Soil fungal isolates produce different organic acid patterns involved in phosphate salts solubilization

Phosphorus availability is a major limiting factor for yield of most crop species. The objective of this study was to compare the solubilization of three sources of phosphorus (P) by different fungal isolates and to determine the possible mechanisms involved in the process. Talaromyces flavus (S73), T. flavus var flavus (TM), Talaromyces helicus (L7b) and T. helicus (N24), Penicillium janthinellum (PJ), and Penicillium purpurogenum (POP), fungal strains isolated from the rhizosphere of crops, are known to be biocontrol agents against pathogenic fungi. The P solubilization efficiency of these fungal strains in liquid media supplemented either with tricalcium phosphate (Ca₃(PO₄)₂; PC), aluminum phosphate (AlPO₄; AP), or phosphorite (PP) depended on the source of P and the fungal species. The type and concentration of organic acids produced by each species varied according to the source of available P. In the medium supplemented with PC, the highest proportion was that of gluconic acid, whereas in the media supplemented with the other P sources, the highest proportion was that of citric and valeric acids. This suggests that the release of these organic compounds in the rhizosphere by these microorganisms may be important in the solubilization of various inorganic P compounds. Results also support the hypothesis that the simultaneous production of different organic acids by fungi may enhance their potential for solubilizing insoluble phosphate.     

Phosphate solubilization potentials of soil Acinetobacter strains

Many phosphate solubilizing microorganisms (PSM) require external pyrroloquinoline quinone (PQQ) for strong phosphorus (P) solubilization in vitro. The objective of this study was to isolate efficient and PQQ-independent PSM. A total of 21 PSM were isolated from the rhizosphere soil of wheat and maize grown in the pots. Acinetobacter strains were the only PQQ-independent and most effective solubilizers of tricalcium phosphate containing agar. The mean P dissolved in liquid cultures of Acinetobacter strains in a 5-day incubation ranged from 167 to 888 μg/ml P. The pH dropped to below 4.7 from 7.8 in six isolates, which produced gluconic acid in concentrations ranging between 27.5 and 37.5 mM. There was a linear regression between soluble P and gluconic acid concentrations in the bacterial cultures (P < 0.05; R ² = 0.59). Inoculation with Acinetobacter sp. WR922 significantly (P < 0.05) increased wheat (Triticum aestivum L.) P content by 27% at 15 days after emergence (DAE) and dry matter by 15% at 30 DAE compared to the control. The plant P content in inoculated plants at 30 DAE was linearly correlated with soluble P of the bacterial cultures (P < 0.05; R ² = 0.69). Gluconic acid production directly affected phosphate solubilization in vitro, which in turn influenced plant P content of inoculated plants in PQQ-independent P-solubilizing Acinetobacter strains.     

Phosphorus uptake and rhizosphere properties of intercropped and monocropped maize, faba bean, and white lupin in acidic soil

Little information is available on phosphorus (P) uptake and rhizosphere processes in maize (Zea mays L.), faba bean (Vicia faba L.), and white lupin (Lupinus albus L.) when intercropped or grown alone in acidic soil. We studied P uptake and soil pH, carboxylate concentration, and microbial community structure in the rhizosphere of maize, faba bean, and white lupin in an acidic soil with 0–250 mg P (kg⁻¹ soil) as KH₂PO₄ (KP) or FePO₄ (FeP) with species grown alone or intercropped. All plant species increased the pH compared to unplanted control, particularly faba bean. High KP supply (>100 mg P kg⁻¹) significantly increased carboxylate concentration in the rhizosphere of maize. The carboxylate composition of the rhizosphere soil of maize and white lupin was significantly affected by P form (KP or FeP), whereas, this was not the case for faba bean. In maize, the carboxylate composition of the rhizosphere soil differed significantly between intercropping and monocropping. Yield and P uptake were similar in monocropping and intercropping. Monocropped faba bean had a greater concentration of phospholipid fatty acids in the rhizosphere than that in intercropping. Intercropping changed the microbial community structure in faba bean but not in the other corps. The results show that P supply and P form, as well as intercropping can affect carboxylate concentration and microbial community composition in the rhizosphere, but that the effect is plant species-specific. In contrast to previous studies in alkaline soils, intercropping of maize with legumes did not result in increased maize growth suggesting that the legumes did not increase P availability to maize in this acidic soil.     

