Vesicular-arbuscular Mycorrhizal Fungal Colonization Research Articles

Temporal and spatial dynamics of vesicular-arbuscular mycorrhizal fungi under the canopy of Zygophyllum dumosum Boiss. in the Negev Desert

The temporal and spatial dynamics of vesicular–arbuscular mycorrhizal (VAM) fungi were investigated in a desert ecosystem. Soil samples under the canopy of Zygophyllum dumosum were collected monthly between November 1999 and October 2000 from 0–10 and 10–20 cm depths. VAM fungal colonization and spore density were used to compare the responses of VAM fungi to soil conditions. The mean percent colonization and spore density of VAM fungi reached maximal values in November 1999 and September 2000. Soil moisture was positively correlated with colonization of VAM structure groups. Vesicular and arbuscular colonization were positively correlated with soil organic matter and total soluble N. Spore density was also positively correlated with total soluble N. Our results suggest that spore density and the extent of vesicular and arbuscular colonization are useful indicators for evaluating changes in desert soil ecosystems.

The mycorrhizal colonization of six wetland plant species at sites differing in land use history

Five wetland prairie sites and six native plant species in western Oregon were examined to determine patterns of vesicular-arbuscular mycorrhizal fungal (VAMF) colonization. The sites differed in type and intensity of past land use. VAMF colonization was tested in situ on seedlings from both field-sown seeds and from transplants. Colonization was measured as the percentage of root length with arbuscles or vesicles. All species (Deschampsia cespitosa, Downingia elegans, Eriophyllum lanatum, Hordeum brachyantherum, Microseris laciniata, and Plagiobothrys figuratus) became colonized by VAMF during the study. This is the first report of mycorrhizal colonization of these important native species. All sites supported mycorrhizal colonization of some of the experimental species. Average VAMF colonization ranged from 58% to 92% but was unrelated to subjective rankings of land use intensity. These results suggest that VAMF inoculum at all sites was sufficient to support revegetation by at least some species of native plants.

An Exploratory Study on the Influence of Vesicular-arbuscular Mycorrhizal Fungi on the Success of Weed Biological Control with Insects

This paper presents the results of an exploratory study to determine whether vesicular-arbuscular mycorrhizal (VAM) fungi affect the success of insects used as weed biological agents. No direct correlation between the colonization by VAM fungi alone and the success of root- and shoot-feeding insects was found. However, in Centaurea diffusa , plants colonized by VAM fungi had a lower shoot root ratio and increased attack and survival of the root beetle, Sphenoptera jugoslavica , in the field. Beetle success was also best on C. diffusa when the plant was growing among determinate grasses that largely cease growing after flowering in the spring or early summer. This is consistent with other reports that VAM hyphae can transfer nutrients between plant species from low to high sinks. From these results and related reports in the literature, it is suggested that plants with many stenophagous root-insects are those in which VAM fungi colonization increases nutrient partitioning to the roots with little or no decrease in plant growth. Root-feeding, rather than foliage- or seed-feeding, insects seem to be particularly effective as biocontrol agents for plants of this type.

Vesicular-arbuscular mycorrhizae in tropical sedges of southern India

Twenty-four species of sedges (representing six genera) from different vegetation types in Western Ghats, South India, were examined for vesicular-arbuscular mycorrhizal (VAM) associations. All the sedges had VAM fungal infections with hyphae and vesicles, but arbuscules were observed only in 42% of the total species. The VAM fungal colonization varied considerably between species, ranging from 9 to 62%. Root colonization was positively and negatively correlated with root diameter and root hair length, respectively. The number of VAM fungal spores in the rhizosphere varied from 5 to 86 g-1 soil. No significant relationship was found between spore numbers or root colonization and either soil pH or moisture. Four Glomus spp., one Acaulospora sp., one Sclerocystis spp., and one Gigaspora sp. were identified among the VAM fungal spores. The results reflect a high incidence of VAM in sedges occurring in Western Ghats and emphasize the need to assess the VAM status of plant species from different ecosystems to understand their mycorrhizal status.

