 3?years, including conventional tillage (CT), no tillage (NT), and alternating tillage (AT). Topsoil samples (0?C15?cm) from four individual plots per treatment were collected for the analysis of chemical properties and fungal parameters. AM fungal spores were isolated using the wet-sieving method and identified to species level based on morphology by light microscopy. The community structure and the diversity of AM fungi were evaluated using the following parameters: spore density, relative abundance, species richness, Shannon?CWiener index (H??), evenness (E), and Simpson's index (D). Jaccard index (J) of similarity was calculated to compare AM fungal species composition under different treatments.

Results and discussion

Twenty-eight species of AM fungi within four genera, Glomus, Acaulospora, Scutellospora, and Entrophospora, were recovered from the 12 plots within the three tillage management treatments. Higher spore density, species richness, and species diversity (H??, E, and D) of AM fungi were observed in the two conservation tillage treatments, and the redundancy analysis showed that the species richness significantly correlated to soil organic carbon content (P?<?0.05). The positive effects of NT and AT on the species richness were very close, while the AT had relatively greater beneficial impacts on the spore density and the evenness of AM fungi compared to the NT. The lowest Jaccard index (J) of similarity in species composition was also observed between the AT and CT treatments.

Conclusions

Soil organic carbon, the spore density, and species richness and diversity of AM fungi increased in the two conservation tillage treatments. The species richness of AM fungi significantly correlated to soil organic carbon content (P?<?0.05). Compared with the CT treatment, the AT rather than the NT significantly increased the spore density and the evenness of AM fungi (P?<?0.05). Thus, alternating tillage practice may be more beneficial to agroecosystem in this region.     

种植方式对玉米不同生长期土壤微生物群落功能多样性的影响

为探讨玉米不同种植方式下土壤微生物群落功能多样性的差异,进行田间定点试验,采用Biolog方法分别研究了4行轮作、4行连作、8行轮作和8行连作的种植方式对玉米种植前、拔节期、抽穗期和收获期土壤微生物功能多样性的影响。结果表明:4种种植方式的土壤微生物均在种植前代谢活性最弱、功能多样性最低,在玉米抽穗期土壤微生物代谢活性最强,功能多样性最高。在种植玉米前,轮作的土壤微生物代谢活性和功能多样性高于连作,8行轮作和4行轮作土壤微生物的物种多样性指数分别比相应的连作高22.93%和11.42%;4行轮作的土壤微生物物种多样性指数比8行轮作低3.17%,而4行连作比8行连作高6.83%。在玉米拔节期、抽穗期及收获期连作土壤微生物功能多样性略高于轮作,且有4行连作大于8行连作的趋势,但差异均未达显著水平。种植前,4种种植模式的土壤微生物对6大类碳源的利用程度整体上都较低,降解碳水化合物类、羧酸类和聚合物类碳源的微生物是种植方式影响的主要土壤微生物类群;随着玉米的生长,土壤微生物对6大类碳源的利用都逐渐增强,玉米拔节期、抽穗期和收获期之间土壤微生物特征碳源没有较大差异,4种种植方式的土壤微生物对聚合物类碳源利用程度差异都不显著。PLS-EDA分析结果表明种植方式对土壤微生物产生较大影响,种植前8行轮作和4行连作的土壤微生物碳源利用模式具有相似性;种植玉米后4种种植方式的土壤微生物对碳源的利用模式存在较大差异,其中4行连作的土壤微生物在玉米拔节期和收获期对碳源的利用模式与其他3种种植方式差异最大。试验说明作物长期连作栽培会影响土壤微生物群落功能,降低土壤微生物物种多样性,引起土壤微生物群落结构与功能的失调。     

