Arbuscular mycorrhizae and the phosphorus nutrition of maize: A review of Guelph studies

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1 Arbuscular mycorrhizae and the phosphorus nutrition of maize: A review of Guelph studies Murray H. Miller Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Received 11 December 1998, accepted 5 July Miller, M. H Arbuscular mycorrhizae and the phosphorus nutrition of maize: A review of Guelph studies. Can. J. Plant Sci. 80: The role of mycorrhizae in phosphorus nutrition of maize (Zea mays L.) is related to the fact that the P concentration in maize shoots at the four- to five-leaf stage affects final grain yield. In the early 1980s we observed greater early-season shoot-p concentration (mg g 1 ) and P absorption (mg plant 1 ) from a notill compared to a conventional tillage system. Further studies established that the greater P absorption is due to a more effective arbuscular mycorrhizal (AM) symbiosis when the soil is not disturbed. The greater P absorption is largely a result of the undisrupted mycelium present in an undisturbed soil, rather than to increased colonization. This mycelium retains viability through extended periods in frozen soil. In the spring this mycelia network is able to acquire P from the soil and deliver it to the plant immediately upon becoming connected to a newly developing root system. Increased P absorption has not resulted in increased grain yield in field trials. Some additional factor limits yield with no-till maize preventing the advantage of early P absorption from being realized as yield. When maize follows a non-mycorrhizal crop such as canola (Brassica napus L.), mycorrhizal colonization is delayed, reducing early-season P absorption. Yield reductions may occur. In summary, AM mycorrhizae are involved in P nutrition of maize and an understanding of their functioning will assist us in modifying management practices to maximize economic returns through increased fertilizer efficiency. Key words: Maize, arbuscular mycorrhizae, phosphorus nutrition, tillage, canola Miller, M. H Mycorhizes à arbuscules et alimentation phosphorée du maïs; revue bibliographique des travaux réalisés à Guelph. Can. J. Plant Sci. 80: Le rôle des mycorhizes dans l alimentation phosphorée du maïs (Zea mays L.) tient au fait que la concentration du phosphore (P) dans les parties vertes du maïs au stade de 4 à 5 feuilles influe sur le rendement grainier. Au début des années 1980, nous avons constaté une plus forte concentration (mg g 1 ) dans les parties vertes et une absorption (mg plante 1 ) plus importante de P en régime de culture sans travail du sol qu en régime de travail classique. Des recherches ultérieures ont confirmé que cette plus forte absorption de P est due à la plus grande efficacité symbiotique des mycorhizes à arbuscules (MA) lorsque la terre n a pas été dérangée, efficacité qui tient davantage au maintien de l intégrité du mycélium mycorhizien en place que d une colonisation mycélienne plus abondante. Le mycélium garde alors sa vitalité durant plusieurs mois en sol gelé et, au printemps, il est capable de soutirer le phosphore du sol et de le transférer à la plante dès qu il est relié au système radiculaire en voie de renouvellement. Dans des essais au champ, l augmentation de l absorption de P ne s est cependant pas répercutée par un accroissement du rendement grainier. Il semble qu un autre facteur limite le rendement, la culture en semis direct empêchant la reprise précoce de l absorption de P de s exprimer par un gain de rendement. Lorsque le maïs succède à une culture non mycorhizienne comme le colza canola (Brassica napus L.), la mycorhization est retardée et, avec elle, l absorption de P en début de saison. Des baisses de rendement peuvent en résulter. En résumé, les mycorhizes à arbuscules jouent un rôle dans l alimentation en phosphore du maïs; la compréhension de leur fonctionnement nous aidera elle dans la mise au point de pratiques culturales susceptibles d améliorer l efficience des engrais et ainsi de maximiser la rentabilité des cultures. Mots clés: Maïs, mycorhizes à arbuscules, alimentation phosphorée, travail du sol, colza canola Arbuscular mycorrhizae were first observed more than a century ago, but studies on their role in P absorption began only about 40 yr ago (Mosse 1986). Since then, while there have been attempts to demonstrate their importance in crop production, much of the AM research has been devoted to understanding the symbiosis. Most of the production-oriented studies have involved inoculation rather than management of the indigenous AM fungal population. Menge (1983) stated that commercial use of mycorrhizal fungi is probably economically feasible at the present time in only Presented at the Symposium Nutrient Cycling in Crop Cultural Systems, 78th Annual Conference of the Agricultural Institute of Canada, Vancouver, British Columbia, 8 July three major agricultural areas: (1) disturbed sites, (2) fumigated soils, and (3) greenhouses. There is little reason to alter that statement today. This does not mean, however, that mycorrhizae are not important in nutrition of field crops, nor that their effectiveness cannot be altered by crop management practices. Agricultural crops under intensive management are generally well fertilized with P. Because AM colonization generally decreases as the P concentration in the root system increases (Amijee et al. 1989; Braunberger et al. 1991; Lu et al. 1994) it was assumed that AM would not contribute significantly to the P nutrition of intensively managed crops. The effectiveness of AM, however, depends not so much on Abbreviations: AM, arbuscular mycorrhizal

