Observations on soil fungistasis

Summary Fungistasis property of soil samples from forest area, cultivated field, and grassland field has been studied. Soil samples from different depths were collected in summer, rainy, and winter seasons. The effect of amendment of certain fungal isolates, culture filtrate, mycelial extract and certain plant materials in sterilized soil has also been studied. Generally the fungistasis property was found to be higher in soil samples of upper profiles and it gradually decreased with an increase in soil depth in all the three localities. Highest fungistasis was observed during rainy season when fungal population was also found to be higher. Amongst three localities, higher values were always noticed in forest soil samples. Physico-chemical characters seem to play an indirect role in inducing the soil fungistasis. Fungal spores, culture filtrates and mycelial extracts of certain fungi caused different levels of fungistasis in sterilized soil. Sterilized soil samples amended with various plant parts also caused fungistasis after certain period. Thus fungistasis seems to be a combined effect of various factors existing in the soil.     

Germination of conidia ofVerticillium dahliae in soil

Summary A study was made of the effect of soil fungistasis on the germination of conidia ofVerticillium dahliae Kleb. Conidia germinated on steam or propylene oxide sterilized soil, but not on unsterilized soil of six types tested. Peptone (0.5 per cent) nullified fungistasis. Glucose (0.5 per cent) had no effect on fungistasis. Wheat straw or lignin amended to unsterilized soil had no effect on spore germination after sterilization. Fungistasis was restored to sterilized soil by incorporating natural soil at various ratios or by lowering the soil pH. Fungistasis was also restored by incubating various micro-organisms in sterile soil. The fungistasis so obtained was nullified by incorporation of nutrients or by sterilization. The results suggest that nutrient deficiency as well as inhibitory substances of microbial origin might play a role in soil fungistasis. 5012 plant and soil     

Fungal diversity increases soil fungistasis and resistance to microbial invasion by a non resident species

Biodiversity decline is a major concern for ecosystem functioning. Recent research efforts have been mostly focused on terrestrial plants, while, despite their importance in both natural and artificial ecosystems, little is known about soil microbial communities. This work aims at investigating the effects of fungal species richness on soil invasion by non resident microbes. Synthetic fungal communities with a species diversity ranging from 1 to 8 were assembled in laboratory microcosms and used in three factorial experiments to assess the effect of diversity on soil fungistasis, microbial invasion of soil amended with plant litter and of plant rhizosphere. The capability of different microbes to colonize environments characterized by different resident microbial communities was measured. The number of microbial species in the microcosms positively affected soil fungistasis that was also induced more rapidly in presence of synthetic communities with more species. Moreover, the increase of resident fungal diversity dramatically reduced the invasibility of both soil and plant rhizosphere. We found lower variability of soil fungistasis and invasibility in microcosms with higher species richness of microbial communities. Our study pointed out the existence of negative relationships between fungal diversity and soil invasibility by non resident microbes. Therefore, the loss of microbial species may adversely affect ecosystem functionality under specific environmental conditions.     

土壤抑真菌作用与细菌群落结构的关系

自然清洁土壤所具有的抑真菌作用,是健康土壤的一种自然属性,也是土壤质量的重要指标之一,对控制农作物土传病原真菌的爆发具有积极的生态学意义.本试验以中国科学院沈阳生态实验站近10年未受农药和肥料影响的撂荒地土壤作为自然清洁土壤样品,通过高温（对照、100 ℃、110 ℃、121 ℃）处理得到具有不同抑真菌作用的土壤模型,采用PCR DGGE(变性梯度凝胶电泳)方法对上述土壤样品的细菌群落结构进行分析.结果表明：土壤抑真菌作用与土壤细菌群落结构紧密相关;对照清洁土壤抑真菌作用最强,处理后土壤细菌群落结构偏离自然清洁土壤愈远,土壤抑真菌能力愈差;DGGE特异性条带切胶测序结果表明,Sphingomonas asaccharolytica、Nitrospira sp.、Hyphomicrobiaceae sp.、Bacillus megaterium和Micrococcus sp.可能与土壤抑真菌作用密切相关.     