棉花连作对北疆土壤酶活性、致病菌及拮抗菌多样性的影响

本文通过测定土壤酶活性与微生物PCR-DGGE指纹图谱研究了北疆棉区5年棉花连作(CtN5)、10年棉花连作(CtN10)及15年棉花连作(CtN15)对土壤过氧化氢酶、蔗糖酶、芳基硫酸酯酶、脱氢酶和蛋白酶酶活性的影响,分析了土壤细菌、真菌、镰刀菌和枯草芽孢杆菌群落结构多样性对北疆棉田长期连作的响应。结果表明:过氧化氢酶、蔗糖酶、脱氢酶活性随棉花连作年限延长而下降。CtN15处理的过氧化氢酶、蔗糖酶和脱氢酶活性分别比CtN10处理下降15.0%、6.4%和12.0%,比CtN5处理下降16.8%、58.6%和49.5%(P0.05);芳基硫酸酯酶与蛋白酶活性随连作年限的增加呈先下降后升高的特点。土壤细菌、真菌多样性指数随连作年限的增加明显下降。CtN15的细菌条带数比CtN10下降7.41%,Ct N10比CtN5降低1.72%。CtN15真菌条带数和Shannon-Wiener多样性指数分别为78和3.22,比CtN5处理低17.02%和5.29%。土壤镰刀菌和枯草芽孢杆菌的条带数、多样性指数均表现为先下降后升高。CtN15枯草芽孢杆菌Shannon-Wiener和Simpson指数分别比CtN10处理高54.8%和14.5%。北疆长期连作棉田的土壤酶活性和土壤微生物群落多样性总体呈下降趋势,长期连作对棉田土壤生物性状有明显负面影响。     

Effects of inoculation with ectomycorrhizal fungi on microbial biomass and bacterial functional diversity in the rhizosphere of Pinus tabulaeformis seedlings

This study investigated the effects of inoculation with three individual ectomycorrhizal (ECM) fungal species on soil microbial biomass carbon and indigenous bacterial community functional diversity in the rhizosphere of Chinese pine (Pinus tabulaeformis Carr.) seedlings under field experimental conditions. The results showed that ECM fungal inoculation significantly increased the ectomycorrhizal colonization compared with non-inoculated seedlings. ECM fungal inoculations have higher soil microbial biomass carbon than that of control, ranging from 49.6 μg C g^?1 dry soil in control to 134.02 μg C g^?1 dry soil in treatment inoculated with Boletus luridus Schaeff ex Fr. Multivariate analyses (PCA) of BIOLOG data revealed that the application of ECM fungi significantly influenced bacterial functional diversity in the rhizosphere of P. tabulaeformis seedlings. The highest average well-color development (AWCD) and functional diversity indices were also observed in treatment inoculated with B. luridus. A wider range of sole carbon sources were utilized by the bacterial community in the rhizosphere of inoculated seedlings. The data gathered from this study provides important information for utilization of ECM fungi in forest restoration project in the Northwestern China. The present study will also significantly broaden our understanding of practical importance in the application of ECM fungal inoculum to promote soil microbial community diversity of soil.     

The effect of electrokinetics on soil microbial communities

There is growing interest in the potential of applying an electric field to soil to move and stimulate the degradation of contaminants, but we know little of the impact of this approach on exposed microbial communities. The effect of electrokinetics (3.14 A m⁻²) on soil bacterial and fungal communities was studied using soil cartridge microcosms (13 cm×5.4 cm×5.9 cm). After 27 days of electrokinetics, a zone of low pH (<4) was detected close to the anode. Soil exposed to electrokinetics and immediately adjacent to the anode demonstrated an increase in carbon substrate utilisation potential (≤290%) and microbial respiration rates. The diversity and structure of the bacterial community showed little response to electrokinetics, with the exception of soil close to the anode. Here, an increase in the percentage of Gram-positive species isolated was identified, most notably of Bacillus megaterium. Overall, the only detectable response of the microbial community was observed in soil immediately adjacent to the anode. The results of this study provide evidence that the application of electrokinetics has no serious negative effect on ‘soil microbial health’, thus endorsing its validity as a viable soil remediation technology.     

Three decades of soil microbial biomass studies in Brazilian ecosystems: Lessons learned about soil quality and indications for improving sustainability

Soil microbial biomass plays important roles in nutrient cycling, plant-pathogen suppression, decomposition of residues and degradation of pollutants; therefore, it is often regarded as a good indicator of soil quality. We reviewed more than a hundred studies in which microbial biomass-C (MB-C), microbial quotient (MB-C/TSOC, total soil organic carbon) and metabolic quotient (qCO₂) were evaluated with the objective of understanding MB-C responses to various soil-management practices in Brazilian ecosystems. These practices included tillage systems, crop rotations, pastures, organic farming, inputs of industrial residues and urban sewage sludge, applications of agrochemicals and burning. With a meta-analysis of 233 data points, we confirmed the benefits of no-tillage in preserving MB-C and reducing qCO₂ in comparison to conventional tillage. A large number of studies described increases in MB-C and MB-C/TSOC due to permanent organic farming, also benefits from crop rotations particularly with several species involved, whereas application of agrochemicals and burning severely disturbed soil microbial communities. The MB-C decreased in overgrazed pastures, but increased in pastures rotated with well-managed crops. Responses of MB-C, MB-C/TSOC and qCO₂ to amendment with organic industrial residues varied with residue type, dose applied and soil texture. In conclusion, MB-C and related parameters were, indeed, useful indicators of soil quality in various Brazilian ecosystems. However, direct relationships between MB-C and nutrient-cycling dynamics, microbial diversity and functionality are still unclear. Further studies are needed to develop strategies to maximize beneficial effects of microbial communities on soil fertility and crop productivity.     

