Microbial community development in the rhizosphere of apple trees at a replant disease site

Apple replant disease (ARD) is a complex syndrome that affects young trees in replanted orchard sites causing necrotic lesions on feeder roots, stunted tree growth and reduced cumulative yields. Use of ARD-tolerant rootstocks is an emerging control strategy. We studied the bacterial, fungal, and oomycetes populations in the rhizosphere of five rootstock cultivars (M.7, M.26, G.16, G.30 and CG.6210) planted into the old tree row or grass lanes of a previous orchard in Ithaca, NY, to better understand the role of rhizosphere microbial communities in the prevalence and control of ARD. The possible involvement of antagonistic Pseudomonas species, Pythium spp., Phytophthora spp. and rhizosphere cyanide concentrations in ARD were also examined. The rootstocks M.7, M.26 and G.16 were susceptible to ARD, while G.30 and CG.6210 were more tolerant. Tree growth on the rootstocks M.7, M.26 and G.16 was reduced by 10% when planted in the old tree rows, but this did not significantly reduce yields in the first fruiting year. The susceptible rootstocks, M.7 and M.26, supported higher densities of culturable rhizosphere fungi and bacteria than G.16, G.30 and CG.6210. Over 2 years, microbial densities were highest in July, lower in May and lowest in September. The composition of bacterial and fungal communities in the rhizosphere was highly variable and changed over seasons and years, as assessed by terminal restriction fragment length polymorphism (T-RFLP) analyses. Initial differences in fungal rhizosphere communities between the two planting positions converged 2 years after the trees were replanted. In contrast, the bacterial rhizosphere community composition still differed significantly between the two planting positions 3 years after the orchard was replanted. The bacterial and fungal rhizosphere community compositions of susceptible rootstocks, M.7 and M.26, differed from those of the tolerant rootstocks, G.30 and CG.6210; G.16, differed from all the other rootstocks. The observed effects of rootstocks, planting positions and time on microbial community composition were small relative to the high variability observed overall. Pythium spp. and Phytophthora spp. infestations were high and similar for all rootstocks and planting positions. Neither potentially antagonistic Pseudomonas nor rhizosphere cyanide concentrations appeared to be involved in the ARD-complex at the studied site. Avoiding replanting into the old tree rows coupled with use of tolerant rootstocks appear to be the best strategies for reducing ARD in replanted orchards. Changes in rhizosphere microbial communities are among the many factors that contribute to improved tree growth when these management strategies are used.     

Rhizosphere bacterial community composition in natural stands of Carex arenaria (sand sedge) is determined by bulk soil community composition

The relative importance of specific plant properties versus soil characteristics in shaping the bacterial community structure of the rhizosphere is a topic of considerable debate. Here, we report the results of a study on the bacterial composition of the rhizosphere of the wild plant Carex arenaria (sand sedge) growing at 10 natural sites in The Netherlands. The soil properties of the sandy soils at these sites were highly disparate, most notably in pH, chloride and organic matter content. Rhizosphere and bulk soil bacterial communities were examined by culture-independent means, namely, 16S rDNA-directed PCR-DGGE profiling. Large differences were observed between the bacterial communities of the different sites for both bulk and rhizosphere soil. Cluster analysis of bacterial profiles revealed that the rhizosphere community of each site was generally more closely related to the bulk soil community of that site rather than to rhizosphere communities of other sites. Hence, bacterial community structure within the rhizosphere of C. arenaria appeared to be determined to a large extent by the bulk soil community composition. This conclusion was supported by a reciprocal planting experiment, where C. arenaria shoots of different sites yielded highly similar rhizosphere communities when planted in the same soil.     

黑土区大豆基因型的根际细菌群落结构时空动态变化

The dynamics of rhizosphere microbial communities is important for plant health and productivity, and can be influenced by soil type, plant species or genotype, and plant growth stage. A pot experiment was carried out to examine the dynamics of microbial communities in the rhizosphere of two soybean genotypes grown in a black soil in Northeast China with a long history of soybean cultivation. The two soybean genotypes, Beifeng 11 and Hai 9731, differing in productivity were grown in a mixture of black soil and siliceous sand. The bacterial communities were compared at three zone locations including rhizoplane, rhizosphere, and bulk soil at the third node (V3), early flowering (R1), and early pod (R3) stages using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) of 16S rDNA. The results of principal component analyses (PCA) showed that the bacterial community structure changed with growth stage. Spatially, the bacterial communities in the rhizoplane and rhizosphere were significantly different from those in the bulk soil. Nevertheless, the bacterial communities in the rhizoplane were distinct from those in the rhizosphere at the V3 stage, while no obvious differences were found at the R1 and R3 stages. For the two genotypes, the bacterial community structure was similar at the V3 stage, but differed at the R1 and R3 stages. In other words, some bacterial populations became dominant and some others recessive at the two later stages, which contributed to the variation of the bacterial community between the two genotypes. These results suggest that soybean plants can modify the rhizosphere bacterial communities in the black soil, and there existed genotype-specific bacterial populations in the rhizosphere, which may be related to soybean productivity.     