2 48 CANADIAN JOURNAL OF PLANT SCIENCE the ultimate level of colonization or the over-all effect on P absorption, as on the timing of the colonization and P absorption in relation to the P requirements of the plants. Maize (Zea mays L.) is highly responsive to early absorption of P. Studies at Guelph in the late 1960s established that application of a small amount of P directly with the seed resulted in greater P absorption (mg plant 1 ) during very early growth than did a greater amount side banded (Miller et al. 1971). Seed-applied P frequently resulted in increased yield (Bates 1971). Barry and Miller (1989) using an outdoor hydroponic system, established that P deficiency in maize shoots prior to the six-leaf stage reduced grain yield regardless of the subsequent P nutrition. Lauzon and Miller (1997) concluded from field studies that yield responses to seedplaced P could be attributed to the enhanced P concentration (mg g 1 ) at the four- to five-leaf stage, which was greater for seed-placed compared to side-banded P. The increased yield appears to result more from an increase in kernel number per ear, rather than from increased kernel weight. SOIL DISTURBANCE AND THE MYCORRHIZAL SYMBIOSIS Discovery of the Effect Our mycorrhizal program at Guelph began in a serendipitous manner. We had hypothesized that P absorption by notill maize would be less than that under conventional tillage because of restriction in root growth, but we found the reverse (O Halloran et al. 1986). After several experiments eliminated soil bulk density, soil temperature and soil moisture as explanations, we investigated mycorrhizae. Evans and Miller (1988) collected disturbed and undisturbed soil cores from three long-term no-till trials and grew maize in a growth room. Phosphorus absorption over a 3-wk period from the undisturbed cores was more than double that from the disturbed cores. Mycorrhizal colonization of the roots was also much greater in the undisturbed cores. Strong evidence that this was a causal relationship was obtained in subsequent experiements in which disturbance reduced the P absorption by maize and wheat (Triticum aestivum L.) but not canola (Brassica napus) or spinach (Spinacea oleracea L.), which are non-mycorrhizal. In addition, elimination of AM by either irradiation or application of a fungicide minimized the effect of disturbance (Evans and Miller 1988). Fairchild and Miller (1988) found that the manifestation of the effect did not require an extended period without disturbance. They began with disturbed soil and grew maize in a growth room for four cycles of 3-wk each, disturbing half the pots after each cycle. Mycorrhizal colonization and P absorption were significantly greater on undisturbed soil at the end of the second cycle, the first with a non-disturbed treatment. The effect was greater in the third cycle, and in the fourth cycle P absorption between 14 and 21 d on the undisturbed soil was 10 times that on the disturbed. In a subsequent experiment using a similar approach but with varying soil P amendments, Fairchild and Miller (1990) found that, although AM colonization decreased with increasing soil P amendment, the effect of disturbance on AM colonization and P absorption was independent of soil Fig. 1. Shoot P content and arbuscular colonization (AC) of maize roots during a fourth 3-wk growth cycle as influenced by frequency of disturbance during three previous growth cycles. D123, disturbed after each cycle; D3, disturbed after third cycle only; U, undisturbed. Bars with same letter (a, b for AC and X, Y, Z for shoot P) are not significantly different at P = P amendment and both were twofold greater on the undisturbed soil. Role of Extraradical Mycelium In the initial studies, there was a consistent relation between the effect of disturbance on AM colonization and P absorption, so it was assumed that the increased P absorption was a result of the increased colonization. However, in a later greenhouse and a field study, the increased P absorption was observed in the absence of any effect of disturbance on AM colonization (McGonigle et al. 1990). Evans and Miller (1990), using a compartmentalized system in a growthchamber study, concluded that the mycelia network appeared to be an important component of the inoculum potential in an undisturbed soil and that its destruction was directly responsible for much of the negative effect of soil disturbance on mycorrhizal colonization as well as on P absorption. Further evidence that the disruption of the extraradical mycelium rather than reduced colonization was the major cause of reduced P absorption in disturbed soil was obtained in growth chamber studies using timing of disturbance to separate the effects of soil disturbance on colonization and P absorption (Miller and McGonigle 1992; McGonigle and Miller 1993a). Growing maize for four 3- wk cycles beginning with disturbed soil, and disturbing after either intermediate cycles, or the penultimate cycle only, we succeeded in creating similar levels of colonization in the final cycle in the presence and absence of an undisrupted mycelium. Shoot dry matter and shoot P concentration and hence absorption were much greater when the mycelium was undisrupted regardless of the degree of colonization (Fig. 1). We suggested that roots of a newly developing plant became attached to the intact mycelium, which then served as a nutrient acquisition system obviating the need to develop a new mycelium from primary infections.