Fungal abundance and diversity in earthworm casts and in uningested soil

Earthworm casts and adjacent uningested soil from 30 different locations were compared to determine the abundance and diversity of fungal species. The casts contained larger fungal populations (g^-1 dry soil weight) and numbers of fungal species than the soil. Variations in these parameters between casts and soil were statistically significant (P=0.05). Fungal populations and the number of fungal species in casts and soil also varied significantly (P=0.05) between samples from different locations. A total of 27 fungal species were recorded from the casts and soil. Indices of dominance (0.084 casts; 0.14 soil) and general diversity (2.53 casts; 2.02 soil) demonstrated that the casts displayed more diverse fungal flora than the soil. The diversity of fungal species increased in earthworm casts after passing through the earthworm gut.     

不同土壤类型下AM 真菌分布多样性及与土壤因子的关系

以禾本科植物群落为研究对象, 研究了宁夏六盘山林地、银川农耕地、暖泉农耕地、固原农耕地、盐池沙地、灵武沙地6 个采样地点5 种土壤类型(黑垆土、灌淤土、黄绵土、灰钙土、风沙土)下AM 真菌物种多样性及其与土壤因子的关系。结果表明: 5 种土壤类型采样点的植被根际土壤中共鉴定出5 属48 种AM真菌, 其中, 无梗囊霉属(Acaulospora)1 种, 巨孢囊霉属(Gigaspora)3 种, 球囊霉属(Glomus)37 种, 类球囊霉属(Paraglomus)1 种, 盾巨孢囊霉属(Scutellospora)6 种, 各采样点土壤均以球囊霉属为优势属。地球囊霉(G.geosporum)和木薯球囊霉(G. manihotis)是6 个采样地点中的优势种。不同土壤类型各采样点AM 真菌各属的频度存在明显差异, 球囊霉属在各点均有出现, 频度值最高。具有较高植被多样性的暖泉样点, AM 真菌的种属数量较多。土壤环境因子对AM 真菌孢子密度的影响因所处土壤、植被类型不同而异。pH、全盐、速效钾、速效磷等土壤肥力因子, 在PCA 轴上能最大程度地解释AM 真菌孢子密度与土壤环境因子之间相互关系的大部分信息。宁夏不同土壤类型区域中AM 真菌种类及分布一定程度上与该采样点的植被类型、植物多样性和土壤肥力特征相对应。     

Arbuscular mycorrhizal fungal diversity, external mycelium length, and glomalin-related soil protein content in response to long-term fertilizer management

Purpose

Arbuscular mycorrhizal (AM) fungi are crucial for ecosystem functioning and can contribute to the formation and maintenance of soil aggregates through the exudation of glomalin by extraradical hyphae. Monitoring fertilization effects on AM fungi may help us to develop sound management strategies. The objectives of this study were to investigate the impacts of long-term fertilization on AM fungal parameters and to find out the key factor that affects the diversity and function of AM fungi.

Materials and methods

A long-term fertilization experiment established in a sandy loam soil at northern China has received continuous fertilization treatments for 21 years, including control; mineral fertilizers of NK, PK, NP, and NPK; organic manure (OM); and half organic manure N plus half mineral fertilizer N (1/2 OMN). Top soil samples (0–15 cm) from three individual plots per treatment were collected for the analysis of chemical properties and fungal parameters. The population size of soil AM fungi was determined by real-time PCR, and the community composition was analyzed using PCR-denature gradient gel electrophoresis (DGGE), cloning, and sequencing techniques. The external mycelium of AM fungi was assessed using the grid-line intersect method, and the glomalin-related soil protein (GRSP) was extracted with citrate solution using bovine serum albumin as a standard.