Phosphate-solubilizing microorganisms associated with the rhizosphere of oil palm (Elaeis guineensis Jacq.) in Colombia

This study determined the cell density in the field and the biological activity of culturable phosphate-solubilizing microorganisms (PSMs) present in the Elaeis guineensis Jacq. rhizosphere at two locations in a commercial plantation. Promising isolates found in two soils under different agronomic management conditions were selected. We first calculated the cell density of cultivable PSMs grown in SRS medium (Sundara-Rao and Sinha, 1963) supplemented with the insoluble phosphate sources Ca₃(PO₄)₂, AlPO₄, and FePO₄. Twenty-two bacteria, ten filamentous fungi, and eight yeast isolates were found. The 16 isolates with the clearest P solubilization halo in Petri dishes were selected to estimate their P solubilization potential in SRS medium with Ca₃(PO₄)₂. No solubilization activity was registered using AlPO₄ or FePO₄ as the P sources. Ten of the isolates presented solubilization efficiencies between 20 and 82%. Some of these isolates showed high percentages of identity with the 16S and ITS rDNA sequences of the genera Aspergillus, Penicillium, Klebsiella, Burkholderia, and Staphylococcus according to the NCBI and EzTaxon-e databases. The solubilization activity of the isolates was associated with a decrease in the pH and the release of organic acids, such as gluconic, citric, succinic, and acetic acids. Gluconic acid was mainly released by the genera Aspergillus and Penicillium, and these isolates also showed the highest solubilization activities (82 and 80%, respectively). Therefore, these isolates were selected as the most promising isolates present in the oil palm rhizosphere for phosphate solubilization.     

Effect of plant growth-promoting bacteria and soil compaction on barley seedling growth, nutrient uptake, soil properties and rhizosphere microflora

Inoculants are of great importance in sustainable and/or organic agriculture. In the present study, plant growth of barley (Hordeum vulgare) has been studied in sterile soil inoculated with four plant growth-promoting bacteria and mineral fertilizers at three different soil bulk densities and in three harvests of plants. Three bacterial species were isolated from the rhizosphere of barley and wheat. These bacteria fixed N₂, dissolved P and significantly increased growth of barley seedlings. Available phosphate in soil was significantly increased by seed inoculation of Bacillus M-13 and Bacillus RC01. Total culturable bacteria, fungi and P-solubilizing bacteria count increased with time. Data suggest that seed inoculation of barley with Bacillus RC01, Bacillus RC02, Bacillus RC03 and Bacillus M-13 increased root weight by 16.7, 12.5, 8.9 and 12.5% as compared to the control (without bacteria inoculation and mineral fertilizers) and shoot weight by 34.7, 34.7, 28.6 and 32.7%, respectively. Bacterial inoculation gave increases of 20.3–25.7% over the control as compared with 18.9 and 35.1% total biomass weight increases by P and NP application. The concentration of N and P in soil was decreased by increasing soil compaction. In contrast to macronutrients, the concentration of Fe, Cu and Mn was lower in plants grown in the loosest soil. Soil compaction induced a limitation in root and shoot growth that was reflected by a decrease in the microbial population and activity. Our results show that bacterial population was stimulated by the decrease in soil bulk density. The results suggest that the N₂-fixing and P-solubilizing bacterial strains tested have a potential on plant growth activity of barley.     

磷供应对玉米根际微生物碳源利用和功能多样性的影响

磷有效性能够改变根分泌物的组成和数量,调节土壤微生物的群落结构和多样性,但磷添加如何影响土壤微生物碳源利用和功能多样性尚不清楚。本研究通过盆栽土培试验,设置2个磷处理[低磷5.7 mg(P)?kg?1和高磷200 mg(P)?kg?1],以生长35 d的玉米根际土壤为研究对象,采用Biolog微平板法,分别在培养后240 h内每隔24 h检测具有31种不同碳源的微孔溶液颜色变化,揭示磷供应对玉米根际微生物碳源利用模式和功能多样性的影响。结果表明:随着培养时间的延长,土壤微生物对土壤碳源的利用呈现增加的趋势,直至碳源消耗殆尽;高磷供应显著增加了玉米根际土壤微生物群落平均颜色变化率(average well color development,AWCD),提高了对糖类及其衍生物、氨基酸和代谢产物的利用,但没有显著提高对脂肪酸和脂类的利用;在培养前72 h内,高磷供应显著增加了玉米根际微生物多样性指数、优势度指数和均匀度指数,但培养72 h后,磷供应对其没有显著的影响。主成分分析结果表明,提取的前3个主成分解释了75.15%的碳源利用,高磷和低磷处理具有显著不同的土壤微生物碳源利用模式。总之,糖类及其衍生物、氨基酸和代谢产物是玉米根际土壤微生物利用的主要碳源,短期磷添加能够显著增加土壤微生物对碳源的利用,在一定程度上能够提高土壤微生物群落功能多样性。     