Structure and function of microbial communities in the rhizosphere of Scots pine after tree-felling

We evaluated changes occurring in the rhizosphere microbial communities of Scots pine (Pinus sylvestris L.) due to tree-felling and decrease of the photosynthetic C flow into the soil under field conditions over one growing season. Samples were taken from tree rhizospheres, freshly felled stump rhizospheres and bulk soil. We used culture dependent (CFU counts, community level physiological profiles, CLPPs) and independent methods (fluorogenic MUF-substrates, PLFA pattern and PCR-DGGE) to monitor the microbial communities in soil samples. The numbers of cultivable bacteria and amounts of phosphatase activity in the rhizosphere of trees were significantly higher compared with those in the bulk soil. The organic C consuming community measured by CLPP was stimulated directly after the tree-felling in stump rhizospheres; utilization of the disintegration components of cellulose, hemicellulose and chitin increased. Furthermore, bacterial and fungal biomass as well as chitin decomposers (CFU) increased in the stump rhizosphere. After 11 weeks of tree-felling the stump rhizosphere soluble PO₄-P and NH₄-N as well as amounts of total C and N began to resemble the concentrations measured in the bulk soil. However, the stump rhizosphere community structure detected by PLFA and PCR-DGGE still resembled that of the tree rhizosphere.     

A 3-year field investigation of impacts of Monsanto’s transgenic Bt-cotton NC 33B on rhizosphere microbial communities in northern China

Monsanto’s Bt-cotton NC 33B, planted in northern China for more than one decade, effectively controls cotton bollworms and decreases the use of chemical insecticides. Because of the concern about undesirable ecological side-effects of transgenic Bt-cottons, it is important to assess Bt-cotton NC 33B’s effects on soil microorganisms in this zone. Microbial communities in the rhizosphere soil of Bt and non-Bt cottons were monitored under field conditions by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprints of eubacteria, fungi and actinomycetes at six growth stages after three-year cultivation. Results showed that the population sizes and community structures of eubacteria, fungi and actinomycetes in rhizosphere soil were markedly affected by natural variations in the environment related to cotton growth stages. However, there was no significant difference in eubacterial, fungal and actinomycete population size and community structures in rhizosphere soil between NC 33B and its non-transgenic parent. In general, Bt-cotton NC 33B did not show evident effects on microbial communities in the rhizosphere soil under field conditions after three-year cultivation. This study provides a theoretical basis for environmental impact monitoring of transgenic Bt cottons.     

盐碱胁迫对黄瓜嫁接苗根际土壤细菌和真菌群落结构及丰度的影响

为了解盐碱胁迫对黄瓜嫁接苗根际土壤细菌和真菌群落结构的影响,本研究以2种耐盐碱砧木‘华砧108’(T1)、‘神力铁木砧’(T2)和2种盐碱敏感砧木‘辉太郎’(S1)、‘京欣砧6号’(S2)为试材,自根苗作为对照,以混合盐(盐分摩尔比为NaHCO_3∶Na_2SO_4∶NaCl∶Na_2CO_3=4∶2∶2∶0.15)浓度为100 mmol·L~(-1)、pH 9.0的处理液处理20 d、30 d、40 d(定植30 d、40 d、50 d),利用PCR-DGGE技术,研究了盐碱胁迫对不同砧木嫁接的黄瓜幼苗根际土壤微生物群落结构和丰度的影响。结果表明,耐盐碱的砧木品种T1、T2根际土壤真菌DGGE图谱条带数显著高于盐碱敏感的S2和自根苗对照CK,并且耐盐碱的品种T2土壤细菌的Shannon-Wiener指数与均匀度指数均显著高于盐碱敏感的品种S1、S2和自根苗对照CK。耐盐碱品种T1的细菌16S rDNA基因拷贝数在定植50 d时显著高于盐碱敏感的品种及自根苗;在定植40 d时,耐盐碱的砧木T2真菌ITS基因拷贝数显著高于盐碱敏感的品种以及黄瓜自根苗;定植50 d时,耐盐碱的砧木真菌ITS基因拷贝数显著高于盐碱敏感的品种,但与自根苗差异不显著。不同耐盐碱性砧木嫁接黄瓜幼苗根际土壤微生物群落结构组成和丰度存在差异。以上研究表明,随着盐碱胁迫时间的增加,耐盐碱性不同的砧木嫁接苗根际土壤微生物群落丰度与结构多样性产生了较大差异,间接改变了土壤微生态环境,致使土壤微生物数量和丰富度也发生改变。耐盐碱的砧木品种可能通过改善土壤微环境来加强其自身的耐盐碱特性。     