Results and discussion

Long-term fertilization significantly increased (P?<?0.05) soil organic C content, AM fungal population, species richness (R), Shannon–Wiener index (H), and GRSP content, except for the P-deficiency (NK) fertilization treatment. OM had a significantly greater (P?<?0.05) impact on AM fungal population and GRSP content compared to mineral fertilizers but significantly decreased the length of external mycelium compared to the control (P?<?0.05). Fertilization also changed the community composition of AM fungi, and the P-deficiency treatment again had the slightest influence. In addition, most species recovered from the DGGE profiles belonged to three genera, Glomus, Diversispora, and Archaeospora. Redundancy analysis showed that the population size and species richness of AM fungi and the GRSP content all significantly correlated to soil organic C content (P?<?0.05).

Conclusions

Long-term P-containing fertilization, especially the application of OM, greatly increased the population size, species richness, and species diversity of AM fungi, as well as the contents of GRSP and soil organic C, but tended to decrease the length of external mycelium, while the P-deficiency fertilization had no such effect, suggesting that P was the key factor to maintain soil fertility as well as soil AM fungal diversity in this sandy loam soil.     

Inputs of nitrogen and organic matter govern the composition of fungal communities in soil disturbed by overwintering cattle

Overwintering cattle outdoors causes soil surface disturbance, substantial increases of soil N_tot, C_org, and P and a shift in pH to alkaline levels. Since fungi predominate in unfertilized soils with acidic pH and have filamentous hyphae, we hypothesized that changes caused by overwintering cattle outdoors (trampling, excreta returns, and changes in soil chemistry) will lead to suppressed species richness, lower biomass, and alter the structure of fungal communities. The research was conducted on an upland pasture used more than 10 years for cattle overwintering. Both culture-dependent and -independent methods were used for the determination of either fungal species composition (cultivation; DGGE) or biomass (numbers of CFU; concentration of fungal PLFA marker 18:2ω6,9). Soils under three different levels of cattle disturbance (S - severe, M - moderate, C - no disturbance/control) were investigated during three subsequent years. In addition, the DGGE analysis of soils was completed by comparison with analysis of fresh cattle excrements (Ex). The composition of fungal communities showed significantly higher richness and a substantial shift in species composition in cattle-disturbed soils (S, M) in comparison to the non-disturbed soil (C). The number of separated DGGE bands was significantly higher in S (30.67 ± 1.63; mean ± SD) and M (25.50 ± 1.64) soils than in the C soil (19.33 ± 1.75). Sequencing of typical bands revealed common fungal genera - Alternaria, Penicillium, Fusarium, Rhizopus, Isaria, and Metarhizium. Profiles of the S soil were enriched by bands of rumen-born anaerobic fungi (Neocallimastix, Cyllamyces) occurring mainly in profiles of excrements, where relatively low band richness (14.33 ± 1.15) was observed. The increasing level of cattle disturbance induced an increase in the biomass of complex fungal community over the three-year experimental period from 3.39 ± 2.11 (mean ± SD) nmol of fungal PLFA per gram of the C soil to 5.87 ± 3.16 in the M soil and 9.21 ± 4.69 in the S soil. Concentrations of soil N_tot and C_org were evaluated as the parameters significantly correlating with biomass as well as composition of the fungal community.     

Converting Australian tropical rainforest to native Araucariaceae plantations alters soil fungal communities

Native rainforest tree plantations are increasingly viewed as potentially important for high value timber production and provision of a range of ecological services in tropical and subtropical areas. In order to determine the extent to which conversion of rainforest to native Araucariaceae plantation influences soil fungi, we compared soil fungal communities under native rainforest and 73-74 year-old Araucaria bidwillii, Araucaria cunninghamii and Agathis robusta plantations at Gadgarra State Forest, Queensland, Australia. Following direct extraction of DNA from soil, terminal restriction fragment length polymorphism (T-RFLP) analysis of rDNA internal transcribed spacer (ITS) regions was conducted. Ordination analysis of the T-RFLP data revealed significant separation of the fungal communities according to forest type along the first canonical axis, with the native rainforest samples separating from the three Araucariaceae plantations along the second axis. Overall, the most abundant ITS sequences in clone assemblages from the four forest types were Ascomycota, followed by Basidiomycota, Zygomycota and Chitridomycota, however their relative importance varied in individual forest types. The results indicate that conversion of tropical rainforest to monoculture plantations of native trees can significantly alter soil fungal diversity.     

