High densities of arbuscular mycorrhizal fungi maintained during long fallows in soils used to grow cotton except when soil is wetted periodically

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1 Neto Phytol. (1997), 136, High densities of arbuscular mycorrhizal fungi maintained during long fallows in soils used to grow cotton except when soil is wetted periodically BY G. S. PATTINSON* AND P. A. McGEE School of Biological Sciences All, University of Sydney, NSW, 26, Australia {Received 26 August 1996; accepted 2 April 1997) SUMM.^RY Sequential harvests of cotton seedlings grown in soil cores enabled the quantification of the density of arbuscular mycorrhizal fungi to detect the effects of time, cultivation and periodic wetting of the soil. Cotton seedlings grown in soil cores from three locations formed arhuscular mycorrhizas at similar rates when cores were stored dry for up to 18 months. Disturbance of dry cores followed by dry storage for 18 months did not reduce the rate of establishment of mycorrhizas. Periodic wetting and drying of the cores, especially if the cores had first been disturbed, significantly reduced the rate of establishment of mycorrhizas. We suggest that long fallow disorder is possibly caused by falls of rain in clay soils of eastem Australia used to grow cotton. The proportion of the root with mycorrhizas at 3 wk was strongly correlated with the infection at. We also suggest that it might be possible to predict maximum levels of infection and early uptake of phosphate of seedlings by determining the proportion of roots that are mycorrhizal 3 wk after emergence of cotton seedlings. Key words: AM fungi, survi\al, long fallows, cotton. INTRODUCTION Cotton plants grown in the field are thought to be dependent on arbuscular mycorrhizas for maximum growth and production of lint (Rich & Bird, 1974). Arbuscular mycorrhizal (AM) fungi are thought to derive all organic energy from the host plant. The host, in return, may gain increased uptake of less mobile nutrients, in part, owing to the AM fungus forming an extensive hyphal network beyond the root (Abbott & Robson, 1991). In Australia, cotton is grown during the summer months. A winter crop may be planted to follow cotton if climatic conditions are appropriate. If the weather conditions are unfavourable, the period between crops, called the fallow^ can be much greater than 12 months. During the fallow, weeds are totally suppressed. Poor growth of seedlings of many crop plants has been detected following fallows even shorter than 12 months, in a syndrome known as 'long fallow disorder'. The disorder is associated * To whom correspondence should be addressed. ; fbiouajbio.usvd.edu.au with low densities of AM fungal propagules in soil, resulting in reduced mycorrhizal infection of the plant (Sanders & Reed, 1978; Thompson, 1987, 1994). By implication, the quantity of propagules of AM fungi had decreased with time. In the absence of a host plant, the fungus sur\'ives in soil as propagules from which future infection can arise. Propagules consist of spores and hyphae in soil and root pieces. In cotton fields, root density is low, roots decompose rapidly and are an unimportant source of propagules (McGee et al., 1997). Thus the survival of spores and hyphae in soil during the fallow will determine the propagule density of AM fungi following cotton. \'^iability of AM fungi can decline over time (Thompson, 1987; McGee et al., 1997), be reduced by cultivation or disturbance (Evans & Miller, 1988, 199; Fairchild & Miller, 1988, 199; Jasper, Abbott & Robson, 1989; McGonigle, Evans & Miller, 199; McGee et al., 1997), environmental factors such as soil temperature and relative humidity (Sylvia & Schenck, 1983; Daft, Spencer & Thomas, 1987; Ruiz-Lozano & Azcon, 1996), and application of pesticides (Trappe, Molina & Castellano, 1984) and perhaps fertilizers

2 572 G. S. Pattinson and P. A. McGee (Johnson & Pfleger, 1992). The rate or extent to which each factor affects \'iability of AA'I fungi is unclear. Methods used to quantify fungal propagules in the soil include spore counts, estimates of hyphal length, dilution series such as the most probable numbers (MPN) estimate and baiting assays (Abbott & Robson, 1991; Tommerup, 1992). There are limitations to each method. Spore counts, hyphal estimations and MPN estimates do not quantify the total number of mycorrhizal propagules in the soil. Bioassays or infectivity assays use progressive infection of roots to indicate the relative capacity of the fungal inoculum to establish infection, as a measure of the density of propagules in soil. Plants are sown in soil and harvested sequentially (Sutton, 1973; Moorman & Reeves, 1979; Abbott & Robson, 1982) or only once or twice following initiation of the experiment (Harris et