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SUMMARYVariation among VA mycorrhizal fungi in their ability to enhance phosphorus uptake and plant growth could be due to differences in the length, distribution and phosphorus uptake of external hyphae. Trifolium subterraneum L. was grown in a sandy soil in association with the VA mycorrhiza] fungi Acaulospora laevis Gerdemann & Trappe, Glomus sp., Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders or left uninoculated. When mycorrhizas had become well established, plants were transferred to a two-compartment system, where root growth into a hyphal compartment was restricted by a fine nylon mesh. Spread of hyphae into the hyphal compartment was monitored by sequential sampling of soil cores at difFerent distances from the root compartment. Plants were harvested at three times.The spread of hyphae differed between fungi. The length density of hyphae of S. calospora declined approximately exponentially with increasing distance from the roots, whereas A. laevis maintained a plateau of constant hyphal density up to 7 and 11 cm from the roots after 28 and 47 d, respectively. Glomus sp. had an intermediate pattern of spread with a plateau closest to the roots followed by an exponential decline. The average rate of hyphal spread (mm d"') was 3-1 for A. laevis but only 0'7-0-8 for Glomus sp. and S. calospora. In addition to its more extensive hyphal spread, A. laevis produced the largest increases in phosphorus uptake and plant growth. All mycorrhizal plants had a larger phosphorus inflow than the non-mycorrhiza! controls, but the phosphorus inflow per unit mycorrhizal root length was 2-5-3 times as high with A. laevis as with the two other fungi. These differences between the fungi became even more pronounced when the phosphorus inflow was expressed on the basis of hypha! length.This work indicates that the efficiency of phosphorus uptake by a VA mycorrhizal fungus is strongly affected by its spatial distribution of hyphae in the soil, and possibly also by differences in capacity for uptake by unit length of hypha.

Advances in ecology during the past decade have led to a much more detailed understanding of the potential negative consequences of species' introductions. Moreover, recent studies of mycorrhizal symbionts have led to an increased knowledge of the potential utility of fungal inoculations in agricultural, horticultural and ecological management. The intentional movement of mycorrhizal fungal species is growing, but the concomitant potential for negative ecological consequences of invasions by mycorrhizal fungi is poorly understood. We assess the degree to which introductions of mycorrhizal fungi may lead to unintended negative, and potentially costly, consequences. Our purpose is to make recommendations regarding appropriate management guidelines and highlight top priority research needs. Given the difficulty in discerning invasive species problems associated with mycorrhizal inoculations, we recommend the following. First, careful assessment documenting the need for inoculation, and the likelihood of success, should be conducted prior to inoculation because inoculations are not universally beneficial. Second, invasive species problems are costly and often impossible to control by the time they are recognized. We recommend using local inoculum sources whenever possible. Third, non-sterile cultures of inoculum can result in the movement of saprobes and pathogens as well as mutualists. We recommend using material that has been produced through sterile culture when local inoculum is not available. Finally, life-history characteristics of inoculated fungi may provide general guidelines relative to the likelihood of establishment and spread. We recommend that, when using non-local fungi, managers choose fungal taxa that carry life-history traits that may minimize the likelihood of deleterious invasive species problems. Additional research is needed on the potential of mycorrhizal fungi to spread to non-target areas and cause ecological damage.

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Abstract1. Biofertilizers are a large part of the global agricultural economy.2. Recently, there has been an increase in the number of companies producing fungal inoculants.3. Whether these inoculants are useful is not clear; they are difficult to monitor in the field. The unintended consequences of inoculants in natural systems is not known, but ifinvasive, they may pose a threat to soil and plant biodiversity and ecosystem functioning. K E Y W O R D Sarbuscular mycorrhizal fungi, biofertilizers, fungal inoculants, invasive fungi, sustainable agriculture

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Summary Hyphae formed in soil by the vesicular‐arbuscular (VA) mycorrhizal fungus, Glomusfasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe were extracted using the membrane filter techniqe and their length was estimated by the grid intersection method. The effect of phosphorus on the formation of this external mycelium was assessed after sampling procedures had been investigated. Phosphorus supply was varied from amounts severely deficient to those adequate for the growth of subterranean clover. After 6 weeks, the alleviation of severe phosphorus deficiency increased both the length of infected root and the length of external hyphae per centimetre of infected root. Further additions of phosphorus decreased both of these measurements. However, the level of added phosphorus at which the most external hyphae was formed per centimetre of infected root was higher than the level of phosphorus which gave the greatest length of infected root. The increase in phosphorus supply which gave the greatest increase in the length of external hyphae per centimetre of infected root also decreased the formation of vesicles within infected roots. At phosphate levels adequate for growth of mycorrhizal plants, there was little development of either external hyphae in soil or vesicles within the mycorrhizal roots.

SUMMARYThe infectivity of vesicular-arbuscular (VA) mycorrhizal fungi m three soils from different vegetation types was compared before and after disturbance. The relative quantities of infective propagules in the disturbed soils were estimated by an infectivity test after the soils were diluted. Spore numbers and mycorrhizal roots were also quantified in each soil.The mycorrhizal colonization of clover roots in tbe infectivity test was not decreased after soil from an annual pasture had been disturbed. In contrast, in both a forest soil and a heathland soil, the percentage root length colonized of test plants was almost halved if the soils had been disturbed. In the pasture soil up to 10 times or 25 times more propagules survived disturbance than in the forest soil or the beatbland soil respectively. The large number of propagules may allow the VA mycorrbizal fungi in the pasture soil to maintain maximum infectivity after soil disturbance.