Influence of grass species and soil type on rhizosphere microbial community structure in grassland soils

A number of studies have reported species specific selection of microbial communities in the rhizosphere by plants. It is hypothesised that plants influence microbial community structure in the rhizosphere through rhizodeposition. We examined to what extent the structure of bacterial and fungal communities in the rhizosphere of grasses is determined by the plant species and different soil types. Three grass species were planted in soil from one site, to identify plant-specific influences on rhizosphere microbial communities. To quantify the soil-specific effects on rhizosphere microbial community structure, we planted one grass species (Lolium perenne L.) into soils from three contrasting sites. Rhizosphere, non-rhizosphere (bulk) and control (non-planted) soil samples were collected at regular intervals, to examine the temporal changes in soil microbial communities. Rhizosphere soil samples were collected from both root bases and root tips, to investigate root associated spatial influences. Both fungal and bacterial communities were analysed by terminal restriction fragment length polymorphism (TRFLP). Both bacterial and fungal communities were influenced by the plant growth but there was no evidence for plant species selection of the soil microbial communities in the rhizosphere of the different grass species. For both fungal and bacterial communities, the major determinant of community structure in rhizospheres was soil type. This observation was confirmed by cloning and sequencing analysis of bacterial communities. In control soils, bacterial composition was dominated by Firmicutes and Actinobacteria but in the rhizosphere samples, the majority of bacteria belonged to Proteobacteria and Acidobacteria. Bacterial community compositions of rhizosphere soils from different plants were similar, indicating only a weak influence of plant species on rhizosphere microbial community structure.     

Bacterial communities in soybean rhizosphere in response to soil type, soybean genotype, and their growth stage

Three experiments were conducted in this study in order to investigate the impacts of soil type, soybean genotype, and the reproductive growth stage on bacterial communities in the soybean rhizosphere. Communities were evaluated by principal component analysis of denaturing gradient gel electrophoresis (DGGE) banding patterns and sequencing of partial 16S rDNA polymerase chain reaction (PCR) amplicons. A pot experiment analyzing three soybean genotypes grown in two different types of soil (Black soil and Dark Brown soil) indicated that soil type was the major factor in influencing the bacterial communities in the soybean rhizosphere, with a more significant effect observed in the Black soil samples than in the Dark Brown soil samples. A field experiment was conducted in Dark Brown soil using three soybean genotypes, and the results gleaned from both pot and field experiments indicated that bacterial communities in the soybean rhizosphere changed with growth stages, and higher number of DGGE bands observed in early reproductive growth stages, while surprisingly, a significant impact of genotype on the bacterial communities was not observed in these experiments. However, a plate culture experiment targeting the culturable bacterial communities detected a remarkable difference in the community structures of the rhizosphere between the two soybean genotypes, suggesting that a small portion of the total bacteria was influenced by genotype. Sequence analysis of DGGE bands indicated that bacterial phyla of Proteobacteria, Actinobacteria, Bacteroidetes, Nitrospirae, Firmicutes, Verrucomicrobia and Acidobacteria commonly inhabit the soybean rhizosphere.     