al., 1987; Jasper, Abbott & Robson, 1987; Scheltema, Abbott & Robson, 1987; Shivcharn & Anderson, 1993). Potentially all propagules contribute to the assessment and the rate at which infection develops from each propagule in the soil is accounted for. However, the technique does not quantify fungal densities greater than those which result m maximum rates of infection. The technique also fails to allow for different rates of germination and hyphal growth between isolates where single harvests are used. Although the technique reflects the density of fungal propagules and describes the pattern of infection, the basis for choosing the length of the assay or the number of harvests in an experiment is rarely provided, thus limiting the predictive value of the data. Generally speaking, the pattern of infection of annual plants has three phases. Infection is initiated following a lag phase. The proportion of the root infected then increases markedly, in the log phase, and finally reaches a constant proportion of the root system (Sutton, 1973) in what has been described as a sigmoid curve (Tinker, 1975). The first phase can be attributed to the germination of propagules, their growth to the root surface and the establishment of primary infection. After the primary infection units have been formed, secondary spread takes place and this probably constitutes the most important component of the log phase. Primary infection continues during the log phase. The final plateau is usually constant when infection is measured as a proportion of the root length infected. Thus having an infectivity assay that harvests plants sequentially over the entire pattern of infection might be used to develop predictive information concerning the effects of inoculum density on plant development and production, especially where a correlation between infection and production can be demonstrated. An appropriate model is required to describe the changes if we are to quantify a pattern of infection under different experimental conditions. The pattern of mycorrhizal infection in roots appears to follow that outlined for population growth in a limited environment (Brower, Zar & von Ende, 199). The typical patterns differ in that the population of mycorrhizal fungi at time zero is zero, and that the root system, the environment, is also growing during spread of the fungus. The pattern appears to fit a generalized logistic curve, a relatively simple sigmoid curve. From this curve, we would predict that the time taken to reach the carrying capacity, or plateau, will be determined by the density of propagules in soil (Brower et al., 199). We would also predict that the carrying capacity will be determined by the time taken to reach maximum infection. Where initiation of infection is delayed or reduced, the final density of infection would also be reduced. We fitted logistic curves to data collected on the de-^'elopment of infection to determine whether and by how much, time, cultivation and rainfall reduce the population of propagules of AM fungi. M.4TERI.'\LS.A.N'D METHODS Soil cores were collected from the ' Auscot' property, 2 km west of Narrabri, NSW, 'Millawa', 15 km east of Warren, NSW and 'Warra', 3km northwest of Dalby, Qld, Australia. T h e soil from Narrabri and Dalby are fertile black cracking clays with blocky structure. The soil from Warren is a cracking clay loam. The cores were collected from within a crop of cotton, immediately before harvest, during late summer, The cotton cv. Sicala V-2 was grown at Narrabri and Dalby and the cv. CS5 at Warren. The cotton crop at Dalby had been defoliated but not harvested, before collection of cores. The top 5 cm of soil was remo\'ed and the tubes (5 cm diameter and 15 cm long) driven vertically into the soil as far as they would travel using the weight of the collector. Soil in the tubes was between 12 and 15 cm deep. Soil cores from each site were air-dried and stored in the laboratory. Cores were randomly removed at, 6, 12 and 18 months after collection to determine the effect of time on the survival of fungal propagules. To simulate cultivation, soil from half the cores collected at Auscot were individually removed and put through a 5 mm sieve. The disturbed soil was then placed back into the same tubes. The effect of cultivation was examined at, 6, 12 and 18 months after collection. A rainfall treatment was applied to both undisturbed and disturbed tubes collected from Auscot. The tubes were watered from the bottom for 48 h and then drained, and subsequently left in the growth room for 6 wk to drj'. The tubes were wetted every 3 months until used at 18 months. The data required for fitting to a logistic model should, ideally, be collected at intervals matching one life cycle of the organism (Brower et al., 199).