Long-term plant growth legacies overwhelm short-term plant growth effects on soil microbial community structure

Plant-soil feedbacks are gaining attention for their ability to determine plant community development. Plant-soil feedback models and research assume that plant-soil interactions occur within days to weeks, yet, little is known about how quickly and to what extent plants change soil community composition. We grew a dominant native plant (Pseudoroegneria spicata) and a dominant non-native plant (Centaurea diffusa) separately in both native- and non-native-cultivated field soils to test if these species could overcome soil legacies and create new soil communities in the short-term. Soil community composition before and after plant growth was assessed in bulk and rhizosphere soils using phospholipid fatty acid analyses. Nematode abundance and mycorrhizal colonization were also measured following plant growth. Field-collected, native-cultivated soils showed greater bacterial, Gram (−), fungal, and arbuscular mycorrhizal PLFA abundance and greater PLFA diversity than field-collected, non-native-cultivated soils. Both plant species grew larger in native- than non-native-cultivated soils, but neither plant affected microbial composition in the bulk or rhizosphere soils after two months. Plants also failed to change nematode abundance or mycorrhizal colonization. Plants, therefore, appear able to create microbial legacies that affect subsequent plant growth, but contrary to common assumptions, the species in this study are likely to require years to create these legacies. Our results are consistent with other studies that demonstrate long-term legacies in soil microbial communities and suggest that the development of plant-soil feedbacks should be viewed in this longer-term context.     

Possible contributions of volatile-producing bacteria to soil fungistasis

Soil fungistasis can adversely affect the germination and growth of most fungal species in the field. Among the inhibitors, volatiles of microbial origins are potentially very important. In this study, we investigated the frequency and identity of bacteria producing fungistatic volatiles. Among the 1018 bacterial isolates tested, 328 were found to produce antifungal volatiles that could inhibit spore germination and mycelial growth of two nematicidal fungi Paecilomyces lilacinus and Pochonia chlamydosporia. A phylogenetic analysis based on restriction fragment length polymorphism (RFLP) and 16S rDNA sequence placed the 328 bacteria in five groups: Alcaligenaceae, Bacillales, Micrococcaceae, Rhizobiaceae and Xanthomonadaceae. Volatile compounds of 39 bacterial isolates were identified by gas chromatography/mass spectrum (GC/MS). Tests with commercially available antifungal compounds suggested that seven volatile compounds of bacterial origins (acetamide, benzaldehyde, benzothiazole, 1-butanamine, methanamine, phenylacetaldehyde and 1-decene) likely play important roles in soil fungistasis.     

广谱抗真菌蛋白转基因水稻秸秆模拟还田对土壤真菌群落结构的影响

为探讨广谱抗真菌蛋白转基因水稻秸秆降解对土壤真菌群落结构的影响,本文在室温条件下进行田间秸秆还田模拟试验,设不添加秸秆(S)、添加转基因水稻‘转品1’秸秆(S-Z1)、添加转基因水稻‘转品8’秸秆(S-Z8)、添加非转基因水稻‘七丝软粘’秸秆(S-CK)4个土壤处理,采用传统的平板计数法和变性梯度凝胶电泳(denatured gradient gel electrophoresis,DGGE)技术,分析广谱抗真菌蛋白转基因水稻秸秆模拟还田过程中土壤可培养真菌数和土壤真菌群落的变化情况。平板计数结果表明,在秸秆降解的第40 d,转基因水稻秸秆处理(S-Z1、S-Z8)与非转基因水稻秸秆处理(S-CK)土壤之间的可培养真菌数差异显著,但秸秆降解中后期(50~90 d),S-Z1、S-Z8和S-CK之间土壤可培养真菌数的差异均不显著。真菌18S r RNA的PCR-DGGE图谱显示,S-Z1、S-Z8和S-CK在秸秆降解过程中没有显著不同的条带出现,仅有个别条带在亮度上存在差异。DGGE图谱条带多样性分析结果表明,在秸秆降解的个别时间段,S-Z1、S-Z8和S-CK之间在丰富度和Shannon-Wiener多样性指数上存在显著差异,而在秸秆降解的整个过程均匀度指数差异均不显著。对DGGE主要条带和差异性条带进行克隆测序后发现,子囊菌占最大比重,其次为担子菌、壶菌,而在转基因和非转基因土壤处理间亮度上存在差异的条带属于子囊菌。以上研究结果表明,广谱抗真菌蛋白转基因水稻秸秆降解对土壤真菌群落结构的影响是短暂的、不持续的。