Rhizosphere fungal communities are influenced by Senecio jacobaea pyrrolizidine alkaloid content and composition

The contents and the compositions of the pyrrolizidine alkaloid (PA) complex of ragwort (Senecio jacobaea L.) were examined as potential drivers of fungal community structure in the rhizosphere. S. jacobaea plants within the coastal sand dune reserve of Meijendel (the Netherlands) were assayed for concentration and composition of PAs in roots. Rhizosphere soil was collected from pre-flowering plants, which differed up to 8-fold in PA production, and represented both jacobine and senecionine/seneciphylline chemotypes. Bulk soil samples from the same site were also collected for comparative examination. A culture-independent approach, involving direct DNA isolation, PCR of fungal 18S rRNA genes, and denaturing gradient gel electrophoresis (DGGE), was applied to compare the fungal communities of plants with different PA contents, as well as differences between bulk and rhizosphere samples. Cluster analysis of PCR-DGGE profiles revealed no clear evidence for PA-induced selection of specific fungal communities. However, canonical variance analysis showed that fungal communities associated with high-PA jacobine chemotypes could be discriminated from low PA samples and from the senecionine/seneciphylline chemotypes. The diversity of DGGE banding patterns, both in terms of band number and evenness, showed a trend toward lower diversity in the rhizosphere of high-PA plants as compared to low-PA plants and bulk soil. These results indicate that PA chemotypes of S. jacobaea differ in their influence on soil-borne fungal communities, with jacobine-containing plants exerting a greater selection in the rhizosphere than plants containing senecionine/seneciphylline.     

Impact of fresh root material and mature crop residues of oilseed rape (Brassica napus) on microbial communities associated with subsequent oilseed rape

Glasshouse bioassays were conducted to assess the impact of different inputs of oilseed rape plant material on soil and rhizosphere microbial diversity associated with subsequently grown oilseed rape (Brassica napus) plants. The first bioassay focussed on the effect of oilseed rape rhizodeposits and fresh detached root material on microbial communities, in a rapid-cycling experiment in which oilseed rape plants were grown successively in pots of field soil for 4 weeks at a time, with six cycles of repeated vegetative planting in the same pot. Molecular analyses of the microbial communities after each cycle showed that the obligate parasite Olpidium brassicae infected the roots of oilseed rape within 4 weeks after the first planting (irrespective of the influence of rhizodeposits alone or in the presence of fresh detached root material), and consistently dominated the rhizosphere fungal community, ranging in relative abundance from 43 to 88 % when oilseed rape was grown more than once in the same soil. Fresh detached root material also led to a reduction in diversity within the soil fungal community, due to the increased relative abundance of O. brassicae. In addition, rhizosphere bacterial communities were found to have a reduced diversity over time when fresh root material was retained in the soil. In the second glasshouse experiment, the effect of incorporating mature, field-derived oilseed rape crop residues (shoots and root material) on microbial communities associated with subsequently grown oilseed rape was investigated. As before, molecular analyses revealed that O. brassicae dominated the rhizosphere fungal community, despite not being prevalent in either the residue material or soil fungal communities.     

Relationship between root-endophytic microbial communities and replant disease in specialized apple growing areas in Europe

Apple replant disorders are one of the causes of the downward trend of land planted with apple orchards in Central Europe. A specific transnational survey was thus conducted on apple root associated microorganisms, aimed at increasing the knowledge regarding crop management to counteract this tendency. Soil health was evaluated using a bioassay test with root cuttings of the clonal M9 rootstock, one of the most commonly used rootstocks in Europe. Plant growth response in replant, fallow and gamma ray-sterilized soil was evaluated using soil samples taken from nine orchards selected (three per country) from specialized apple growing areas of Germany, Austria and Italy. Plant growth significantly differed between treatments but not between countries. Root endophytic fungi were confirmed as one of the main components of growth reduction, while endophytic nematodes (Pratylenchus sp.) were not. The large set of quantitative and qualitative data of root endophytic fungi allowed to clarify that Cylindrocarpon-like fungi (Ilyonectria spp. and Thelonectria sp.) had a major pathogenic role in the three countries, while Pythium spp. prevailed as pathogen only in German orchards. Gamma ray-sterilized soil resulted in a more-than-proportional increase of plant growth (42% and 31% respectively vs. replant and fallow) as compared to that observed between fallow and replant soil (18%). This best performance in gamma ray-sterilized soil, in addition to the lowest root colonization by Cylindrocarpon-like fungi in this treatment, appeared to be due to the different composition of fungal communities as compared to two other treatments (fallow and replant soil), which instead were highly similar to each other. This difference was due to a shift of community composition toward Fusarium and binucleate Rhizoctonia, which prevailed in sterilized soil.     