3 AM fungi and long fallows 573 -a cu o oc 9: 8 i 7^ 6: 5 i 4 ; 3 i 2: 1: ^ (c) / y q 9 ; id) i/ ( Y e * I S ^ c r t r Figure 1. Logistic curves fitted to data on arbuscular mycorrhizal infectivity in cotton plants grown in soil collected from Narrabri fresh (a), or stored for 6 (b), 12 (r) or 18 (d) months. ft i r-r-t * Infection was initiated within 5 d and maximum infection was reached in c. 3 d in undisturbed cores of uncultivated soil using tbe trap plant Trifolium subterraneati L. (McGee et al., 1997). In severely disturbed soil, maximum infection bad not been reacbed in 4 d. Tberefore a bioassay tbat measures infection at weekly inter\ als for at least 42 d would appear to be necessary to detect tbe normal infection curve and variations from it. Into each core a surface-sterilized, germinated seedling of the cotton cv. CS5 was transplanted. The tubes containing seedlings were randomly placed in a growtb room with a day length of 13 h at 1/^mol m"' s"' at 25 C. The night temperature was set at 2 C. The tubes were watered every second day witb deionized water. Five tubes from eacb site were destructively barvested every 7 d for. At harvest the tubes were soaked in 2 o Calgon in tap water for 3 b to disperse tbe clay. Tbe soil was tben rinsed from tbe roots, and tbe sboots and roots weigbed. A subsample of roots was taken, cleared and stained (Philips & Hayman, 197), and the total length of roots and mycorrhizas determined by tbe grid intersect metbod (Giovannetti & Mosse, 198). The remaining roots and sboots were dried at 7 C for 24 h and weighed. Phosphate (Pj) content of leaves minus petioles was determined after 4 and using the ammonium-molybdate metbod (Allen et al., 1974), except after in the 6 month harvest owing to possible interactions witb insecticide use. Seedlings sown in soil stored for 6 niontbs became heavily infested with thrips. Sixteen days after sowing, all seedhngs were sprayed with the insecticide Alaldison* at recommended rates. Data analysis Logistic curves were fitted to tbe change of infection with time using Systat 5.4 (Systat, Inc). Parallel curve analysis (Ross, 199; Mead, Curnow and Hasted, 1993 p. 261) was used to compare logistic curves. The residual sums of squares about tbe fitted models for eacb individual set of data and for tbe combined data were compared by analysis of residual variation. Linear regression was used to predict final infection () from intermediate infection (3 wk). At 2 wk, data varied enormously, and at, tbe data had reached the asymptote in some treatments and were therefore unsuitable. R E s u L T s Infectivity profiles Infection developed in roots of plants grown in undisturbed cores in a similar pattern at all sites over tbe 18 montb period (Figs 1-3). Disturbance did not significantly reduce infection over 18 months (Fig. 4). Periodic wetting and drying of the soil reduced infection (Fig. 5), particularly when the soil was also disturbed (Fig. 5). Tbe pattern of infection at all sites, at all barvests, and at Narrabri wben disturbed

4 574 G. S. Pattinson and P. A. McGee 1 T 1 Figure 2. Logistic curves fitted to data on arbuscular mycorrhizal infectivity in cotton plants grown in soil collected from Warren fresh (a), or stored for 6 (b), 12 {c) or 18 {d) months. T3 CD ^ * c.c D) c _ o o DC ; (a) : (c) \/ 1 s c t! * " ^ 1 ^ 1 8 e s ft 1 1 Figure 3. Logistic curves fitted to data on arbuscular mycorrhizal infectivity in cotton plants grown in soil collected from Dalby fresh (a), or stored for 6 {h), 12 (c) or 18 (d) months. and/or when periodically wetted, can be represented by the logistic curve y = a/(l -fe'"'''^). Location. The change in degree of infection over time (infectivity profile) at harvest, 12 and 18 months at Warren and Dalby and at 6, 12 and 18 months at Narrabri were similar {P > -5). The infectivity profiles at 6 months at Warren and Dalby and at months at Narrabri had a greater maximum growth rate (Table 1) at the inflection point compared with the other harvests, but the final level of infection reached was similar. The plants at

5 AM fungi and long fallows Figure 4. Logistic curves fitted to data on arbuscular mycorrhizal infectivity in cotton plants grown in soil collected from Narrabri and passed through a 5 mm sieve. Plants were sown in fresh {a) soil or after storage for 6 (b), 12 (c) or 18 {d) months. 1 Figure 5. Logistic curves fitted to data on arbuscular mycorrhizal infectivity in cotton plants grown in soil from Narrabri wetted and dried every 3 months. The soil was stored for 18 months either undisturbed (a) or passed through a 5 mm sieve (b) immediately after collection. 6 months were affected by a severe thrip infestatioti, which might account for the variation of the infectivity profiles. The thrips were controlled hefore the later harvests, preventing further thrip damage. As there were no differences hetween profiles at Warren and Dalhy for, 12 and 18 months and at Narrahri at 6, 12 and 18 months, we combined the data to determine if infection developed differently at each site. The infection profiles at the three locations were not significantly different (P > O'OS). Disturbance. The infectivity profiles of disturbed soils at harvest, 6 and 18 months were similar {P > -5). The logistic curve fitted the 12 months data very tightly, with a very small residual, compared with the other curves (Fig. 4). We assumed the small residual accounted for differences detected in the statistical analysis, because the curves had a similar infection profile. The infectivity profiles of cores of disturbed soil at, 6 and 18 months and undisturbed soil at 6, 12 and 18 month harvests, from Narrabri, were combined and compared. There was no significant difference between the disturbed and undisturbed treatments {P > -5). Wetting and drying. The infectivity of disturbed and undisturbed soil from Narrabri was compared after either wetting cycles or storage dry. Wetting of undisturbed or disturbed soil significantly altered the infectivity profile, reducing both the infection at and delaying the point of infiection of the

6 576 G. S. Pattimon and P. A. McGee Table 1. Optimized values of the parameters a, b and c of the logistic curve y = a/(\-\- e''~") fitted to the infectiinty profiles, value of x {wk)for the point of inflection (b/c) and the maximum rate of growth of the curve at that point {ac/4) Treatment Parameter \'alues a b c Inflection point (wk) Max. growth (% wk-') Narrabri Narrabri Dis Warren Dalby Narrabri Wet Narrabri Dis Wet , (a) I o 7 6^ % Infection (3 wk) 6 7 Figure 6. Regression line fitted to a scatter plot of mean arbuscular mycorrhizal infection at 3 and for each treatment at each site for, 12 and 18 months. curve (Figs. Id, 5a; Table 1). Disturbance before periodic wetting of the soil cores further reduced the final level of infection and the time taken for the point of inflection to be reached (Table 1). Prediction of maximum infectioti The linear regression of infection at 3 wk to predict infection at was significant (P < -1) with }-' = -68 (Fig. 6). As well, secondary infection was first observed after 3 wk indicating that nearly all infection points at 3 wk resulted from propagules in the soil. Shoot dry weight Shoot d. wt increased over the 8-wk period in all treatments. Variation within and between treatments was large, especially after the fourth week (Fig. 7). The shoot d. wt of plants grown in soil stored for 6 months were lower, owing to severe thrip infestation. The shoot d. wt of the plants grown in the periodically wetted cores was similar to those grown in the cores stored dried (Fig. 8). Phosphate concentration Concentrations of Pj in leaves of plants grown in soil from Narrabri and Dalby were similar, and consistently lower in leaves of plants grown in the soil from Warren (Table 2). Disturbance had no significant effect on the concentration of Pj. Periodic wetting and drying of the soil was consistently associated with a reduction in the concentration of Pj in leaves. After, the concentration of Pj was significantly reduced in plants grown in undisturbed cores which were periodically wetted (P < -5). The reduction was not statistically significant in disturbed, periodically wetted cores (P > -5). After, seedlings grown in the undisturbed and disturbed soils had significantly (P < -5) higher concentrations of Pj than the undisturbed and

7 A]\d fungi and long fallows Figure 8. Mean shoot d. wt of plants grown in undisturbed ( ) and disturbed ( -) soil cores collected at Narrabri and stored for 18 months, witb periodic wetting and drying of the cores e\ ery 3 months. Standard error in undisturbed soil = -19 and disturbed soil = Figure 7. Mean shoot d. wt of plants grown in cores of soil at ( ), 6 ( ), 12 ( ) and 18 ( ) months, (ci) Narrabri (undisturbed): SE at harvest months = -21, 6 months = -9, 12 months = -14 and 18 months = -16; ih) Narrabri (disturbed): SE at harvest -25 ( months), -6 (6 months), -11 (12 months), -19 (18 months); (c) Warren: SE at harvest -32 ( months), -23 (6 months), -32 (12 months), -31 (18 months); id) Dalby: SE at harvest -32 ( months), -8 (6 months), -11 (12 months), -19 (18 months). disturbed periodically wetted soils. Disturbance had no statistically significant effect on the concentration of Pj in seedlings grown m periodically wetted soils after either 4 or. DISCUSSION The sequential harvesting of cotton plants grown in soil cores collected from Narrabri, Warren and Dalby enabled the quantification of the effects of time, disturbance and periodic wetting of the soil on the density of AM fungi. The pattern of mycorrhizal development in the cotton plants grown in the soil cores collected at each site was similar to that described in other studies and followed a logistic curve in all cases. The patterns observed in soils from three geographically distant locations, were similar, leading us to suggest that variation from the initial patterns would indicate influences of various factors on propagules densities. The patterns did not change over 18 months, indicating that sufficient propagules of mycorrhizal fungi survived for 18 months in dry soil to establish maximum rates of infection in the cotton plants. Implicit in the description of long fallow disorder (Thompson, 1987, 1994) is a decline in viability as the propagules age leading to reduced infection in crops following fallows longer than 12 months. In these experimental soils, time was not the only factor in the depletion of propagules of mycorrhizal fungi. Sieving and then storing soil dry for 18 months did not significantly reduce the rate of mycorrhizal infection in cotton plants, or the concentration of Pj in the leaves, indicating that cultivation alone is unlikely to reduce infectivity. This contrasts with other research where disturbance was associated with a reduction in mycorrhizal infectivity (Jasper et al., 1989) and nutrient uptake (Evans & Miller, 1988; Fairchild & Miller, 1988). Maintenance of infectivity suggests that the mycorrhizal fungi are adapted to soil disturbance, or that the soil contained a high density of fungal propagules initially. The heavy clay soils used to grow cotton in eastern Australia expand when wetted and contract when drying. The expansion and contraction would select fungi which are adapted to disturbance, possibly

8 578 G. S. Pattitison and P. A. McGee Table 2. Mean {and SD) coneetitration of Pj {%) in leaves of cotton plants at either 4 or after sowing in undisturbed cores of soil frotn Narrabri, Warren and Dalby, and at Narrabri in soil that has been disturbed {Dis.), periodically wetted {Wet/Dry) or both {Wet/Dry Dis.) Soil Narrabri Narrabri Dis. Warren (months) Mean SD 6 Mean SD 12 Mean SD 18 Mean SD Narrabri Wet/Dry Narrabri Wet/Dry Dis Dalby Storage Soil cores were stored for, and 18 months. Data not available () for at 6 months owing to thrip infestation of plants. enabling survival through cultivation witb an insignificant depletion of propagule numbers. It IS likely that the soils contained more propagules tban were necessary to initiate maximum rates of infection as suggested by McGee et al. (1997). Tbe result of excess propagules would be to mask effects of factors tbat deplete the number of propagules. The minimum density at whicb maximum rates of mycorrhizal infection are found is unknown. Periodic wetting of the soil reduced rates of initiation of arbuscular mycorrhizas, decreased tbe maximum mycorrhizal infection, and reduced the concentration of P, in leaf tissue in tbe cotton plants grown in tbe undisturbed and disturbed soil cores collected from Narrabri. Tbe effects w'ere most pronounced if the soil had first been disturbed, an effect not observed in soil stored dry. These data lead us to suggest tbat rainfall is tbe most significant factor causing long fallow disorder. We would also suggest tbat the disorder becomes significant wben the density of propagules is reduced below the critical level for maximum infection. These comparisons were made using infectivity profiles or curves. Determination of infection profiles is cumbersome and time-consuming. If one of tbe parameters of tbe function describing tbe curve could be used predictively to indicate quantity of propagules or the final level of infection reacbed without the need to derive the entire curve, then the process could be reduced. Parameters of tbe curves whicb could be used include tbe time corresponding to the point of inflection {b/c. Table 1), or maximum rate of increase of infection {ac/4. Table 1), and