Phosphate solubilizing bacteria and fungi in desert soils: species,limitations and mechanisms

Desert soils are infertile, and the ability to improve them by P-fertilization is limited by the solubility of phosphate. We aimed to understand the function of phosphate solubilizing bacteria and the mechanisms behind phosphate solubilization in desert soils. Vegetated and barren desert soils, mine spoil and a fertile temperate grassland loam were sampled. Bacteria and fungi were isolated and identified, and their phosphate-solubilizing abilities were measured in vitro. The release of plant available PO₄, SO₄, NO₃ and NH₄ from desert soils did not compare with that of a grassland soil. Desert soils had substantially lower solubilization than grassland, 162 and 99–121 µg PO₄-P g^?1 dry soil, respectively. Phosphate-solubilizing bacteria and fungi were inhabiting the soils. Si addition increased phosphate solubilization of fungi by 50%. The isolated microbes were shown, using ³¹P nuclear magnetic resonance (NMR) analysis, to rapidly take-up both intracellular and extracellular phosphate during the phosphate solubilizing process. Desert soil had potentially active microbial populations that are capable to solubilize inorganic phosphorus; S and Si as the limiting factors. Acidification as the main mechanism to solubilize mineral phosphate was not as evident in our desert soils as in former studies dealing more fertile soils.     

K3解磷菌的解磷机理及其对缓冲容量的响应

以对磷酸三钙具有高效溶解作用且对玉米苗生长有促生效果的假单胞菌K3为模式菌株,采用NBRIP液体培养基研究了解磷菌K3的解磷机制及缓冲容量对其解磷量的影响。结果表明,解磷菌K3液体摇瓶培养7 d后,培养液中水溶性磷从6.54 μg/mL增加至655.23 μg/mL,pH从7.00降至3.99。高效液相色谱测定发现,K3菌液中的主要代谢产物为苹果酸、乳酸和草酸,浓度分别为47.39 mmol/L、25.67 mmol/L和1.89 mmol/L。人工模拟K3菌株产生的有机酸及调节培养基不同pH值对磷酸三钙溶解度影响的试验表明,有机酸的螯合作用是解磷细菌K3菌株解磷的主要机理,而调节培养基pH对解磷的作用有限。液体摇瓶和土培试验结果显示,土壤缓冲容量对K3解磷菌的解磷效应有显著的抑制作用。     

Inoculation of phosphate-solubilizing bacteria Bacillus thuringiensis B1 increases available phosphorus and growth of peanut in acidic soil

Phosphate-solubilizing microorganisms play an important role in plant nutrition by enhancing phosphorus (P) availability to roots through converting the insoluble phosphates into soluble ions. We isolated phosphate-solubilizing bacteria (PSB) from acidic soil (Ultisols) in the field from the layer of 0–150 mm at a tea garden located at 28°38′26″ N and 116°24′27″ E. The capacity of bacterial isolates to solubilize mineral phosphate was tested on aluminum phosphate (AlPO₄) in liquid medium. Among these PSB, isolate B1 (identified as Bacillus thuringiensis) exhibited the maximum P-solubilizing ability and was particularly efficient at solubilizing AlPO₄ (up to 321 mg L^?1) in vitro. The isolate B1 was inoculated to an acidic soil to study its effect on phosphate solubilization and growth of peanuts (Arachis hypogeae). The Olsen-P in the tested soil increased from 14.7 to 23.4 mg kg^?1, with solubilization of 16.4 mg kg^?1 soil of Occluded-P after 14-day incubation. The inoculation by B1 significantly increased plant height (from 37.7 to 45.7 cm), number of branches (from 34.0 to 52.7 per plant), hundred-seed weight (from 42.1 to 46.9 g) and crude protein content (from 243.5 to 268.2 g kg^?1 dry weight). The phosphate-solubilizing B. thuringiensis strain B1 showed potential as a biological phosphorus fertilizer.     