不同施氮水平下配施硅肥对水稻根部周围土壤微生物群落结构的影响

为研究不同水平氮肥、硅肥配施对水稻根部周围土壤微生物群落结构影响,为水稻的硅肥、氮肥合理配施提供参考。以Na₂SiO₃作为硅肥硅源,设置了6组硅肥、氮肥处理组,采用变性梯度凝胶电泳（PCR-DGGE）技术探究不同施氮水平下配施硅肥处理对水稻根部周围土壤细菌、真菌和古细菌群落结构的影响。结果显示在施加低氮和中氮处理组中水稻根部周围土壤细菌群落多样性没有显著变化,施高氮处理组的细菌群落多样性呈上升趋势,但在不同施氮水平下加硅处理对细菌的群落结构影响不明显。施低氮水平和中氮水平下硅对水稻根部周围土壤真菌群落结构的影响亦不明显;与施中低氮水平相比,在施高氮水平下,水稻根部周围土壤中真菌的多样性显著下降,但施硅能显著提高水稻根部周围土壤真菌的多样性,能促进真菌类群（Coniochaeta prunicola）、粉质拟青霉（Paecilomyces farinosus）、子囊菌（Ascomycota sp.）与浅黄褐色嗜热菌（Thermothelomyces hinnuleus）等类群的生长,同时抑制水稻病原真菌木贼镰刀菌（Fusarium equiseti）类群的生长。随着施加的氮水平升高,古细菌群落多样性呈升高趋势,各氮水平下加硅处理对水稻根部周围土壤的古细菌群落结构没有显著影响。研究表明,在不同施氮水平下,施加以Na₂SiO₃为硅源的硅肥对水稻根部周围土壤的细菌、古细菌群落结构没有显著影响;但在施加高氮水平后,加硅肥能显著提高水稻根部周围土壤真菌群落的多样性。     

苹果树根际促生细菌种群分析

利用选择性培养基, 对多年生苹果树根际与连作幼树根际促生细菌进行了分离和测数, 并采用BOX-PCR技术进行聚类分析。结果表明: 多年生苹果树根际细菌总量及固氮细菌、解磷细菌、硅酸盐细菌、拮抗细菌4类根际促生细菌的数量均高于连作幼树根际。在多年生苹果树根际, 硅酸盐细菌的数量最大, 解磷细菌和固氮细菌的数量次之, 拮抗细菌的数量最小; 在连作幼树根际, 解磷细菌的数量最大, 硅酸盐细菌和固氮细菌的数量次之, 拮抗细菌的数量最小。从两种土壤中获得的促生细菌分离株的BOX-PCR图谱最大的相异百分数都在1.25以上, 说明这些细菌分离株的遗传进化距离比较接近。在细菌BOX-PCR图谱相异百分数为0.25的水平上, 多年生苹果树根际促生细菌分为79个聚类群, 其中固氮细菌18个聚类群, 解磷细菌29个聚类群, 硅酸盐细菌19个聚类群, 拮抗细菌18个聚类群; 连作幼树根际促生细菌分为46个聚类群, 其中固氮细菌15个聚类群, 解磷细菌19个聚类群, 硅酸盐细菌8个聚类群, 拮抗细菌9个聚类群。多年生苹果树4类根际促生细菌的多样性、丰富度和均匀度指数均高于连作幼树根际, 而优势度指数低于连作幼树根际。与连作幼树相比, 多年生苹果树根际促生细菌具有丰富的种属多样性。     

Diversity and antagonistic potential of Pseudomonas spp. associated to the rhizosphere of maize grown in a subtropical organic farm