Studies on soil fungistasis: Effect of certain physical and biological factors

Summary The effect of different physical and biological factors like soil sterilization, incubation period of soil, spore age, amendment of certain fungal species and their metabolites on soil fungistasis has been investigated.Different degree of sterilization affected the fugistasis differently. Soil heating above 80°C completely annulled the fungistasis. No fungistasis was recorded in soil samples steamed for 15 mts in an autoclave.Incubation of soil samples to longer duration resulted in increased fungistasis. Maximum fungistatic value was noted in samples incubated for 15 days at 25±1°C.Spore age also played important role in fungistasis. A positive relation was noted in the spore age and fungistasis upto 30 days of age and thereafter the increase in fungistasis was not well marked.Varying inhibitory effect was noted on the spore germination of the test fungi in relation to amendment of certain fungi individually and in different combinations to the soil.Aspergillus flavus alone and in combination ofAspergillus niger proved most inhibitory. The filtrate of the different fungi also induced fungistasis in soil. In this case alsoA. flavus was most effective.     

Microbial community composition affects soil fungistasis

Most soils inhibit fungal germination and growth to a certain extent, a phenomenon known as soil fungistasis. Previous observations have implicated microorganisms as the causal agents of fungistasis, with their action mediated either by available carbon limitation (nutrient deprivation hypothesis) or production of antifungal compounds (antibiosis hypothesis). To obtain evidence for either of these hypotheses, we measured soil respiration and microbial numbers (as indicators of nutrient stress) and bacterial community composition (as an indicator of potential differences in the composition of antifungal components) during the development of fungistasis. This was done for two fungistatic dune soils in which fungistasis was initially fully or partly relieved by partial sterilization treatment or nutrient addition. Fungistasis development was measured as restriction of the ability of the fungi Chaetomium globosum, Fusarium culmorum, Fusarium oxysporum, and Trichoderma harzianum to colonize soils. Fungistasis did not always reappear after soil treatments despite intense competition for carbon, suggesting that microbial community composition is important in the development of fungistasis. Both microbial community analysis and in vitro antagonism tests indicated that the presence of pseudomonads might be essential for the development of fungistasis. Overall, the results lend support to the antibiosis hypothesis.     

Microbial Community Composition Affects Soil Fungistasis

Most soils inhibit fungal germination and growth to a certain extent, a phenomenon known as soil fungistasis. Previous observations have implicated microorganisms as the causal agents of fungistasis, with their action mediated either by available carbon limitation (nutrient deprivation hypothesis) or production of antifungal compounds (antibiosis hypothesis). To obtain evidence for either of these hypotheses, we measured soil respiration and microbial numbers (as indicators of nutrient stress) and bacterial community composition (as an indicator of potential differences in the composition of antifungal components) during the development of fungistasis. This was done for two fungistatic dune soils in which fungistasis was initially fully or partly relieved by partial sterilization treatment or nutrient addition. Fungistasis development was measured as restriction of the ability of the fungi Chaetomium globosum, Fusarium culmorum, Fusarium oxysporum, and Trichoderma harzianum to colonize soils. Fungistasis did not always reappear after soil treatments despite intense competition for carbon, suggesting that microbial community composition is important in the development of fungistasis. Both microbial community analysis and in vitro antagonism tests indicated that the presence of pseudomonads might be essential for the development of fungistasis. Overall, the results lend support to the antibiosis hypothesis.     

固相微萃取-气质法测定土壤挥发性抑菌物质

运用固相微萃取．气相色谱,质谱法(SPME-GC／MS),测定了参与土壤抑真菌作用的土壤挥发性成分和土壤细菌挥发性代谢物。通过比较土壤来源和土壤细菌来源的挥发性抑菌成分,发现在强挥发性抑菌土壤和土壤细菌代谢物中普遍存在着三甲胺、二甲基二硫醚、3-甲基-2-戊酮、甲基吡嗪、2,5-二甲基吡嗪、N,N-二甲基辛胺、十九烷等化合物。这些化合物很有可能就是参与土壤抑菌作用,特别是挥发性物质抑菌作用的主要成分。另外,为深入了解土壤中参与抑菌作用的挥发性化合物提供了简便有效的方法。     

FUNGISTASIS IN SOILS

1. Fungistasis in soil is a widespread phenomenon affecting most fungal propagules, though some are insensitive. In most instances, it is coexistent with the presence of living microorganisms, and is annulled by energy-yielding nutrients. Fungistasis with characteristics similar to that in soil may also occur on leaves of plants. 2. Germination and growth of bacteria and actinomycetes is also restricted in soils. The characteristics of their inhibition appear to be the same as those for fungi. Therefore, the concept of a widespread microbial inhibition in soil can be applied to all three groups of microorganisms. 3. Fungistasis can be detected by various direct methods, or indirectly by methods involving the use of porous or permeable carriers. It may be expressed as a restriction on the final amount of germination (the usual parameter), germination rate (with time), and rate of germ-tube or hyphal growth. Since the expression of fungistasis is often complete in soil, titration with nutrients may be required to distinguish between the sensitivities of different fungi. 4. Fungistasis generally is expressed most strongly at soil moisture contents somewhat less than saturation. Its expression usually is maximal in neutral or slightly alkaline soils. In acidic conditions fungistasis may be lessened because of suppression of bacterial and actinomycete activity. Increased sensitivity of some fungi in soils of pH > 7.0 may be caused by a directly unfavourable effect of pH on the fungus. 5. Fungal species with small spores tend to be highly sensitive to fungistasis. These spores tend to germinate slowly and to require exogenous nutrients for germination. By contrast, species with larger spores and sclerotia often do not require exogenous nutrients for germination. The larger spores tend to germinate rapidly and to exhibit low sensitivity, as compared with small spores. A few nutrient-independent spores are insensitive to fungistasis. At least a part of the difference in sensitivity is related to germination time; spores which germinate slowly compete poorly with the soil micro-flora for their nutrients. 6. Fungistasis is often temporarily annulled by enriching the soil with energy-yielding nutrients. Usually, complex materials such as plant residues are most effective. A few weeks after such treatment, the level of fungistasis may, however, be increased. Annulment of fungistasis by compounds not utilized as energy sources has not yet been demonstrated. 7. Several soils naturally suppressive to Fusarium wilt diseases were more fungistatic to Fusarium than soils conducive to wilt. Potential means by which fungistasis may be manipulated to control root-infecting fungi are (a) through stimulation of germination with nutrients, thus exposing the germ tube to lysis, and (b) by increasing the fungistatic level of soil through appropriate amendments. 8. Volatile substances identified in soils, some of which are potentially inhibitory to fungi include (a) ammonia, which apparently is evolved from ammonium salts in some arid soils of high pH, (b) ethylene, which has been identified in some soils of pH < 7.0 (though high levels of this gas seem to be tolerated by most fungi), (c) allyl alcohol, and (d) other unidentified substances. Non-volatile inhibitors include high molecular weight substances revealed by molecular sieve chromatography of soil extracts. Microbial metabolites such as those present in staled fungal cultures also have been proposed to account for fungistasis. In a few soils fungistasis persists after sterilization because of the presence of inhibitory concentrations of calcium carbonate, iron or aluminium. Inherent in the proposition that inhibitory substances provide the primary mechanism of fungistasis is the concept of a highly complex phenomenon, involving various highly specific inhibitory and counteracting stimulatory substances, with the outcome for the fungus depending on the kinds and relative amounts of each present. 9. By the nutrient-deficiency hypothesis, the level of available nutrients in soil is insufficient to support germination of nutrient-dependent propagules, except in nutrient-rich microsites. Inhibition of nutrient-independent propagules is explained by loss of endogenous nutrients required for germination, through microbial nutrient competition. Evidence for this hypothesis is (a) the imposition of fungistasis on numerous nutrient-independent propagules during incubation on leaching model systems designed to simulate microbial nutrient competition in soil, (b) similar losses of endogenous nutrients occurring on soil and the leaching system, and (c) the fact that soils are chronically deficient in energy in relation to the microbial populations present, with the consequence that enforced inactivity is imposed upon most of the population at any given time for this reason alone, regardless of the presence or absence of fungistatic substances. Journal series article no. 7747 from the Michigan Agricultural Experiment Station.     

Fungistatic activity of different Indian soils againstFusarium udum Butler

Summary The fungistatic activity of different Indian soils againstF. udum, causing wilt disease of pigeon-pea, has been assessed in relation to soil pH and organic matter. Correlation co-efficient between soil characteristics and fungistasis has been calculated to evaluate relationship. High fungistatic activity was exhibited generally by the soils having low pH but high organic matter. The soils exhibiting high fungistatic activity againstF. udum have low incidence of wilt disease of pigeon-pea. There was no definite correlation between volatile and non-volatile fungistasis and, therefore, the origins of volatile and non-volatile fungistasis are different. In the usual biological limit pH had insignificant impact on fungistasis.     

Cross-Site Soil Microbial Communities under Tillage Regimes: Fungistasis and Microbial Biomarkers

The exploitation of soil ecosystem services by agricultural management strategies requires knowledge of microbial communities in different management regimes. Crop cover by no-till management protects the soil surface, reducing the risk of erosion and nutrient leaching, but might increase straw residue-borne and soilborne plant-pathogenic fungi. A cross-site study of soil microbial communities and Fusarium fungistasis was conducted on six long-term agricultural fields with no-till and moldboard-plowed treatments. Microbial communities were studied at the topsoil surface (0 to 5 cm) and bottom (10 to 20 cm) by general bacterial and actinobacterial terminal restriction fragment length polymorphism (T-RFLP) and phospholipid fatty acid (PLFA) analyses. Fusarium culmorum soil fungistasis describing soil receptivity to plant-pathogenic fungi was explored by using the surface layer method. Soil depth had a significant impact on general bacterial as well as actinobacterial communities and PLFA profiles in no-till treatment, with a clear spatial distinction of communities (P < 0.05), whereas the depth-related separation of microbial communities was not observed in plowed fields. The fungal biomass was higher in no-till surface soil than in plowed soil (P < 0.07). Soil total microbial biomass and fungal biomass correlated with fungistasis (P < 0.02 for the sum of PLFAs; P < 0.001 for PLFA 18:2ω6). Our cross-site study demonstrated that agricultural management strategies can have a major impact on soil microbial community structures, indicating that it is possible to influence the soil processes with management decisions. The interactions between plant-pathogenic fungi and soil microbial communities are multifaceted, and a high level of fungistasis could be linked to the high microbial biomass in soil but not to the specific management strategy.     

Compromised Chemical Toxicity in Year-Weathered Soils: Implications for Compromised Toxicity Factors

With contaminated terrestrial sites always being multiple decades old before they first submit to health risk assessments for humans and ecological receptors, there is great opportunity for soils to elicit markedly lesser chemical toxicity than would be expected. Soil aging and weathering foster various physico-chemical processes that reduce the toxic potency or bioavailability of sequestered chemicals. Because only brand new and unadulterated chemicals with seemingly maximum potencies are used in animal dosing that supports toxicity factor derivation, measured chemical concentrations in soil can be misleading, producing exaggerated risk and hazard outcomes. We sought to determine the extent to which toxicity reduction occurs in experimentally amended soils, working with large soil volumes exposed to the unimpeded ambient condition for a calendar year. A broad toxicity testing matrix for two chemicals (i.e., multiple test species, endpoints, effect level concentrations, and soil types), found species’ responses in contaminated soils to be indistinguishable from those in control soil 80% and 98% of the time for the inorganic and organic compounds used, respectively; a case in point was lead with a soil concentration of 11,000 mg/kg. The results suggest that incorporating a toxicity reduction term is an indispensable task when deriving toxicity factors.     

Colonization capacity of the soil micro-organisms in relation to fungistasis

Summary Evans' tube and cellophane agar-disc techniques were used to evaluate the colonization capacity of the soil microbes and fungistasis, respectively. Spore germination ofAlternaria tenuis, Curvularia geniculata, Helminthosporium rostratum andPestalotia sp. was studied on a natural soil. Aspergillus fumigatus was the pioneer colonizer, followed in the order of decreasing colonizing capacity by,A. flavus, A. niger, A. terreus andPenicillium nigricans. Sensitivity of test spores to soil fungistasis declined in the order,C. geniculata → Pestalotia sp. →A. tenuis → H. rostratum. The fungistatic level of the soil samples decreased gradually from side arms 1 to 7 of the Evans tube, inoculated with unsterilized soil from one end. Studies conducted with an inoculum of four soil fungi (A. fumigatus, A. flavus, A. niger, andP. nigricans) as well as those involving fumigation of Evans tube by CS₂ also proved the greater colonizing capacity ofA. fumigatus. The level of fungistasis followed closely the colonization pattern of this fungus. When added individually to the soil, all fungi were able to inhibit spore germination of test fungi to some degree or other.     

Movement of water and heavy metals (Zn, Cu, Pb and Ni) through sand and sandy loam amended with biosolids under steady-state hydrological conditions

New guidelines for using biosolids in UK agriculture favour the use of enhanced treated biosolids, such as dried and composted cakes, due to concerns about the potential for transfer of pathogens into the food chain. However, there is a need to ensure that their use is environmentally acceptable and does not increase the risk to potable water supplies or the food chain from other contaminants such as heavy metals and xenobiotic organic chemicals. The objective of this study was to determine whether the use of composted and dried mesophilic anaerobically digested dewatered (MADD) biosolids would increase the risk of heavy metal leaching from cultivated horizons when compared to more conventionally used MADD cake. Three biosolids (MADD sewage sludge cake - fresh, dried and composted) were mixed with a sand (typic quartzipsamments, %OM = 3.0, pH = 6.5) or a sandy loam (typic hapludalf, %OM = 4.8, pH = 7.6) at an application rate equivalent to 250 kg N/ha/y resulting in loadings of approximately Zn: 6 microg, Cu: 2 microg, Pb: 5 microg and Ni: 0.2 microg/g of soil dry weight basis. These amended soils were repacked into columns (0.4 m by 0.1 m internal diameter) and leaching of Zn, Cu, Pb and Ni was investigated following application of two 24 h simulated rainfall events of 4.5 mm/h. Water balance data and the use of conservative tracers (Cl- and Br ) showed that the hydrological regimes of each core were comparable and, thus, unlikely to account for differences in metal leaching observed. Although no significant difference (P = 0.05) was observed between biosolid amended and control soils, those amended with composted sludge consistently gave higher loss of all metals than did the control soils. Total losses of metals from compost amended soil over the two rainfall events were in the ranges, Zn:20.5-58.2, Cu:9.0-30.5, Pb:24.2-51.2 and Ni:16.0-39.8 microg metal/kg amended soil, compared with Zn:16.4-41.1, Cu:6.2-25.3, Pb:16.9-41.7, and Ni:3.7-25.4 microg metal/kg soil from the control soils. Losses of Zn, Cu, Pb and Ni from fresh MADD cake amended soils (19.8-41.3, 3.2-25.8, 21.6-51.6 and 7.6-36.5 microg metal/kg amended soil, respectively) and from dry MADD cake amended soils (10.7-36.7, 1.8-23.8, 21.2-51.2 and 6.8-39.2 microg metal/kg amended soil, respectively) were similar to the controls. Generally, quantities of metals leached followed the order Zn = Pb > Cu > Ni, which was consistent with the levels of metals in the original sludge/soil mixtures. These results suggest that composting or drying MADD biosolids is unlikely to increase the risk of groundwater contamination when compared to the use of MADD cake; therefore, the changes in UK sludge use in agriculture guidelines are satisfactory in this respect.     

Microbial formation of secondary and tertiary amines in municipal sewage.

Dimethylamine was formed in municipal sewage that was amended with creatinine, and trimethylamine was formed from choline or phosphatidylcholine. The maximum level of product that accumulated was equivalent to 0.13, 0.096, and 6.7% by weight, respectively, of the added chemicals. No dimethylamine or trimethylamine was detected in sewage amended with betaine, and no dimethylamine was found in sewage that was amended with methylamine and methionine.     

Impact of dibenzofuran/dibenzo-<Emphasis Type="Italic">p</Emphasis>-dioxin amendment on bacterial community from forest soil and ring-hydroxylating dioxygenase gene populations

The impact of dibenzofuran (DF) and dibenzo-p-dioxin (DD) on the changes in bacterial community structure and the transition of catabolic genes were studied using forest soil. The bacterial community structure of soil suspensions amended with 1 μg/g of either DF or DD was analyzed by 16S rRNA and functional gene sequencing. To analyze the functional genes in the communities, we targeted a gene sequence that functions as the binding site of Rieske iron sulfur center common to ring-hydroxylating dioxygenases (RHDs) for monocyclic, bicyclic, and tricyclic aromatic compounds. The gene fragments were polymerase chain reaction-amplified from DNAs extracted from soil suspensions spiked with either DF or DD, cloned, and sequenced (70 clones). Bacterial community analysis based on 16S rRNA genes revealed that specific 16S rRNA gene sequences, in particular, phylotypes within alpha-Proteobacteria, increased in the soil suspension amended with DF or DD. RHD gene-based functional community analysis showed that, in addition to two groups of RHD genes that were also detected in unamended soil suspensions, another two groups of RHD genes, each of which is specific to DF- and DD-amended soil, respectively, emerged to a great extent. The DD-specific genotype is phylogenetically distant from any known RHDs. These results strongly suggest that soil microbial community potentially harbors a wide array of organisms having diverse RHDs including those previously unknown, and that they could quickly respond to an impact of contamination of hazardous chemicals by changing the microbial community and gene diversity.     

Arthrobacter globiformis and Its Bacteriophage in Soil

Bacteriophages in soil for Arthrobacter globiformis were rarely detected unless the soil was nutritionally amended and incubated. In amended soil, phage were continuously produced for at least 48 h, and this did not require the addition of host cells. Rod and spheroid stage host cells added to the amended soil encountered indigenous bacteriophage, but added phage did not encounter sensitive indigenous host cells for some time, if at all. The indigenous phage in nonincubated soil seemed to be present in a masked state which was not merely a loose physical adsorption to soil materials but required growth conditions other than lysogeny for them to increase their titers. The possibility is discussed that the indigenous host cells in nonamended soil are present in a nonsensitive spheroid state, with the cells becoming sensitive to the phage in a rate-limiting fashion as nonsynchronous outgrowth occurs for a portion of the spheroid cells.     

New approaches to establish optimum moisture content for compostable materials

This paper reports on the persistence of total and immobilised enzyme activities (urease and phosphatase) in a soil amended with organic wastes containing high levels of total-urease and phosphatase activity. Fresh organic materials showed the highest values for both total-enzymatic activities. The addition of organic waste to soil increased both total-enzymatic activities in the soil, which, after 360 days, showed values above those of the control. Immobilised enzymes were also higher in the fresh wastes than in the soil with compost, while the specific enzymatic activity levels (enzymatic activity per unit of carbon) were similar. The immobilised urease activity was greater in the amended soil than in the control. At the beginning of the incubation period, the immobilised urease activity was significantly higher in the soil amended with fresh organic wastes than with compost. However, this activity decreased with incubation, whilst the compost-immobilised urease activity increased with time. The effect of organic amendment on immobilised phosphatase activity was similar to that shown by immobilised urease but less pronounced. The persistence of both enzymes was significantly higher in the soil amended with compost than in that amended with fresh materials.     

Epidemiological studies on jute diseases

Summary Hyphae of M. phaseoli failed to grow on unsterilized natural soil and were completely lysed within 4 days of exposure. Germination of sclerotia in natural soil was inhibited indicating soil fungistasis. Lysis of mycelium and inhibition of germination of sclerotia could be annulled by addition of various organic nutrients and fertilizers to natural soil or by autoclaving the soil. Germination of dormant sclerotia in natural soil was stimulated by root-exudates of host and non-host plants. Population of sclerotia buried in unsterilized natural soil gradually declined and after 15 months only 35 per cent of the initial number could be recovered; more than 80 per cent of these germinated when nutrients were added. Data suggest poor saprophytic ability of M. phaseoli in mycelial form and the involvement of dormant sclerotia in the survival of the organism in soil.     

土壤抑菌作用对食线虫真菌及其制剂的影响*

通过我国东北及黄淮海地区主要大豆种植区11种土壤对4株定殖于大豆胞囊线虫真菌淡紫拟青霉（Paecilomyceslilacimus）M-14,厚垣孢子轮枝菌（Verticilliumchlamydosporium）JN-11SH-26和BQL－1孢子的抑制作用及其影响因子的研究,表明土壤抑菌作用普遍存在,但依土壤条件、菌种类型而异,其中以安徽宿县、济南农科院、北京、黑龙江宝泉岭农场的土壤抑菌作用最强,洗涤悬液中孢子萌发率多小于30％,济南敬家、河南开封、黑龙江八五三农场等地土壤抑菌现象不明显,孢子萌发率一般在40～80％。同一土壤对4个菌株的影响是一致的。将菌种液体发酵培养,其滤液孢子萌发率一般可提高至50％以上。加入复合添加剂制得菌剂后,孢子萌发率进一步提高。土壤中微生物数量对其抑菌作用的强弱有很大影响。一般来说土壤中微生物数量越多,抑菌作用表现得也越明显。机理探讨表明土壤中存在着一些水溶性及挥发性的抑制因子,其中水溶性成分是主导因素。