Soil–Phosphorus Mobilization Potential of Phytate Mineralizing Fungi

Four most efficient phytase and phosphatase producing fungi belonging to genera Aspergillus, Trichoderma, and Penicillium were isolated from the rhizosphere soil of leguminous, cereal, and vegetable crops. Efficacy order of fungi in terms of phytate hydrolysis under laboratory conditions was Aspergillus > Penicillium > Trichoderma. The test fungi released more of extracellular (E) phytase than intracellular (I) phytase (E: I- 3.44 - 6.03:1) and produced acid phosphatase activity ranging from 367- 830 μmol pNP ml^?1 h^?1. Aspergillus niger possessed the twin ability of phosphate mineralization and solubilization. The incubation studies in compost-amended soil exhibited the higher competence of Penicillium chrysogenum to improve the soil available P and increase the level of extractable organic P under alkaline soil to benefit P nutrition. Developing microbial inoculant using P. chrysogenum strain and its subsequent application to soil may help the marginal farmer to replenish soil P more economically compared to chemical fertilizer.     

The maize root system in situ: Evaluation of structure and capability of utilization of phytate and inorganic soil phosphates

• 1 The dependence of the morphology of the maize (Zea mays L.) seminal root system on physical, chemical and biotic parameters was investigated with pot cultures in quartz sand and in a natural loamy sand soil. Low O₂-supply to the soil resulted in a substantially smaller root biomass despite a relative increase in total root length. Reduced N-supply also stimulated root length growth, but also enhanced the formation of laterals. The presence of soil microorganisms, in comparison to sterile cultures, resulted in a reduced length of the main roots, and the production of slender laterals with a decreased root hair density. Generally, the structural variability of laterals in response to different growth conditions was much more pronounced than that of the main roots.
• 2 A major part of the work reported here was dedicated to a detailed study of phosphate (P) acquisition by the maize root system under field conditions. Radioactive labelling of the roots and radioautography of soil cores revealed the in situ distribution pattern of the maize root system. Controlled labelling of the soil with radioactive phosphate allowed the documentation of the development and replenishment of the phosphate depletion zone around roots. Finally, the longevity and phosphate uptake activity of the different parts and tissues of the primary root system of maize was examined by electron microscopy and tracer studies including pulse chase experiments. From these studies the phosphate-acquiring strategy of the maize root system appears as follows: The capability of P uptake decreases in the order: root hairs, 1^st order laterals, 2^nd order laterals, main root. The life-spans of the components of the maize root system increase by the sequence: root hairs, laterals, main root. Inorganic P uptake, therefore, mainly occurs during the first weeks of root development. Dying back of the root occurs in an ordered manner resulting in a relocation of stored P predominantly into the main root cortex. Furthermore, it could be shown that competition for P between roots of the same or of adjacent maize and/or lupin plants virtually does not occur in situ.
• 3 The utilization of phytate-P was studied with ¹⁴C/³²P-labelled Camyo-inositol-hexaphosphate supplied to maize plants grown in sterile quartz sand or in hydroponic cultures. The ratio of P- and C-uptake as well as the incidence of phytate hydrolysis products in the rooting medium indicated the capability of maize roots to acquire P from phytate by enzymatic hydrolysis. This was confirmed by enzyme studies of the root tissues. A specific hydrolyzing enzyme (phytase; molecular weight 51 kD) could be detected in the cell wall of the root, especially in the root tip, which initiates phytate dephosphorylation. Further breakdown is presumably accomplished by monophosphoric phosphohydrolases.