3 MILLER MYCORRHIZAE AND P NUTRITION OF MAIZE 49 Fig. 2. Arbuscular colonization and ratio of soil-derived shoot P of maize grown for a 3-wk period in disturbed (D) or undisturbed (U) soil cores collected from a no-till soybean field. Fig. 3. Arbuscular colonization of bioassay plants grown in pouches containing either undisturbed mycelia (n), disturbed mycelia (m) or isolated spores ( ). Prior to the bioassay the pouches were either frozen or maintained at 13 C (n = 5; bars indicate ±1 SE). Time-zero data were analyzed in a separate ANOVA, because there was not a time and temperature variable. Means for the time 0 treatment surmounted by different letters are significantly different at P = All means for other treatment effects surmounted by different letters are significantly different at P = Reprinted with permission from Addy et al. (1997). New Phytol. 135: Winter Survival of Extraradical Mycelia The studies described in the preceding section were short term, with only a few days between cycles. We realized that if disruption of the extraradical mycelium was the main cause of reduced P absorption in the field, the mycelium must over-winter and retain the capacity to initiate infection as well as to absorb P and transfer it to the host plant. Although in earlier studies similar effects of disturbance had been observed with cores collected in the spring and in the fall, we had not conducted a systematic assessment of the effect of time of collection on the disturbance effect. This was accomplished by collecting disturbed and undisturbed cores from two no-till sites one in soybean (Glycine max (L.) Merr.) and one in maize at several times ranging from when the crop was actively growing through to the following spring (Miller et al. 1995). Maize was grown on the cores for 3-wk in a growth room. The colonization of maize roots in undisturbed cores collected in the spring from the soybean field was at least as great as that in cores collected the previous summer or fall (Fig. 2). Although the effect of disturbance on colonization was inconsistent, P absorption from the disturbed cores ranged from 30 to 60% of that from the undisturbed cores at all sampling times including spring when the AM colonization was similar for the two disturbance treatments (Fig. 2). These differences in P absorption were significant (P = 0.05) at all five times. Similar results were obtained using cores from the maize field. Although this supported the hypothesis that the increased P absorption was due to the presence of functional extraradical mycelium following the winter period, it was not conclusive. In these studies, the inoculum consisted of old root pieces with their AM fungal components as well as extraradical spores and mycelia. A second approach to assessing winter survival eliminated old root pieces and extraradical spores as a source of inoculum. The procedure involved producing mycelia in pasteurized soil in pouches made from a 43-µm nylon mesh that permitted the entry of hyphae but not roots. Following exposure to a range of conditions for varying periods of time, the soil in half of the pouches was disturbed by hand, replaced in the pouch, and the infectivity of the disturbed and undisturbed systems determined in a bioassay using sudangrass (Sorghum sudanese Staph). Addy et al. (1997) exposed pouches containing mycelia of Glomus species to freezing conditions either in the field or in controlled temperature chambers. The mycelia remained infective following prolonged periods of freezing if it was not disrupted prior to the bioassay (Fig. 3). This survival was not dependent on either the presence of root pieces or on the connection of mycelia to roots of the host plants on which they developed. Disruption of the mycelia prior to the bioassay reduced the infectivity at time zero. The magnitude of the disturbance effect on colonization in the pouches maintained at 13 C was similar to that at time zero, but this