Pseudomonas spp. are one of the most important bacteria inhabiting the rhizosphere of diverse crop plants and have been frequently reported as biological control agents (BCAs). In this work, the diversity and antagonistic potential of Pseudomonas spp. in the rhizosphere of maize cultivars Nitroflint and Nitrodent grown at an organic farm in Brazil was studied by means of culture-dependent and -independent methods, respectively. Sampling of rhizosphere soil took place at three different stages of plant development: 20, 40 and 106 days after sowing. A PCR-DGGE strategy was used to generate specific Pseudomonas spp. fingerprints of 16S rRNA genes amplified from total community rhizosphere DNA. Shifts in the relative abundance of dominant populations (i.e. PCR-DGGE ribotypes) along plant development were detected. A few PCR-DGGE ribotypes were shown to display cultivar-dependent relative abundance. No significant differences in diversity measures of DGGE fingerprints were observed for different maize cultivars and sampling times. The characterisation and assessment of the antagonistic potential of a group of 142 fluorescent Pseudomonas isolated from the rhizosphere of both maize cultivars were carried out. Isolates were phenotypically and genotypically characterised and screened for in vitro antagonism towards three phytopathogenic fungi and the phytopathogenic bacterium Ralstonia solanacearum. Anti-fungal activity was displayed by 13 fluorescent isolates while 40 isolates were antagonistic towards R. solanacearum. High genotypic and phenotypic diversity was estimated for antagonistic fluorescent Pseudomonas spp. PCR-DGGE ribotypes displayed by antagonists matched dominant ribotypes of Pseudomonas DGGE fingerprints, suggesting that antagonists may belong to major Pseudomonas populations in the maize rhizosphere. Antagonists differing in their genotypic and phenotypic characteristics shared the same DGGE electrophoretic mobility, indicating that an enormous genotypic and functional diversity might be hidden behind one single DGGE band. Cloning and sequencing was performed for a DGGE double-band which had no corresponding PCR-DGGE ribotypes among the antagonists. Sequences derived from this band were affiliated to Pseudomonas stutzeri and P. alcaligenes 16S rRNA gene sequences. As used in this study, the combination of culture-dependent and -independent methods has proven to be a powerful tool to relate functional and structural diversity of Pseudomonas spp. in the rhizosphere.     

Assessing insecticide and fungicide effects on the culturable soil bacterial community by analyses of variance of their DGGE fingerprinting data

To assess the effects of three insecticides (aldicarb, chlorpyrifos, deltamethrin) and two fungicides (tebuconazole and metalaxyl + mancozeb) on the PCR-DGGE fingerprints of culturable soil bacterial communities (CSBC), a greenhouse experiment was carried out with soil samples from an Integrated System for Agroecological Production (ISAP), a Conventional Potato Production Area (CPPA) and a Secondary Forest Area (SFA) close to the CPPA. Samples were obtained at 15 day intervals starting at 32 until 77 days after sowing (DAS) to perform the PCR-DGGE analysis of the CSBC cultured on media amended with soil suspension. Analysis of variance from PCR-DGGE data indicated significant differences among treatments. Regardless the type of pesticide applied, CSBC was disturbed and similarity values varied from 5% to 90% in comparison to the control. Significant shifts on CSBC were only detected among treatments in the first two harvests, while CSBC tended to be more akin to each other at the last two harvest dates. The most significant responses observed were due to different soil sample origins, where values of 5% of similarity to the control were observed on CPPA soil. The use of analysis of variance on PCR-DGGE data was useful to a better understanding of the changes on CSBC induced by pesticides applications.     

Response of ‘Jonagold’ Apple Trees in the First Three Years after Planting to Mono-Ammonium Phosphate Fertilization Under Replant Problem Conditions

Abstract

The objective of the experiment was to examine response of immature apple trees to application of mono-ammonium phosphate (MAP) fertilizer on replant problem soil. The study was carried out during 2001–2003 under a greenhouse on ‘Jonagold’ apple trees/M.9 EMLA planted singly in 50 L polyethylene containers filled with a sandy loam soil with low status of both organic matter and phosphorus (P) in soil solution. This soil originated from an apple orchard unfertilized with P for 23 years. The biological test showed the presence of specific replant disease in the soil. Immediately before apple tree planting, the soil was mixed with MAP at rates of 1, 2, and 3 g L^{? 1}. Trees grown in the soil untreated with MAP served as a control. Each year apple trees were drip-irrigated and supplied with nitrogen (N) at differentiated rates to achieve a level of 50 g N per plant. The results showed that MAP application increased soil solution P status. Simultaneously, MAP supply at rates of 2 and 3 g L^{? 1} caused a drop in soil pH value in the last two years of the experiment. MAP treatments increased both dry weight and length of fine roots (< 2 mm in diameter), vigor of trees, the number of flower clusters per tree, flower intensity, the number of fruits per tree, and P concentrations in leaf and fruit tissues. Fruits from MAP-supplied trees were firmer than those of the control trees. Mean fruit weight, titratable acidity, and soluble solids concentration of ‘Jonagold’ apples at harvest were not influenced by MAP treatment. Fruits from MAP-supplied apple trees had increased calcium concentration only in one year. It is concluded that pre-plant application of MAP at a rate of 1g L^{? 1}can be recommended on coarse-textured soils with low P status in soil solution to increase precocity of apple trees. However, MAP-supplied apple trees have to be watered to avoid the risk of osmotic stress.     