tbe time the asymptote is reached and the infection at this point {a. Table 1). Altbougb infection at 3 wk was significantly related to infection at, the data appear incomplete. Examination of Figure 6 suggests that two separate groups of data contributed to tbe regression, data between 7 and 85 o at in the main group and the two points between 5 and 6 o at. More data at tbe lower part of tbe regression are needed before tbe relationsbip could be considered indicative. At present, tbe association suggests tbat infection of at 3 wk will result in infection of 6 o at 8 w-k. Clearly, nil infective units will result in zero infection. It is quite possible tbe relationsbip between infection at 3 and is curvilinear. Further data are needed to clarify the hypothesis. Infection at 3 wk probably indicates tbe density of propagules necessary to establish maximum infection because of tbe relationsbip between propagule density and primary infection witbin the first 3 wk (Pugh, Roncadon & Hussey, 1981; Walker & Smith, 1984). If the relationship exists it may be possible to predict plant growth rates, uptake of Pj by seedlings, and possibly, productivity, as occurs with long fallowdisorder following a single harvest of seedlings at 3 wk. Patterns of mycorrhizal infectivity in the soil cores also indicates spatial homogeneity of AM fungi in soils of tbe cotton growing regions of eastern Australia. Similar densities of AM fungi were found m the upper soil profile at all sites. Tbe uniformity of density of fungal propagules in tbese cultivated soils is in contrast to otber studies of distribution of propagules. Brundrett and Abbott (1994, 1995) found substantial spatial beterogeneity of AM fungi in a mixed plant forest. The variability was as great between soil cores collected in one location as between sites. Tbe soil was sandy. Similarly, Boerner, DeMars & Leicbt (1995) observed variation in AM infection in a trap plant after growtb in soils from an undisturbed mixed plant community to be between 4 and 8 %. Spatial beterogeneity increased in progressively disturbed sites. Tbe soil

9 AM fungi and long fallows examined in their investigation was a silty loam. Explanations for the spatial variability in mixed plant communities include variation in the degree of mycotrophy or dependence between plants of mixed species communities, and spatial variation in the organic matter and mineral nutrients which cause spatial heterogeneity of the root system, leading to clumping of spores and hyphae (Boerner et al., 1995; Brundrett & Abbott, 1995). These explanations do not apply to monocultures. Single plant species are sown over large areas and treated in a similar manner. Highly reactive clay soils were present at all three sites, cotton was the main vegetative cover and the management practices were similar. Similar selection pressures would apply to fungal species within fields and between fields. The homogeneity might also be caused by high densities of propagules in the cultivated soils masking variation. The hypothesis that propagules of AM fungi are denser in agricultural than undisturbed soils is more speculative. Boerner et al. (1995) noted that mean AM infection at the prairie site was lower than the more disturbed sites. We have not examined the density of propagules in undisturbed soils in a comparative manner. It might be that the highly reactive clay soils have high propagule densities at all times, or that the climate, especially the long periods of high PAR during the growing period, are important factors. Disturbance would select for fungi that react rapidly to the presence of roots of potential hosts. Uniformity of infection and high densities of propagules do not necessarily indicate a uniform response by cotton plants to fungal infection or the extent to which mycorrhizas influence uptake of minerals (Hetrick & Wilson, 1991; Safir, 1994), and the factors might interact. The reason for high densities of fungal propagules remain unclear. The analysis of the results has ignored data from plants grown in soil stored for 6 months at Warren and Dalby. These seedlings became variably infested with thrips and were treated with insecticide. The shoot d. wt (Fig. 7) indicates the extent of predation of the cotton seedlings by thrips. Predation might have influenced mycorrhizal infection in the plants, owing to a decrease in photosynthates transported through the plant to the fungi (Smith & GianinazziPearson, 1988). For these reasons, the data on infectivity in these plants might also indicate the effect of insect predation and were therefore excluded. The results of the experiments were partly determined by the methods used. The maximum level of infection was c. 8 o by 6 wk in undisturbed soil at all sites. In the fleld, the maximum level of infection in cotton plants is in the range 6 7 o (Rich & Bird, 1974; Brown & Allen, 199). Cores were collected from the upper soil profile; the response of the fungi in this layer might difter from that of fungi lower in the soil profile. This possibility 579 is being examined elsewhere. The high level of infection could mask some of the treatment eftects. The cotton plants were grown in a relatively small volume of soil, which restricted the growth of roots and access to nutrients, perhaps leading to a greater rate of secondary infection of the neighbouring roots. The small volume of soil should not affect the initiation of infection, or the initial spread of infection within the roots. It is likely that the small volume of soil affected the shoot growth of the seedlings, and the data obtained do not indicate the mycorrhizal effect on plant growth which we have demonstrated elsewhere (Pattinson et al., 1997). The logistic curve (Brower et al., 199) fitted the infectivity data at all sites and treatments. This enabled the quantification of variation associated with factors likely to cause long fallow disorder in field soils. Although the logistic model adequately describes mycorrhizal infectivity, more work is required to determine whether other equations provide a better representation of infection patterns. Long fallow disorder is a decline in plant productivity, associated with reduced rates of plant growth and a decline in level of mycorrhizal infection, following a period of fallow (Thompson, 1987). The disorder varies both spatially and temporally. The importance of time, cultivation and falls of rain on long fallow disorder were quantified by comparing mycorrhizal infectn'ity of soils. Provided the soils remained dry, the density' of AM fungi did not decline below that required for maximum rates of infection. Cultivation of the soil which was then stored dry did not significantly reduced the density of propagules. However, when cultivated and uncultivated soils were wetted and then dried several times, densities of fungi were significantly reduced. When all data were combined, a correlation between the density of infection at 3 and suggests that infection at 3 wk may be used to predict final levels of infection and perhaps rates of uptake of Pj and plant growth in different environments..acknowledgements We thank.\ssociate Professor Mick O'Neill for advice on statistics and Dr S. J. Allen on the cotton industry. The research was funded by the Cotton Research and Development Corporation. REFERENCES Abbott LK, Robson AD Infectivity of vesicular-arbuscular mycorrhizal fungi in agricultural soil. Australian Journal of Agricultural Research 33: Abbott LK, Robson AD Factors influencing the occurrence of vesicular-arbuscular mycorrhizas. Agriculture, Ecosystems and Environment 35: Allen SE, Grimshaw HM, Parkinson JA, Quarmby C Chemical analysis of ecological materials. Oxford: Blackwell Scientific Publications.

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Symposium of the Society for Experimental Biology Sur\-i\-al of propagules of arbuscular mycorrhizal fungi in 29: soils in eastern Australia used to grow cotton. Neit; Phytologist Tommerup IC Methods for the study of the population 135, biology of vesicular-arbuscular mycorrhizal fungi. In: Methods McGonigle TP, Evans DG, Miller MH Effect of soil in Microbiology 24: disturbance on mycorrhizal colonization and phosphorus ab- Trappe JM, Molina R, Castellano M Reactions of sorption by maize in growth chamber and field experiments. mycorrhizal fungi and mycorrhiza formation to pesticides. New Phytologist 116: Annual Review of Phytopathology 22: Mead R, Curnow RN, Hasted AM Statistical methods in Walker NA, Smith SE The quantitative study of agricultural and experimental biology, 2nd edn. London : Chap- mycorrhizal infection. II. The relation of rate of infection and man & Hal!. speed of fungal growth to propagule density, the mean length of Moorman T, Reeves FB The role of endomycorrhizae in the infection unit and the limiting value of the fraction of the re\-egetation practices in the semi-arid west. II. A bioassay to root infected. New Phytologist 96:

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