4 50 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 4. Arbuscular colonization (AC) and shoot-p content of sudangrass bioassay plants growing in root-free pouches of soil containing mycelia produced under corn plants in the field and collected at different times. Pouches were either disturbed (D) or not (U) prior to the bioassay. Within collection dates, means for plants in the disturbed and undisturbed treatments were significantly different at the 0.05 (*) and 0.01 (**) probability levels as shown; n =6. reduction was significant only at P = The magnitude of the reduction was greater in the pouches that had been frozen, and increased with increasing duration in the frozen condition. Spores were not an effective inoculum in these studies (Fig. 3). McGonigle and Miller (1999) produced root-free mycelia in pouches buried beneath corn plants in the field. These pouches were removed from their place of synthesis at various times before and after winter and growth-chamber bioassays were conducted. Although a statistical comparison was not made because the bioassays were conducted at different times, colonization of the sudangrass roots in bioassays on pouches collected after the winter was at least as great as that for bioassays on pouches collected before the winter (Fig. 4). At all times, disturbance of pouch soil after excavation reduced the P content of bioassay shoots relative to pouches with undisturbed soil (Fig. 4). This was a result of reductions in both shoot dry matter and shoot-p concentration. We concluded that extraradical mycelia of arbuscular mycorrhizae can survive over winter and provide a functional inoculum and nutrient acquisition system very early Fig. 5. Effect of soil disturbance on maize grain yield in 1993 at various levels of P applied in the spring of Means within a level of P applied with different letters (a, b) above differ significantly at the 0.05 probability level. Reprinted with permission from McGonigle and Miller. (1996). Soil Sci. Soc. Am. J. 60: in the growth of a succeeding crop. Disruption of the mycelium following freezing markedly reduces the P acquisition capacity, and may or may not reduce AM infectivity. Presumably disturbance prior to freezing would have a similar if not greater effect, but we have not made this comparison. Tillage and Mycorrhizal Effectiveness Several field experiments have been conducted to assess the effect of tillage on the efficacy of AM symbiosis. McGonigle and Miller (1993b) observed that in each of 1990 and 1991, P concentration in maize shoots at the fourto five-leaf stage was greater in a no-till (NT) or ridge-till (RT) system than in a fall moldboard plow (MP) system. This difference disappeared by the seven- to eight-leaf stage. Increases in shoot P content between 25 and 32 d after planting in 1990 were 0.08, 0.33 and 0.71 mg plant 1 in the MP, NT and RT systems, respectively. There was also a greater AM colonization of roots in the two undisturbed systems beginning at the two- to three-leaf stage and continuing to about the 10-leaf stage. In this long-term experiment, tillage did not significantly affect yield in either of the years in which mycorrhizal effectiveness was assessed. The treatments received a P-fertilizer application although the soil test P was above that at which a response would be expected. As a consequence the shoot-p concentrations at the fourto five-leaf stage on the MP treatment were in the range where a positive response to increased shoot-p concentration would be unlikely. To better assess the impact of mycorrhizae on yield, McGonigle and Miller (1996) established blocks of soil varying in soil P level from severely deficient to more than adequate in the spring of 1991 and grew maize. In 1992 and 1993, four soil disturbance treatments were imposed on

5 MILLER MYCORRHIZAE AND P NUTRITION OF MAIZE 51 Table 1. Shoot dry mass, final yield and harvest index of maize as influenced by previous crop Final yield (kg ha 1 ) Previous Shoot dry mass (g plant 1 ) Total Harvest crop 4 5 leaf 6-leaf Silk Stover Grain biomass index Canola 0.20b 0.83b 139a 6555a b Maize 0.28a 2.16b 102b 5943b a a,b Values for each parameter followed by a different letter are significantly different at P = these blocks: CT = moldboard plow plus disking and cultivation: ROTO=CT plus roto-tilled; NT = normal no-till planter: NTHP = no disturbance, seeds placed by hand. Severity of soil disturbance was assumed to decrease in the order: ROTO>CT>NT>NTHP. In both years, shoot-p content (mg plant 1 ) at the four- to five-leaf stage generally increased as the severity of disturbance decreased, although shoot dry mass tended to be lower in the less-disturbed treatments. The effect of disturbance on P absorption was independent of soil P level (McGonigle and Miller 1996). Because of weather-related problems, yields in 1992 were very low. Grain yields in 1993 increased with increasing soil P. At low soil-p levels, yields on undisturbed and