Plant genotype strongly modifies the structure and growth of maize rhizosphere microbial communities

We studied the microbial communities in maize (Zea mays) rhizosphere to determine the extent to which their structure, biomass, activity and growth were influenced by plant genotype (su1 and sh2 genes) and the addition of standard and high doses of different types of fertilizer (inorganic, raw manure and vermicompost). For this purpose, we sampled the rhizosphere of maize plants at harvest, and analyzed the microbial community structure (PLFA analysis) and activity (basal respiration and bacterial and fungal growth rates). Discriminant analysis clearly differentiated rhizosphere microbial communities in relation to plant genotype. Although microorganisms clearly responded to dose of fertilization, the three fertilizers also contributed to differentiate rhizosphere microbial communities. Moreover, larger plants did not promoted higher biomass or microbial growth rates suggesting complex interactions between plants and fertilizers, probably as a result of the different performance of plant genotypes within fertilizer treatments, i.e. differences in the quality and/or composition of root exudates.     

土壤与基质栽培系统对生菜(Lactuca sativa)根际细菌群落的影响

以生菜(Lactuca sativa)"申选5号"与"罗莎红"为材料,采用PCR-DGGE和Real-Time PCR技术,分析了土壤栽培系统和基质栽培系统根际细菌群落的差异。Real-Time PCR检测结果表明,基质栽培的两个生菜品种根际细菌数量均显著高于土壤(P0.05);PCR-DGGE图谱条带结果表明,根际细菌群落多样性基质高于土壤。栽培系统是造成多样性差异的主要原因,但也与品种有关:"申选5号"基质的Shannon-Wiener指数(H),Simpson指数(D)和均匀度(E)均显著高于土壤(P0.05);"罗莎红"基质的H显著高于土壤,而D和E无显著差异。结合土壤和基质理化性质的RDA分析结果,土壤和基质具有不同的细菌群落,p H值与硝态氮是塑造根际细菌群落的主要因子,含水量、碳氮比和有效磷与细菌群落的形成呈正相关。     

In situ dynamics of soil fungal communities under different genotypes of potato, including a genetically modified cultivar

Fungi are key to the functioning of soil ecosystems, and exhibit a range of interactions with plants. Given their close associations with plants, and importance in ecosystem functioning, soil-borne fungi have been proposed as potential biological indicators of disturbance and useful agents in monitoring strategies, including those following the introduction of genetically modified (GM) crops. Here we report on the impact of potato crop varieties, including a cultivar that was genetically modified for its starch quality, on the community composition of the main phyla of fungi in soils, i.e. Ascomycota, Basidiomycota and Glomeromycota in rhizosphere and bulk soil. Samples were collected at two field sites before sowing, at three growth stages during crop development and after the harvest of the plants, and the effects of field site, plant growth stage and plant cultivar (genotype) on fungal community composition assessed using three phylum-specific T-RFLP profiling strategies and multivariate statistical analysis (NMDS ordinations with ANOSIM test). In addition, fungal biomass, arbuscular mycorrhizal colonization of roots and activities of extracellular fungal enzymes (laccases, Mn-peroxidases and cellulases) involved in degradation of lignocelluloses-rich organic matter were determined. Fungal community compositions, densities and activities were observed to differ significantly between the rhizosphere and bulk soil. The most important factors determining fungal community composition and functioning were plant growth stage for the rhizosphere communities and location and soil properties for the bulk soil communities. The basidiomycetes were the most numerous fungal group in the bulk soils and in the rhizosphere of young plants, with a shift toward greater ascomycete numbers in the rhizosphere at later growth stages. There were no detectable differences between the GM cultivar and its parental cultivar in terms of influence on fungal community structure of function. Fungal community structure and functioning of both GM- and parental cultivars fell within the range of other cultivars at most sampling moments.     

Responses of soil microbial communities to manure and biochar in wheat cultivation of a rice-wheat rotation agroecosystem in East China

Soil contamination in agroecosystems remains a global environmental problem. Biochar has been suggested as an organic amendment to alleviate soil pollution, sequester carbon(C), and improve soil fertility. However, information on how bacterial and fungal communities in acidic bulk and rhizosphere soils respond to swine manure and its biochar is still lacking. In this study, biochar and swine manure were applied at two rates of 1.5 and 3 t ha^-1 in a rice-wheat rotation field to assess ...