     

溶磷真菌液体培养条件下对磷矿粉的溶解效果

Rock phosphate （RP） is a low efficiency P fertilizer that is directly applied to the soil and can be solubilized by phosphate-solubilizing microorganisms （PSMs） in fermentation or soil conditions. This study investigated dynamic solubilization of 2 concentrations of rock phosphate in a liquid culture with different dosages of glucose by two fungal isolates,Aspergillus niger P39 and Penicillium oxalicum P66, from soybean and wheat rhizosphere soil. Although during the 20 day culture period A. niger P39 had a stronger ability to acidify the culture media than P. ozalicum P66, soluble P concentrations at glucose dosages of 30 and 50 g L^-1 with RP of 15 g L^-1 in the culture solution were much higher by P. oxalicum P66. The greater effectiveness of P. oxalicum P66 compared to A. niger P39 in the solubilization of RP was strongly associated with the production of organic acids. This study suggested that for RP solubilization the type rather than the concentration of PSM-produced organic acids was more important.     

Glucose uptake by maize roots and its transformation in the rhizosphere

The flow of carbon from roots into the rhizosphere represents a significant C loss from plants. However, roots have the capacity to recapture low molecular weight C from soil although this is in direct competition with soil microorganisms. The aim of this study was to investigate the behaviour of glucose in rhizosphere and non-rhizosphere soil, the plant's potential to recapture sugars from soil and translocation and utilization of the recaptured sugars. In microcosms containing maize plants we injected ¹⁴C-glucose into the rhizosphere and followed its uptake into plants, upward and downward transport in the plant and soil, evolution as ¹⁴CO₂ and incorporation into the soil microbial biomass. These fluxes were compared with non-rhizosphere soil. Glucose was rapidly mineralized in soil and the rate of turnover was significantly greater in the rhizosphere in comparison to non-rhizosphere soil. The amount of glucose captured by the maize plants was low (<10% of the total ¹⁴C-glucose added) in comparison to that captured by the soil microbial biomass. Only small amounts of the ¹⁴C-glucose were transported to the shoot (0.6% of the total). The degree of glucose capture by maize roots whilst in competition with soil microorganisms was similar to similar experiments performed for amino acids. We conclude that while plant roots can recapture low molecular weight C from the rhizosphere, intense competition from soil microorganisms may reduce the efficiency of this process.     

Growth,P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability

The aim of the present study was to assess the role of soil type on growth, P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability. Two wheat (Goldmark and Janz) and two canola genotypes (Drum and Outback) were grown in a calcareous soil (pH 8.5) at two P levels [no P addition (0P) or addition of 200 mg kg⁻¹ P as Ca₃(PO₄)₂ (200P)] and harvested at flowering or maturity. Shoot and root dry weight, root length and shoot P content were greater in the two canola genotypes than in wheat. There were no consistent differences in available P, microbial P and phosphatase activity in the rhizosphere of the different genotypes. Shoot P content was significantly positively correlated with root length, pH and phosphatase activity in the rhizosphere. The microbial community composition, assessed by fatty acid methylester analysis, of the canola genotypes differed strongly from that of the wheat genotypes. The weight percentage bacterial fatty acids, the bacteria/fungi (b/f) ratio and the diversity of fatty acids were greater in the rhizosphere of the canolas than in the rhizosphere of the wheat genotypes. In contrast to the earlier studies in an acidic soil, only small differences in growth and P uptake between the genotypes of one crop were detected in the alkaline soil used here. The results confirmed the importance of root length for P uptake in soils with low P availability and suggest that the rhizosphere microbial community composition may play a role in the better growth of the canola compared to the wheat genotypes.     

几株高效溶磷菌株对不同磷源溶磷活力的比较

在液体培养条件下,研究了4株溶磷菌株(Bmp5、Bmp6、Bmp7和Fmp9)对不同磷源溶解能力的差异并与荧光假单孢菌As1.867和巨大芽孢杆菌As1.223进行了比较,探讨了菌株组合培养对溶磷活力的影响。结果表明,4株菌株对磷酸钙、磷酸铝、磷酸氢钙溶解能力明显高于磷酸铁和卵磷脂。以磷酸钙为磷源时,Fmp9的溶磷量比As1.867和As1.223分别高出约92%和48%;而以磷酸铝为磷源时,As1.223的溶磷量明显高于其他菌株;在磷酸氢钙为磷源的条件下,Bmp6为优势菌株,溶磷量高达785.51mg/L。对比研究发现,Bmp5、Bmp6、Bmp7及Fmp9的优势磷源分别为卵磷脂、磷酸氢钙、磷酸铝和磷酸钙。组合培养表明,Bmp5+Fmp9和Bmp6+Fmp9较单株菌的溶磷量有所增加,为较好的组合。试验得到的溶磷微生物配方已经应用于生物复合肥料的研究,并进行了盆栽实验,得到了较好的效果。该研究可为土壤生物肥料工业的微生物学研究提供借鉴。     