disturbed treatments were similar, but at higher soil-p levels yields on undisturbed treatments were lower (Fig. 5). This can be explained by assuming two opposing factors are affecting yield. Increased early-season P absorption in undisturbed systems increases yield when shoot-p concentration is below a critical value. Another factor, possibly lower soil temperature, reduces yield in undisturbed systems. In this experiment, at lower soil-p levels the effect of the increased absorption in the lesser disturbed treatments was sufficient to overcome the yield-depressing effect. At the higher soil- P levels, the shoot-p concentration in the disturbed treatments was at or above the critical level, so only the yield-depressing factor was operating. We concluded that, while there was potential for the increased AM effectiveness to increase yields in no-till production or to result in similar yields with either lower soil P levels or lower fertilizer P applications, this potential will not be achieved unless the cause of the yield depression with no-till is eliminated. PREVIOUS CROP AND MYCORRHIZAL SYMBIOSIS OF MAIZE To further assess the importance of AM in P nutrition and grain yield of maize, we grew a non-mycorrhizal crop, canola (Brassica napus L.) and maize in blocks of disturbed soil in 1993 and observed the effect on both NT and CT maize in Mycorrhizal colonization of maize in 1994 was markedly reduced at the three-leaf stage when canola rather than maize was the previous crop (Gavito and Miller 1998a). The inhibitory effect of previous cropping with canola on colonization although reduced, was still significant at the six-leaf stage but had disappeared by silking. Tillage had no effect on colonization. Maize shoot P content at the four- to five-leaf stage when maize was the previous crop was more than double that when canola preceded maize (Gavito and Miller 1998b). The greater P absorption in the CT system when maize rather than canola was the previous crop indicates that AM play a significant role in early P nutrition of maize even in disturbed systems and that any management system that reduces the effectiveness of AM will impact negatively on the maize crop. The greater early P absorption when maize was the previous crop was accompanied by increased shoot dry mass at the four- to five- and six-leaf stages (Table 1). By silking, however, this effect had reversed, and shoot dry weight was greater when canola was the preceding crop. At maturity, the previous crop had no impact on total dry matter, but did affect the allocation of carbon to stover and grain. Stover dry mass was greater following canola, but grain yield was greater (P = 0.09) following maize, giving rise to a significant increase in harvest index (Table 1). This response of shoot dry mass and increased grain yield is very similar to that observed in studies where P is applied directly with the seed (Lauzon and Miller 1997). It appears that an effective mycorrhizal system performs a function similar to that of P fertilizer with the seed. SUMMARY Arbuscular mycorrhizal fungi are present in most soils, and thus most cultivated crops are mycorrhizal. Arbuscular mycorrhizae develop very early in the growth of maize and contribute significantly to early P absorption. Because maize yield is affected by shoot P concentration at the fourto five-leaf stage, management practices that alter the effectiveness of the AM may have an impact on final yield. When tillage disrupts the extraradical mycelium produced in association with a previous crop, early-season P absorption is reduced. Although factors other than P nutrition result in reduced maize yield with a no-till system, the increased early P absorption resulting from a more effective mycorrhizal system may allow similar yields to be obtained at a lower soil P level, or with a reduced fertilizer P application. When maize follows a non-mycorrhizal crop such as canola, mycorrhizal development is delayed and earlyseason P absorption is reduced which may reduce grain yield. An effective mycorrhizal system appears to enhance early P absorption similar to P applied directly with the seed of maize. Thus, an increase in the effectivess of the AM system may reduce the importance of this starter fertilizer application in an NT system. Conversely, a decrease in earlyseason effectiveness of AM such as following a non-mycorrhizal crop with maize may increase the importance of an application of P with the seed.