Response of Acacia mangium to vesicular – arbuscular mycorrhizal inoculation, soil pH, and soil P concentration in an oxisol

The vesicular–arbuscular mycorrhizal (VAM) fungus Glomus aggregatum and the legume species Acacia mangium were grown together in a manganese-rich oxisol at pH 4.3 to 6.0 and at soil P concentrations favorable for VAM-host growth (0.02 mg ∙ L−1) and sufficient for non-VAM host growth (0.8 mg ∙ L−1). At the lower P concentration, vesicular–arbuscular mycorrhizal fungal (VAMF) colonization of roots increased as soil pH increased from 4.3 to 5.0. However, colonization of roots was not significantly influenced by further increases in pH. Growth of A. mangium at 0.02 mg P/L in the presence of G. aggregatum was inferior to that observed at 0.8 mg P/L, suggesting that there was some degree of host–endophyte incompatibility. Increasing P concentration from 0.02 to 0.8 mg P/L increased target soil pH in the unlimed soil from 4.3 to 4.8 and reduced the concentration of available soil Mn from 13.2 to 2.1 mg ∙ L−1. Thus, the better plant growth observed at the higher P concentration at pH < 5 was mainly due to the alleviation of Mn toxicity due to the precipitation of the cation directly by excess phosphate and (or) phosphate-induced elevation of soil pH. The poor VAMF inoculation effect observed at the lower soil P level in the unlimed soil was thus largely due to the toxicity of high concentrations of Mn2+ and H+ ions. Keywords: hydrogen ion, calcium, manganese-rich, manganese toxicity, pinnule P, soil acidity, VAMF colonization, VAMF effectiveness.

Mycorrhizal inoculation effect inAcacia mangiumgrown in an acid oxisol amended with gypsum

Abstract Glomus aggregatum and Acacia mangium were interacted in an acid manganese (Mn)‐rich oxisol unamended or amended with hydrated lime [Ca(OH)2] or gypsum (CaSO4) at soil phosphorus (P) concentrations considered optimal for mycorrhizal host growth. Vesicular‐arbuscular mycorrhizal fungal (VAMF) colonization as well as VAMF function was significantly curtailed if soil was unamended with gypsum or lime. The highest mycorrhizal inoculation effect (MIE) was observed in the soil treated with gypsum at the rate of 0.32 g of calcium (Ca)/kg followed by the limed soil. Higher concentrations of gypsum deleteriously affected VAMF infectivity and effectivity. The first increment of gypsum compensated for part of the VAMF colonization and for all of the mycorrhizal inoculation effect that was lost due to low pH. The better MIE observed in the gypsum treated soil compared to that which was amended with lime suggests that the sensitivity of the acacia‐VAMF association to soil acidity was more a function of Ca inad...

Dependency ofCassia siameaon vesicular arbuscular mycorrhizal fungi

Abstract Thirty‐day‐old seedlings of Cassia siamea were transplanted into pots containing a subsurface Oxisol uninoculated or inoculated with Glomus agaregatum at two target soil solution phosphorus (P) concentrations. While no evidence of Vesicular‐arbuscular mycorrhizal fungal (VAMF) colonization was noted in the uninoculated soil, C. siamea roots were colonized to the extent of 63 and 61% at soil P concentrations of 0.02 and 0.2 mg/L, respectively. VAMF colonization led to significant increases in tissue P concentrations measured at harvest at both soil P concentrations. However, shoot dry matter yield was significantly increased only at the first soil P concentration. Shoot dry matter yield of mycorrhizal C. siamea at soil P concentration of 0.02 mg/L was comparable to mycorrhizal growth of C. siamea at soil P concentration of 0.2 mg/L but inferior to the nonmycorrhizal growth of the legume. Based on these response patterns, C. siamea was classified as a highly mycorrhizal dependent species.

Mycorrhizal inoculation effect on nodulation and N accumulation in Cajanus cajan at soil P concentrations sufficient or inadequate for mycorrhiza-free growth

The interaction of Cajanus cajan with Rhizobium and vesicular-arbuscular mycorrhizal fungi (VAMF) was investigated in a greenhouse experiment. C. cajan was planted in soil that had been inoculated with Glomus aggregatum or treated with benlate to suppress VAMF activity. Initial soil solution P concentrations of 0.06, 0.2, 0.4, and 0.8 mg l-1 were established to test the interaction at external P levels that ranged from inadequate to nonlimiting for the host plant. At 0.06 and 0.2 mg P l-1, mycorrhizal inoculation significantly increased plant P concentrations as well as nodule numbers and shoot dry weight. Mycorrhizal inoculation also significantly increased nodule dry weight at a soil P concentration of 0.4 mg l-1 but did not significantly influence any of the other variables. The mycorrhizal inoculation effect observed at this soil solution P concentration could not be explained by any of the measures of plant P status. At 0.8 mg P l-1, none of the measured variables were affected significantly by mycorrhizal inoculation. The results indicate that the enhanced nodulation associated with mycorrhizal inoculation at soil P concentrations lower than 0.4 mg l-1 was explainable by mycorrhizal-mediated P uptake. The small but significant increase in nodule mass due to VAMF inoculation at 0.4 mg P l-1 suggests that factors not related to plant P nutrition may be involved. On the other hand, the lack of a VAMF inoculation effect at 0.8 mg P l-1 despite VAMF colonization at a level comparable to that observed at the former P concentration appear to discount this hypothesis. This observation is also supported by the lack of response of plant N status and nodule number to VAMF inoculation at this soil P concentration.

Limitation of vesicular-arbuscular mycorrhizal activity in Leucaena leucocephala by Ca insufficiency in an acid Mn-rich oxisol

Glomus aggregatum and Leucaena leucocephala were interacted in an acid Mn-rich oxisol unamended or amended with lime [Ca(OH)2] or gypsum (CaSO4) at soil P concentrations considered optimal for mycorrhizal host growth and sufficient for mycorrhiza-free growth. At 0.02 mg P 1-1, both vesicular-arbuscular mycorrhizal fungal (VAMF) colonization and function were significantly curtailed if soil was not amended with gypsum or lime. The highest mycorrhizal effect was observed in the limed soil, followed by the soil treated with gypsum at the rate of 32 g Ca kg-1 soil. Higher concentrations of gypsum deleteriously affected VAMF infectivity and effectivity. The first increment of gypsum compensated completely for the VAMF colonization and for part of the mycorrhizal effect that was lost due to low pH. The superiority of the limed soil to that amended with gypsum apparently lies in the fact that Ca supply was assured in the former and also that the adverse effects of toxic constituents such as H+ and Mn2+ were eliminated. We were unable to separate the effect of Ca on VAMF from its effect on the host because a P concentration sufficient for mycorrhiza-free growth was not attained due to interaction of some of the P with Ca to form insoluble phosphate.

Mycorrhizal and nonmycorrhizal host growth in response to changes in pH and P concentration in a manganiferous oxisol

Glomus aggregatum and Leucaena leucocephala were allowed to interact in a manganese-rich oxisol at pH 4.3–6.0 and at soil P concentrations considered optimal for mycorrhizal host growth and sufficient for nonmycorrhizal host growth. At 0.02 mg P l-1, vesicular-arbuscular mycorrhizal fungal (VAMF) colonization of roots increased as soil pH increased from 4.3 to 5.0. However, VAMF colonization of roots did not respond to further increases in pH. At pH 6.0, growth of mycorrhizal Leucaena observed at 0.02 mg P was comparable with that observed at 0.8 mg P l-1. Increasing P concentration from 0.02 to 0.8 mg P 1-1 increased target soil pH from 4.3 to 4.7 and reduced the concentration of available soil Mn from 15.1 to 1.9 mg 1-1. Thus, the normal plant growth observed at the higher P concentration at pH<5 was mainly due to the alleviation of Mn toxicity as a result of its precipitation by excess P. VAMF colonization levels observed at pH 5.0–6.0 were similar, but maximal plant growth occurred at pH 6.0, suggesting that the optimal pH for mycorrhizal formation was substantially lower than for VAMF effectiveness. The poor growth of Leucaena at the lower P concentration in the unlimed soil was largely due to high concentrations of Mn2+ and H+ ions.

Nitrogen and phosphorus requirements for raising mycorrhizal seedlings of Leucaena leucocephala in containers

A greenhouse study was undertaken to determine the nitrogen and phosphorus fertilization requirements for raising mycorrhizal seedlings in soil in containers. Seedlings of Leucaena leucocephala were grown for 40 days in dibble tubes containing fumigated or nonfumigated soil uninoculated or inoculated with Glomus aggregatum. The soil was fertilized with NH4NO3 solution to obtain 25–200 mg N kg-1 soil, and with a KH2PO4 solution to establish target soil solution P concentrations of 0.015–0.08 mg P l-1. At the end of 40 days, seedlings were transplanted into pots containing 5-kg portions of fumigated soil. Posttransplant vesicular arbuscular mycorrhizal fungal (VAMF) effectiveness, measured as pinnule P content, plant height, shoot dry weight and tissue N and P concentrations, was significantly increased by pretransplant VAMF colonization in both soils. The best posttransplant mycorrhizal colonization and mycorrhizal growth responses were observed if the nonfumigated pretransplant soil was amended with 50 mg N kg-1 soil and 0.04 mg P l-1 or if the fumigated pretransplant soil was amended with 100 mg N kg-1 soil and 0.04 mg P 1-1. There was no relationship between N∶P ratios of nutrients added to the pretransplant soil medium and shoot N∶P ratios observed after transplanting. Shoot N∶P ratio was also not correlated with root colonization level.

Inoculum potential and other VAM fungi parameters in four sugar maple forests with different levels of stand dieback

The inoculum potential of vesicular-arbuscular mycorrhizal (VAM) fungi, spore abundance, and the VAM fungi colonization levels of young feeder roots in sugar maple ( Acer saccharum, Marsh.) were investigated in four Vermont sugar maple forest stands for which different levels of crown dieback status previously were estimated. Physical and chemical properties of soils were determined and the relationships between these properties and maple root colonization and spore abundance were investigated. Propagule densities, estimated by a greenhouse assay, ranged from 0.03 to 5.23 propagules g −1 dry soil among four study sites and were strongly correlated ( r 2 = 0.704, P < 0.01) with VAM fungi colonization levels in maple roots. Colonization levels ranged from 9 to 30% of root length. The VAM fungi structures most frequently observed were intracellular coiled hyphae and arbuscules; vesicles were rarely seen. Counts of VAM fungi spores in the four forest soils ranged from 118 to 217 spores g −1 dry soil. The most frequently occurring species at all four sites were Glomus rubiforme and Glomus fasciculatum. The site with the greatest crown dieback had the lowest viable propagule density and the lowest VAM fungi colonization levels in maple feeder roots. However, the two sites with relatively little dieback differed markedly in their VAM status. Therefore, no clear relationship between sugar maple VAM and forest dieback was indicated. Correlation and multiple regression analyses suggested that concentration of soil potassium was the key predictor for variations observed in colonization levels and for the occurrence of arbuscules in root cortex cells. Spore abundance was correlated with concentrations of soil available and reserve phosphorus and soil pH. The difference in concentrations of soil potassium between the two sites with little dieback might explain the considerable difference in mycorrhizal formation on those sites.

Mycorrhizal Dependency and Nutrient Uptake by Improved and Unimproved Corn and Soybean Cultivars

AbstractPlants in nutrient‐deficient soils often benefit when colonized by vesicular‐arbuscular mycorrhizal fungi (VAMF). Plants vary, however, in responsiveness or mycorrhizal dependency (MD). The objective of this work was to evaluate MD within corn and soybean cultivars. Three improved and three unimproved corn (Zea mays L.) and three improved and two unimproved soybean [Glycine soja Siebold &amp; Zucc. and Glycine max (L.) Merr.] cultivars were evaluated for growth response, nutrient uptake (N, P, K, Ca, Mg, and Zn), and root phosphatase activity. Greenhouse studies were conducted with VAMF (Gigaspora margarita or Glomus intraradices) and without inoculation in a Iow‐P soil. The VAMF colonization of roots with inoculation ranged from 62 to 87% for soybean and 49 to 68% for corn. Soybean had a higher MD than corn, but considerable variation occurred within soybean cultivars. Relative growth of the two unimproved soybean cultivars was significantly greater with VAMF colonization than without (Soja, &gt;1900%; Mandarin, &gt;400%). Among improved cultivars, relative growth was less enhanced with colonization (BSR 201, Richland, and Swift cultivars averaged ≈ 200% greater growth with VAMF than without VAMF colonization). An unimproved corn cultivar, Reid Yellow Dent, was unresponsive to mycorrhizal colonization, whereas another unimproved cultivar, Argentine Pop, increased growth 400% with colonization. Total uptakes of N, P, K, Ca, Mg, and Zn were significantly greater (P &lt; 0.001) in mycorrhizal plants, but the concentrations (mg g−1) of N, Mg, and Ca were lower in mycorrhizal plants than in nonmycorrhizal plants. These results suggest that considerable variability exists in MD of corn and soybean cultivars when grown in P‐deficient soils, and the variability extends across both improved and unimproved cultivars.

Inoculation with vesicular-arbuscular mycorrhizal fungi from cropped soil overcomes long-fallow disorder of linseed ( Linum usitatissimum L.) by improving P and Zn uptake

Long-fallow disorder is a term describing poor crop growth that can occur after extended clean fallows of vertisol soils in the north-east grain belt of Australia. Although many crop species can suffer from this disorder, linseed ( Linum usitatissimum L.) is the most susceptible. A glasshouse experiment with linseed demonstrated that naturally-occurring propagules (spores and colonized roots) of vesicular-arbuscular mycorrhizal fungi (VAMF) extracted from a recently-cropped field soil and added into a long-fallow soil would overcome the disorder. Supplying additional VAMF propagules to the long-fallow soil had a similar effect on linseed as supplying Zn fertilizer, increasing growth in the presence of P fertilizer. Soil irradiation killed all VAMF inoculum in the long-fallow soil, resulting in uncolonized linseed plants that grew very poorly. Inoculation of irradiated soil with VAMF spores from the cropped soil resulted in early VAMF colonization of the linseed roots and good growth of the linseed tops. Inoculation of irradiated soil with VAMF-colonized roots sieved from the cropped soil resulted in delayed VAMF colonization and delayed growth of linseed tops, compared with inoculation with spores. Regression analysis including all treatments showed that dry weight of the linseed at 80 days was linearly dependent on the degree of VAMF colonization of the roots at 36 days. These results demonstrate the key role of natural VAMF inoculum in improving the nutrition and growth of field crops on farms subject to long-fallow disorder.

Soil drainage and distribution of VAM fungi in two toposequences

Much remains unknown about the distribution and abundance of vesicular-arbuscular mycorrhizal (VAM) fungi in natural ecosystems. Our objective was to determine whether soil drainage and position in the toposequence affect VAM fungal spore production and colonization of soybean ( Glycine max L. Merr.) roots. Plant roots and rhizosphere soil from 24 soils, which consisted of sets of three soils each that varied in soil drainage class from four sites located in two toposequences [Clarion-Nicollet-Webster (CNW) and Sharpsburg-Macksburg-Winterset (SMW)], were sampled. Soils were analyzed for chemical properties, and numbers of VAM fungal spores per 50 g soil were determined by a wet-sieving-and-decanting technique. The percentages of root VAM fungal colonization were determined by the grid-line intersect method. The distribution of spores among locations, series and series within locations varied significantly ( P < 0.001). Soil from the CNW toposequence averaged 417 spores (50 g) −1 soil, whereas soil from the SMW toposequence averaged 147 spores (50 g) −1 soil. Within the two toposequences, the poorly to somewhat poorly drained soils had higher spore counts than the well-drained to moderately well-drained soils. In the CNW toposequence, the mean spore counts of Nicollet and Webster soils (poorer drained) were 1.37-fold higher than those of Clarion soils (better drained). In the SMW toposequence, the mean spore counts of Macksburg and Winterset soils (poorer drained) were 2.06 fold higher than those of Sharpsburg soils (better drained). VAM fungal colonization of roots ranged from 74 to 88% in the CNW toposequence and from 68 to 86% in the SMW toposequence. No correlation was found between spore count and VAM fungal colonization.

Growth of Argentine Mesquite Inoculated with Vesicular-Arbuscular Mycorrhizal Fungi

Argentine mesquite (Prosopis alba) seedlings were inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungi, Glomus intraradices , container-grown for five months, and transplanted into a simulated landscape in Tempe, Arizona. After transplanting, mesquite trees were either drip irrigated at regular intervals or nonirrigated for one year. Six months after transplanting, VAM fungal colonization was observed only in the roots of inoculated mesquite but by 12 months, roots of both inoculated and noninoculated mesquite were colonized by VAM fungi. Higher levels of VAM fungal colonization occurred in roots of irrigated mesquite. While irrigation promoted mesquite shoot growth, VAM inoculation inhibited shoot growth of nonirrigated trees. Trunk caliper was greater for irrigated trees than for nonirrigated trees and was not affected by VAM infection. VAM fungi promoted the growth of thinner roots for irrigated trees and thicker roots for nonirrigated trees after six months. At the conclusion of the study, root growth was enhanced by drip irrigation and was not affected by previous VAM fungal inoculation.

Response of Sauropus androgynus to soil phosphorus concentration and mycorrhizal colonization

Abstract Thirty three‐day‐old seedlings of nonmycorrhizal Sauropus androgynus were transplanted into pots containing a subsurface Oxisol uninoculated or inoculated with Glomus aggregation at three target soil solution phosphorus (P) concentrations. While no evidence of vesicular‐arbuscular mycorrhizal fungal (VAMF) colonization was noticed in the uninoculated soil, sauropus roots were colonized to the extent of 54%, 60% and 38% in the inoculated soil if target soil P concentrations were 0.014, 0.02 and 0.2 mg/L. VAMF colonization led to significant increases in tissue P and shoot dry matter accumulation at the first two soil P concentrations but not at the highest concentration. Root dry matter yield of mycorrhizal sauropus was greater than that of nonmycorrhizal sauropus at all soil P concentrations tested. Although mycorrhizal inoculation effects at soil P concentrations of 0.014 and 0.02 mg/L were comparable, growth of mycorrhizal plants was greater at the latter P concentration than at the former. Gro...

Propagation and storage of vesicular–arbuscular mycorrhizal fungi isolated from Saskatchewan agricultural soils

Factors affecting the propagation of vesicular–arbuscular mycorrhizal fungi (VAMF) Glomus clarum, Glomus mosseae, and Glomus versiforme isolated from Saskatchewan soils were assessed in growth-chamber studies. Initially, lentil (Lens esculenta L.), maize (Zea mays L.), and a sorghum (Sorghum bicolor (L.) Moench) and sudangrass (Sorghum sudanense (Piper) Hitch.) hybrid were inoculated with G. clarum and grown in 2 or 6 kg of soil–sand (1:1) substrate until maturity. All three crops supported growth and spore production by G. clarum, but the maize plant yielded the largest number of spores. The ability of G. clarum to produce large numbers of spores reflects either an inherent ability of this VAMF strain to sporulate or a favorable interaction (related to spore production) with a maize host crop. Twice as many G. clarum spores per gram of soil–sand substrate were obtained from 2-kg pots compared with 6-kg pots. Enhanced spore production in the smaller pots was related to poor plant growth and, apparently, nutrient stress. Plants grown in 2-kg pots were stunted, yielded less biomass, and exhibited severe symptoms of P and N deficiency. Monospecific cultures of G. clarum, G mosseae, and G. versiforme were produced using maize plants grown in 2-kg pots. Under these conditions the level of spore production was G. clarum &gt; G. versiforme &gt; G. mosseae. Storage of the VAMF inoculants at 7 °C, compared with 25 °C, enhanced spore viability. This was evident from a reduction in VAMF colonization and spore production in wheat (Triticum aestivum L.) and the sorghum–sudangrass hybrid plants inoculated with G. mosseae spores stored at 25 °C. Osmotic stress exerted on G. mosseae spores during density-gradient centrifugation had no effect on their viability. Key words: vesicular–arbuscular mycorrhizal fungi, Glomus spp., host crops, substrate mass, nutrient stress.

Response of neem (Azadirachta indica) to soil P concentration and mycorrhizal colonization

Abstract Thirty two‐day‐old seedlings of neem (Azadirachta indica) were transplanted into pots containing a subsurface Oxisol uninoculated or inoculated with Glomus aggregatum at three target soil solution P concentrations. While no evidence of Vesicular‐arbuscular mycorrhizal fungal (VAMF) colonization was noted in the uninoculated soil, neem roots were colonized in excess of 80% in the inoculated soil, and soil P did not influence VAMF colonization significantly. VAMF colonization led to significant increases in tissue P and dry matter accumulation at the first two increments of soil P, but not at the highest concentration. Dry matter yield of mycorrhizal neem at soil P concentration of 0.02 mg L‐1 was comparable to nonmycorrhizal growth of neem at soil P concentration of 0.2 mg L‐1. Based on these data neem was classified as a highly mycorrhizal‐dependent species.

Effectiveness of VAM fungi in nonsterile soils before and after optimization of P in soil solution

Five tropical soils were either not inoculated or inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus aggregatum. The degree to which VAM effectiveness was expressed in the soils was evaluated prior and after solution P status was adjusted for optimal VAM activity. VAM effectiveness determined by monitoring P concentrations of pinnules of Leucaena leucocephala leaves as a function of time and as dry matter yield determined at the time of harvest, indicated that in three of the soils VAM effectiveness was either very restricted or altogether unexpressed irrespective of vesicular-arbuscular mycorrhizal fungal (VAMF) inoculation if soil solution P was not optimized for VAM effectiveness. After P optimization, effectiveness was significantly increased by VAMF inoculation although in four of the soils, densities of indigenous VAMF propagules greatly exceeded that attained by the inoculum after it was mixed with soil. Mycorrhizal fungal inoculation effects varied from soil to soil, depending on the extent to which the effectiveness of indigenous and introduced endophytes was enhanced by P optimization and the similarity of inherent soil solution P concentrations to the range known to be optimum for VAM effectiveness. Of the indicator variables monitored, VAMF colonization was least sensitive to treatment effects followed by shoot P concentration measured at the time of harvest.