Influence of low-molecular-weight organic acids on the solubilization of phosphates

A range of low-molecular-weight organic acids were identified in rhizosphere soil, leaf litter, and poultry manure compost. Laboratory and greenhouse experiments were carried out to examine the effects of seven low-molecular-weight organic acids on phosphate adsorption by soils, and the solubilization and plant uptake of P from soil pre-incubated with monocalcium phosphate and North Carolina phosphate rock. Acetic, formic, lactic (monocarboxylic), malic, tartaric, oxalic (dicarboxylic), and citric (tricarboxylic) acids were used in the study. The addition of organic acids decreased the adsorption of P by soils in the order tricarboxylic acid>dicarboxylic acid>monocarboxylic acid. The decreases in P adsorption with organic acid addition increased with an increase in the stability constant of the organic acid for Al (logK _Al). Organic acids extracted greater amounts of P from soils meubated with both monocalcium phosphate and phosphate rock than water did. Although more phosphate was extracted by the organic acids from monocalcium phosphate — than from phosphate rock — treated soils in absolute terms, when the results were expressed as a percentage of dissolved phosphate there was little difference between the two fertilizers. The amount of P extracted by the organic acids from both fertilizers increased with an increase in logK _Al values. The addition of oxalic and citric acids increased the dry matter yield of ryegrass and the uptake of P in soils treated with both fertilizers. The agronomic effectiveness of both fertilizers increased in the presence of organic acids and the increase was greater with the phosphate rock than with the monocalcium phosphate. The results indicated that organic acids increase the availability of P in soils mainly through both decreased adsorption of P and increased solubilization of P compounds.     

The application of phosphate solubilizing endophyte Pantoea dispersa triggers the microbial community in red acidic soil

To confirm whether endophytes are members that play important roles in phosphorus (P) solubilization in red acidic soil, five endophytes that have the potential to dissolve insoluble P were isolated from cassava (Manihot esculenta Crantz) root. Based on the maximum amount of soluble P in Ca₃(PO₄)₂, AlPO₄, and FePO₄ liquid NBRIP, the strain Pantoea dispersa was selected to investigate the ability to solubilize phosphate over time. Our results showed that the solubilizing process of P. dispersa was accompanied by acid production. Beside succinate, oxalic acid, and citric acid, two special organic acids, salicylic acid, and benzeneacetic acid were found during microbial P solubilization. Based on PCR-DGGE (denaturing gradient gel electrophoresis) analysis, soil application of P. dispersa triggered natural soil microbial activity. This phenomenon could be maintained up to 25 days, suggesting that the endophyte P. dispersa would be a suitable candidate for optimizing agro-microecological systems via soluble P release in red acidic soil.     

Improvement of wheat and maize crops by inoculating Aspergillus spp. in alkaline soil fertilized with rock phosphate

Aspergillus tubingensis and A. niger were isolated from the landfills of rock phosphate mines and tested for their efficacy to solubilize rock phosphate (RP), and improve plant growth and phosphate (P) uptake by plants grown in soil amended with RP. The results showed that they effectively solubilized RP in Pikovskaya's (PKV) liquid medium and released significantly higher amounts of P into the medium. A. tubingensis solubilized and released 380.8 μg P mL^?1, A. niger showed better efficiency and produced 403.8 μg P mL^?1. Field experiments with two consecutive crops in alkaline agricultural soil showed that inoculation of these fungi along with RP fertilization significantly increased yield and nutrient uptake of wheat and maize plants compared with control soil. P uptake by wheat and maize plants and the available P increased significantly in the RP-amended soil inoculated with fungi compared with control. These results suggest that the fertilizer value of RP can be increased, especially in alkaline soils, by inoculating P-solubilizing fungi.     

菊芋根系分泌物改良滨海盐土的微生物机制

[目的] 探究菊芋在滨海盐土改良过程中的作用机制,分析菊芋和碱蓬根系分泌物的组分差异,明确土壤微生态环境的变化规律,进一步为盐土改良提供理论依据。[方法] 以种植菊芋和自然碱蓬植被为样地,对菊芋和碱蓬的根系分泌物进行对比分析,研究在根系分泌物作用下土壤微生物数量,微生物量碳氮,微生物群落结构以及土壤酶活性的变化,从而系统地阐明根系分泌物介导下盐土改良的微生物机制。[结果] 菊芋根际土壤中含有果糖（2.343×10^-3 g/kg）、葡萄糖（4.235×10^-3 g/kg）、蔗糖（2.670×10^-3 g/kg）,分别是碱蓬根际土壤的9.28,1.52和2.43倍。而菊芋根际与非根际中的果糖含量存在显著性差异（p<0.05）,其根际中含量为非根际的12.02倍。菊芋土壤还含有低聚果糖（蔗果三糖、蔗果四糖和蔗果五糖）,而碱蓬土壤中未检测出低聚果糖。除糖类外,菊芋根系分泌物还含有烷烃、酚、醛、酯、有机酸、醇、酮、酰胺,其组分较碱蓬土壤更为复杂且某些组分为菊芋特有〔1-氯—十八烷、正十六烷酸、2-甲基-Z-4-十四碳烯、十二酮、（Z）-9-十八碳酰胺、苯丙酸十六烷基酯等〕。功能性根系分泌物（如低聚果糖、果糖、十六烷、十八烷酸等）为根际微生物提供碳源、氮源和营养元素的同时,使菊芋根际土壤中微生物数量显著增加（p<0.05）,土壤微生物量碳、氮显著高于碱蓬土壤（p<0.05）,其值分别是碱蓬土壤的1.95和1.6倍,且菊芋根际的微生物量碳、氮约为非根际的1.69和1.50倍,优势菌群（变形菌门、放线菌门、绿弯菌门、酸杆菌门）所占比重达到90%,土壤有益菌群（Actinobacteria和Acidobacteria）的相对丰度显著增加（p<0.05）,土壤生物活性提升。此外,菊芋根际特有的分泌物（十六烷、烯醛等）,抑制了病原菌的生长,优化了微生物群落结构。除过氧化氢酶外,土壤脲酶、蔗糖酶和碱性磷酸活性显著提高（p<0.05）,其活性分别是碱蓬土壤的1.83,1.88和3.30倍。[结论] 种植菊芋后,通过根际分泌物介导,改善土壤微生物群落结构与功能,增加土壤酶活性,使土壤生物活力得以整体提升,与原生植被碱蓬相比,降低了土壤含盐量,起到了改良盐土的作用。     

Microscale distribution and function of soil microorganisms in the interface between rhizosphere and detritusphere

The rhizosphere and the detritusphere are hot spots of microbial activity, but little is known about the interface between rhizosphere and detritusphere. We used a three-compartment pot design to study microbial community structure and enzyme activity in this interface. All three compartments were filled with soil from a long-term field trial. The two outer compartments were planted with maize (root compartment) or amended with mature wheat shoot residues from a free air CO₂ enrichment experiment (residue compartment) and were separated by a 50 μm mesh from the inner compartment. Soil, residues and maize differed in ¹³C signature (δ¹³C soil −26.5‰, maize roots −14.1‰ and wheat residues −44.1‰) which allowed tracking of root- and residue-derived C into microbial phospholipid fatty acids (PLFA). The abundance of bacterial and fungal PLFAs showed clear gradients with highest abundance in the first 1–2 mm of the root and residue compartment, and generally higher values in the vicinity of the residue compartment. The δ¹³C of the PLFAs indicated that soil microorganisms incorporated more carbon from the residues than from the rhizodeposits and that the microbial use of wheat residue carbon was restricted to 1 mm from the residue compartment. Carbon incorporation into soil microorganisms in the interface was accompanied by strong microbial N immobilisation evident from the depletion of inorganic N in the rhizosphere and detritusphere. Extracellular enzyme activities involved in the degradation of organic C, N and P compounds (β-glucosidase, xylosidase, acid phosphatase and leucin peptidase) did not show distinct gradients in rhizosphere or detritusphere. Our microscale study showed that rhizosphere and detritusphere differentially influenced microbial C cycling and that the zone of influence depended on the parameter assessed. These results are highly relevant for defining the size of different microbial hot spots and understanding microbial ecology in soils.