6 52 CANADIAN JOURNAL OF PLANT SCIENCE ACKNOWLEDGEMENT This paper was presented at a Joint Symposium of The Canadian Society of Agronomy, the Canadian Society of Soil Science and the Canadian Society for Horticultural Science held in Vancouver, BC, 7 July The financial assistance of the Potash Phosphate Institute of Canada and West Co. Fertilizer Ltd. for attendance at the symposium and the Canadian Society of Agronomy for publication costs is gratefully appreciated. Addy, H. D., Miller, M. H. and Peterson, R. L Infectivity of the propagules associated with extraradical mycelia of two AM fungi following winter freezing. New Phytol. 135: Amijee, F., Tinker, P. B. and Stribley, D. P The development of endomycorrhizal root systems. VII. A detailed study of effects of soil phosphorus on colonization. New Phytol. 111: Barry, D. A. and Miller, M. H Phosphorus nutritional requirements of maize seedlings for maximum yield. Agron. J. 81: Bates, T. E Response of corn to small amounts of fertilizer placed with the seed. II. Summary of 22 field trials. Agron. J. 63: Braunberger, P. G., Miller, M. H. and Peterson, R. L Effect of phosphorus nutrition on morphological characteristics of vesicular-arbuscular mycorrhizal colonization of maize. New Phytol. 119: Evans, D. G. and Miller, M. H Vesicular-arbuscular mycorrhizas and the soil-disturbance-induced reduction of nutrient absorption in maize. 1. Causal relations. New Phytol. 110: Evans, D. G. and Miller, M. H The role of the external mycelia network in the effect of soil disturbance upon vesiculararbuscular mycorrhizal colonization of maize. New Phytol. 114: Fairchild, G. L. and Miller, M. H Vesicular-arbuscular mycorrhizas and the soil-disturbance induced reduction of nutrient absorption in maize. III. Influence of P amendments to soil. New Phytol. 114: Gavito, M. E. and Miller, M. H. 1998a. Changes in mycorrhiza development in maize induced by crop management practices. Plant Soil 198: Gavito, M. E. and Miller, M. H. 1998b. Early phosphorus nutrition, mycorrhizal development, dry matter partitioning and yield of maize. Plant Soil 199: Lauzon, J. D. and Miller, M. H Comparative response of corn and soybean to seed-placed phosphorus over a range of soil test phosphorus. Commun. Soil Sci. Plant Anal. 28: Lu, S., Baunberger, P. G. and Miller, M. H Response of vesicular-arbuscular mycorrhizas of maize to various rates of P addition to different rooting zones. Plant Soil 158: McGonigle, T. P. and Miller, M. H. 1993a. Responses of mycorrhizae and shoot phosphorus of maize to the frequency and timing of soil disturbance. Mycorrhiza 4: McGonigle, T. P. and Miller, M. H. 1993b. Mycorrhizal development and phosphorus absorption in maize under conventional and reduced tillage. Soil Sci. Soc. Am. J. 57: McGonigle, T. P. and Miller, M. H Mycorrhizae, phosphorus absorption, and yield of maize in response to tillage. Soil Sci. Soc. Am. J. 60: McGonigle, T. P. and Miller, M. H Winter survival of extraradical hyphae and spores of arbuscular mycorrhizal fungi in the field. Appl. Soil Ecol. 12: McGonigle, T. P., Evans, D. G. and Miller, M. H Effect of degree of soil disturbance on mycorrhizal colonization and phosphorus absorption in maize in growth chamber and field experiments. New Phytol. 116: Menge, J. A Utilization of vesicular-arbuscular mycorrhizal fungi in agriculture. Can. J. Bot. 61: Miller, M. H. and McGonigle, T. P Soil disturbance and the effectiveness of arbuscular mycorrhizas in an agricultural ecosystem. Pages in D. J. Read, D. H. Lewis, A. H. Fitter, and I. J. Alexander, eds. Mycorrhizas in ecosystems. CAB International, Oxford, UK. Miller, M. H., Bates, T. E., Singh, D. and Baweja, A. S Response of corn to small amounts of fertilizer placed with the seed: 1. Greenhouse studies. Agron. J. 63: Miller, M. H., McGonigle, T. P. and Addy, H. D Functional ecology of vesicular arbuscular mycorrhizas as influenced by phosphate fertilization and tillage in an agricultural ecosystem. Crit. Rev. Biotechnol. 15(3/4): Mosse, B Mycorrhiza in sustainable agriculture. Bio. Agric. Hortic. 3: O Halloran, I. P., Miller, M. H. and Arnold, G Absorption of P by corn (Zea mays L.) as influenced by soil disturbance. Can. J. Soil Sci. 66: