CHAPTER 7 Agroecology of Arbuscular Mycorrhizal Activity John C. Zak and Bobbie McMichael CONTENTS Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Factors Impacting Root Growth and AM Symbiosis. . . . . . . . . . . . . . . . . . . 146 Soil Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Soil Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Nutrient Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 Impacts of Management Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Tillage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Crop Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Inoculum Dynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Herbicide and Pesticide Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 Varietal Responses and Breeding Programs . . . . . . . . . . . . . . . . . . . 156 Role of AM Fungi in Soil Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Is Management of AM Fungi Practical? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 INTRODUCTION Arbuscular mycorrhizal (AM) fungi are recognized as important compo- nents of agricultural systems as a consequence of their roles in plant mineral nutrition, root disease dynamics, and soil fertility. While it is generally agreed that AM fungi are a necessary component of agricultural ecosystems, there is only limited understanding as to how to integrate and maintain efficient AM fungi within an annual cropping system. Moreover, our understanding of the 145 0-8493-0904-2/01/$0.00+$.50 © 2001 by CRC Press LLC 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 145 146 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT dynamics of AM fungi within an agricultural context applies only to several types of cropping systems under a limited number of climatic conditions. Based on the information that has been collected over the last decade, the importance of AM fungi in various cropping systems is being taken more seriously, particularly as production of some crops moves toward low-input sustainable systems. The importance of AM fungi seems to be more crucial for these low input systems than in the traditional high input production sys- tems, where breeding has selected for genotypes that respond to high fertil- izer and water inputs. However, as the cost of chemical inputs and irrigation continues to increase and as researchers assess the sustainability of tradi- tional farming practices, the benefits of AM fungi in an overall crop manage- ment plan become economically important. There have been increased efforts over the last decade to understand the interactions among abiotic and biotic factors associated with agricultural sys- tems and to develop management options that can be used to incorporate AM fungi into annual cropping systems. The studies detailed in this chapter point out how much has been learned concerning the impacts of farming practices on AM dynamics. These same investigations also articulate our lim- itations towards integrating AM fungi within a long-term soil management program that maintains crop yields. Our goal in this chapter is to examine those aspects of annual production systems that influence AM dynamics. We state at the outset that our work with AM colonization of cotton in a semi-arid environment does bias some- what the topics we have chosen to examine concerning the ecology of AM fungi in agricultural systems. However, given that arid and semi-arid lands constitute about 40% of the planet’s surface, and that the majority of world- wide cotton production occurs within this climatic zone, we believe that there is the need to expand the discussion of mycorrhizae in agriculture beyond what has been previously discussed for mesic regions. FACTORS IMPACTING ROOT GROWTH AND AM SYMBIOSIS Soil Temperature The influence of soil temperature on root growth has been documented for a number of species (e.g., Cooper, 1973). There is an optimum tempera- ture for maximum root development for all plant species with the general pattern of root growth increasing up to the optimum and then decreasing at higher temperatures. For example, the optimum temperature for root growth in cotton plants is between 28 and 35°C (Pearson et al., 1970) while the opti- mum temperature for forage legumes is significantly lower (Brar et al., 1991). Abbas Al-Ani and Hay (1983) showed that root extension rates increased for each 10°C rise in temperature. However, when soil temperatures deviate sig- nificantly from optimum, root branching (Brouwer and Hoagland, 1964 ) and 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 146 AGROECOLOGY OF ARBUSCULAR-MYCORRHIZAL ACTIVITY 147 water uptake (Nielsen, 1974) can be reduced. Thus, strategies that would enhance root development may also improve AM colonization. Research on impact of soil temperature on AM colonization is limited. Addy et al. (1997) demonstrated that some extraradical hyphae remain alive and are capable of infecting following soil freezing. Working with blocks of field soil, Addy et al. (1998) showed that colonization of AM fungi was greater in soil that was cooled slowly, allowing for apparent acclimation of the AM fungi. The exact mechanism for the acclimation and increase in freez- ing tolerance was not determined. In general, higher temperatures generally result in greater colonization and increased sporulation (Daniels-Hetrick, 1984). Schenck and Schroder (1974) observed that maximum AM develop- ment in soybean occurred near 30°C. In contrast, Forbes et al. (1996) showed that in Plantago the highest level of colonization occurred in roots grown at 15°C with the lowest at 27°C. Menge (1984) indicated that AM colonization is generally inhibited at soil temperatures lower than 15°C. Ferguson and Woodhead (1982) showed that periods of cold stress followed by high soil temperatures increased colonization and sporulation. In recent studies under controlled conditions, McMichael and Zak (unpublished data) showed that AM colonization of cotton was higher when plants were grown at 28°C than at 18°C soil temperature. Managing soil temperature for improved root growth and AM coloniza- tion is very difficult, particularly on a large scale. Plastic mulches have been utilized in some crops, for example, to change soil temperature characteris- tics for improving plant performance (e.g., Ham et al., 1993; Mbagwu, 1991). Wien et al. (1993) also used mulches to improve field performance of toma- toes. Burke and Upchurch (personal communication) used different field row spacings to adjust crop canopy closure to change soil temperatures and growth of cotton. However the impacts of various field manipulations to control soil temperatures on AM colonization have not been investigated. Another approach to field manipulations of temperature would be to alter root growth characteristics of plants for improved root development and AM colonization over a wide range of soil temperatures. McMichael (unpublished data) has shown genetic variability in the temperature response of a number of cotton genotypes. In a preliminary study, Zak and McMichael (2000) found that several lines of cotton that differed in cold tol- erance when soil temperatures were kept at 18°C had lower colonization than cotton lines that were rated as highly cold tolerant. The mechanisms for these effects have not been determined but might reflect differences in root growth and root densities among the cotton genotypes. Soil Moisture Changes in soil moisture can have a direct influence on the growth of plant root systems and subsequent AM colonization levels. In addition, root- ing depth and density may increase in a drying soil (Taylor, 1983), while root 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 147 148 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT elongation rates may significantly decrease (Klepper, et al., 1973), affecting colonization patterns. Zak et al. (1998) indicated that a decrease in soil mois- ture appeared to impact the extent of mycorrhizal colonization of cotton plants only during the later stages of growth under dry-land conditions in west Texas. Ryan and Ash (1996) showed that the decline in AM colonization in wheat in southern New South Wales was due to a reduction in AM inocu- lum as a result of severe drought the previous year, rather than a direct impact on colonization levels. Cade-Menun et al. (1991) reported that for winter wheat growing in British Columbia, Canada, differences in AM colonization levels among winter-wheat fields could have resulted from differences in soil moisture levels with wet conditions inhibiting AM colonization. Soil moisture may impact colonization levels by decreasing spore germination (e.g., Silva and Schenck, 1983) and altering spore abundance (Anderson et al., 1984). The alteration of soil moisture characteristics for improved root develop- ment is less difficult to accomplish on a relatively large scale than genetically altering the root pattern of the crop. Research to study the direct interactions between environmental effects on root development and AM colonization, however, is lacking. Sylvia and Williams (1992), in their review of the impact of environmental stress on AM activity, indicated that stresses that influence plant growth also influence AM colonization. Nutrient Conditions The nutrient status of soil in agroecosystems is modified through fertil- izer applications to enhance production. These fertilizer applications usu- ally have significant negative effects on AM colonization levels and seasonal patterns as the N and P status of the soil increases within a growing season and between years (e.g., Daniels-Hetrick, 1984; Menge, 1984). In addition to the direct negative impacts of fertilizer application, Johnson and Pfleger (1992) suggest that an indirect effect of fertilizer application is to alter AM fungal species occurrences. Moreover, populations of AM fungi may be adapted to specific fertility levels for a particular crop and region resulting in AM fungi that are adapted to a specific level of nutrients responding differ- ently to altered fertilization regimes when crops are rotated through a spe- cific field. In designing fertilizer application rates that not only optimize plant production but that enhance AM colonization and maintain more effective AM species, Johnson and Pfleger (1992) indicated that the ratio of nutrients is important with a balanced fertilizer providing improved AM coloniza- tion. Menge (1984) also reported that high levels of micronutrients, such as manganese and zinc, can also reduce colonization. Therefore, in the manage- ment of agricultural soils, maintenance of the proper nutrient balance appears to be important for optimum performance of plant-mycorrhizal associations. 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 148 AGROECOLOGY OF ARBUSCULAR-MYCORRHIZAL ACTIVITY 149 IMPACTS OF MANAGEMENT PRACTICES Tillage For many agricultural systems, tillage is a necessary management prac- tice that is used to reduce weed competition, reduce soil compaction, enhance water infiltration, and reduce wind erosion of sandy-loam soils. Based on both greenhouse and field investigations, the general conclusion of numer- ous studies is that soil disturbances from tillage result in decreased AM colo- nization, a decline in AM spore numbers, a change in AM species, and a subsequent decline in mycorrhizal infectivity, particularly if fields are not replanted that year. The causal relationships between tillage and reduced AM colonization and effectiveness center on the impact of this management prac- tice on the disruption, fragmentation, and destruction of the extensive net- work of AM extraradical hyphae that develops in soil during the growing season, and on the decreased viability of AM inoculum types (McGonigle and Miller, 1996). The disruption of the AM hyphal network can negatively influence AM-induced enhancement of plant growth (e.g., O’Halloran et al., 1989; Jasper et al., 1989a, b), reduce tissue P concentrations and shoot dry weight (e.g., Fairchild and Miller, 1988; Evans and Miller, 1990), and has been reported to result in the subsequent decline in AM colonization (Evans and Miller, 1988). In greenhouse pot studies the effects of soil disturbance have varied with impacts depending upon the length of time between the distur- bance and planting. Jasper et al. (1989a, b) reported a decline in AM colo- nization following disturbance, while McGonigle et al. (1990) found no effects of soil disturbance. In a field study using corn under tillage and a No- till system, Entry et al. (1996) found that tillage had no impact on coloniza- tion of corn after 7 years. Not only can tillage impact AM infectivity and viability, Kabir et al. (1999) reported a direct decrease in metabolically active hyphae associated with mycorrhizal corn following soil disturbance if soils were subsequently left fal- low for one to three months (Figure 7.1). In their greenhouse study, the destruction of the AM hyphal network also reduced plant phosphorous con- tent and shoot dry weights. The decrease in plant P was attributed to the inability of the fragmented network to explore a sufficient soil volume to maintain adequate plant P levels for optimum plant growth. The maintenance of a continuous AM hyphal network is crucial to supplying the host with suf- ficient P to meet plant demands and support high yields (Kabir et al., 1999). For cropping systems in temperate regions there can be an interval of up to five months before the next crop is planted. During this period of time, AM inoculum can either remain intact or be reduced, depending upon soil prepa- ration needs, the previous impacts of tillage practices on the maintenance of the AM hyphal network, and the interactions of agricultural practices with climatic conditions. Since tillage practices also affect root distributions, it is reasonable to propose that tillage will also affect the subsequent distribution 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 149 150 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT 80 100 60 40 20 0 0 30 60 90 Fallow Period (Days) Hyphal Lengths (cm/g) Parameter and Treatment Total - Disturbed Total - Undisturbed Metabolically Active - Disturbed Metabolically Active - Undisturbed Figure 7.1 The interactive effects of disturbance (mixing) and fallow time on lengths of total and metabolically active AM hyphae associated with Zea mays L. Values are means Ϯ S.E. Data from Kabir, Z., O’Halloram, I. P., and Hamel, C., Soil Biol. Biochem., 307–314, 1999. of AM propagules that have survived from the previous crop. Comparing conventional tillage and No-till systems, Smith (1978) found that AM spores were most abundant in the top 10 cm of soil in the No-till system (drilled wheat) when compared to conventional tillage wheat where the majority of the spores occurred in soil below 10 cm. If the density of the AM inoculum is crucial for the successful colonization of annual crop seedlings such as cot- ton (Zak et al., 1998), the vertical distribution of AM inoculum becomes an issue that should be considered if one is to manage effectively AM fungi. Tillage may also negatively affect mycorrhizal dynamics by influencing AM fungal species composition. Johnson and Pfleger (1992) speculated that through repeated disruption of the mycorrhizal network and the severing of hyphae from roots, tillage would be a strong selective influence in determin- ing AM species composition. Species richness of AM fungi has been shown to decrease when land is first brought into cultivation (Schenck et al., 1989) and as the intensity of the agricultural inputs increases (e.g., Sieverding, 1990). Therefore, it is reasonable to speculate that different types of soil man- agement practices (tillage, minimal tillage, and No-till systems) should affect 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 150 AGROECOLOGY OF ARBUSCULAR-MYCORRHIZAL ACTIVITY 151 AM fungal species composition to different degrees. However, there have been few long-term evaluations of the effects of different degrees of soil dis- turbance on AM fungal species richness and species composition. Certain species of AM fungi (e.g., Glomus mosseae and Glomus aggregatum ) are fre- quently abundant in highly managed agricultural systems (Schenck et al., 1989), suggesting that these species may be adapted to highly disturbed sys- tems. Johnson and Pfleger (1992) and Kurle and Pfleger (1994) previously pointed out the deficiencies in our understanding of the impacts of tillage on AM dynamics in agricultural systems, specifically with respect to changes in species composition. In addition to the disruption of the AM mycelial network, tillage also negatively affects mycorrhizal benefits to crop plants by increasing soil com- paction and through increased decomposition of incorporated plant residues, which includes mycorrhizal root fragments. Intensive tillage exac- erbates soil compaction, requiring annual deep plowing to break up this compacted layer (Soane, 1990), further disrupting the AM fungal network (Entry et al., 1996) and hastening root decomposition. Crop Rotation When compared to undisturbed systems, the species richness of AM fun- gal assemblages in agroecosystems is lower, sometimes substantially, depending upon the amount of human input into the system (e.g., Siqueira et al., 1989, and Sieverding, 1990). Most annual cropping systems are managed as monocultures that are either rotated through a specific cropping sequence (e.g., corn—soybean) or that are continuously planted as a single crop some- times for years. The continuous cropping approach, in conjunction with the use of a single plant species, and cultural practices that are part of the man- agement system (irrigation, tillage, fertilizer and pesticide application) all interact to select for a specific ensemble of AM species that can tolerate and proliferate under the conditions that are dominant in the production system. The combination of type of annual crop plant and the length of cultiva- tion exert a strong influence on the species of AM fungi that are found in a particular field or production system. Schenck and Kinloch (1980) were one of the first to document that, although AM fungi were considered generalists with regard to host species, there were differences in the species composi- tions of AM fungal ensembles among six different crops planted in the same soil type and within the same climatic region. Johnson et al. (1992) showed that three species of Glomus (aggregatum, leptotichum, and occultum) were dominant in a corn cropping system, while in a soybean cropping system in the same region only spores of Glomus microcarpum predominated. Not only can annual crops select certain species of AM fungi from the species pool that would exist for a given region, the species that can prolifer- ate under monocultural conditions have been shown not to be the most 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 151 152 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT efficient mutualists. In their work on understanding rotation effects on yield decline and the involvement of AM fungi, Johnson et al. (1992) proposed that continuous cropping selects for rapidly growing AM species. As these ineffi- cient AM fungal species predominate in the soil, crop vigor begins to decline. Rotations that use either facultatively mycorrhizal crops or species that do not form arbuscular mycorrhizae, such as rapeseed, sugar beet, or buck- wheat, may decrease AM inoculum for a succeeding, highly mycorrhizal- dependent crop to the same degree as fallowing (Thompson, 1991). From a management perspective, crop rotation decisions should consider the ability of the current crop to maintain inoculum of effective fungi at high densities as well as the mycorrhizal dependency of the succeeding crop type (Thompson, 1994). Inoculum Dynamics AM propagules include spores, hyphal fragments, and dead roots that contain hyphae and vesicules. The survival and abundance of these propag- ules in an annual cropping system are influenced by a suite of abiotic factors and management considerations that includes crop rotations, tillage, water- ing schedules, fertilizer type and application rates, and pesticide use. These factors either negatively impact AM inoculum production or decrease viabil- ity with the successive crop suffering the greatest adverse affect. While AM fungal spores can be found in most agricultural systems, it is unclear to what extent AM spores maintain colonization levels of annual crops from season to season (Abbott and Gazey, 1994). In semiarid regions, where spore pro- duction is generally low (Stutz and Morton, 1996), mycorrhizal root frag- ments can be critical sources of inoculum for the succeeding crop (Friese and Allen, 1991). Any management practice or change in climate that accelerates decomposition of colonized root fragments can result in a decline in subse- quent AM colonization levels. Changes in AM fungal species and spore densities in annual cropping systems have been reported to occur in response to tillage practices. In a No- till corn and soybean system, Glomus occultum predominated while in a con- ventional tillage system, spores of Glomus etunicatum were the most numerous (Douds et al., 1995). The negative effects of tillage on AM fungal spore production and densities have been primarily observed to occur in the top 5 cm of soil where the disturbance effects are the most severe. Deep plow- ing to more than 15 cm will reduce colonization of roots by AM fungi, thereby reducing inoculum densities (Kabir et al., 1999) which in turn may result in a decrease in seedling establishment during the following year. Depending upon the crop, climate, and rainfall patterns for a particular region, annual cropping systems are either followed by the same cash crop, rotated with a second cash crop, planted in a winter cover crop, or left fallow. 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 152 AGROECOLOGY OF ARBUSCULAR-MYCORRHIZAL ACTIVITY 153 The decisions that are made at this management level can have profound negative and positive effects on the production and survival of AM inocu- lum. The types of propagules that are present in a production system and their rates of survival are crucial pieces of information for subsequently max- imizing colonization of developing seedlings of annual crops under conven- tional cropping systems. Since AM fungi differ in their ability to produce spores (Abbott and Gazey, 1994), the importance of the AM hyphal network in the soil and the survivability of AM hyphae contained within living and dead roots become critical if one is to develop management strategies of AM fungi in an annual cropping system. Walker and Smith (1984) showed that the rate of AM colonization was determined primarily by the density of AM propagules in the soil. In the southern parts of Australia, AM fungi appear to survive as hyphal networks depending on the degree of disturbance (e.g., Jasper et al., 1987), hyphae in dried root fragments (Tommerup and Abbott, 1981), and as spores (e.g., McGee et al., 1997). The form of inoculum that best survives from year to year is highly dependent upon the degree of soil dis- turbance. In arid and semiarid regions, fallowing is a necessary component of a water management plan. The length of time that a suitable AM host is absent from a field can result in a significant decline in AM propagules and limited colonization of the subsequent crop. Long-fallow disorder has now been attributed to declines in AM propagule densities due to the extended periods without a suitable host (Harinikumar and Bagyaraj, 1988; Thompson, 1987). Johnson and Pfleger (1992) emphasized that crops that generate large quan- tities of AM propagules are more effective in alleviating long-fallow disorder in subsequent crops than do crops that are only facultatively mycorrhizal. Using a combination of vital staining of AM fungal hyphae and AM fungal spores, McGee et al. (1997) determined that, for cotton production systems in southern Australia on a cracking, heavy clay soil, the viability of AM fungal spores is low and declines during the growing season (Figure 7.2). Furthermore, the infectivity of mycorrhizal propagules appeared to decline over time (32 wks) for dry soil in the absence of any direct impact on AM propagules. In addition, when fields were left fallow, any rainfall that occurred during the fallow resulted in germination of nondormant pro- pagules further exasperating the decline in AM inoculum (Pattinson and McGee, 1997). Paradoxically, McGee et al. (1997) reported that while long- fallow disorder should be a major problem in cotton production systems in southern Australia, the phenomenon is uncommon. They suggest that either current methods used to quantify fungal survival do not reflect the ability to initiate colonization in the field, or that the decline, while substantial during fallowing, does not reduce the level of AM inoculum below a threshold needed for colonization of cotton in southern Australia. While fallow alone may or may not have a negative impact on subse- quent mycorrhizal colonization, fallowing is usually followed by tillage. 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 153 154 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Figure 7.2 Changes in viability of AM fungal spores in stored field soils compared with freshly collected samples. Soil was obtained from a paddock that had never been cultivated and was used for grazing. Data from McGee, P. A., Pattinson, G. S., Heath, R. A., Newman, C. A., and Allen, S. J., New Phytol., 773–780, 1997. Kabir et al. (1999) were the first to show that when these two practices were combined, there were substantial declines in AM hyphal infectivity and metabolic activity leading to subsequent declines in crop growth and nutri- ent content. Herbicide and Pesticide Effects Many annual crops require several applications of pesticides during the growing season to maintain crop vigor and enhance yield quality. In addi- tion, herbicide applications are routinely made either during the growing season or during the fallow periods to ensure effective weed control. For cot- ton production systems on the Southern High Plains of west Texas, for exam- ple, cotton is treated with a variety of biocides during the growing season (Table 7.1) to control weeds and disease organisms. The recent introduction of Round-Up Ready Cotton to cotton production systems ensures that Round-Up herbicide will be applied to cotton fields for weed control when the genetically altered plant is used. The long-term effects of these and other genetic modifications of cotton (Table 7.1) on mycorrhizal development have not been examined in detail. 920103_CRC20_0904_CH07 1/13/01 10:54 AM Page 154 [...]... Swift, M.J and Anderson, J M., 1993 Biodiversity and ecosystem function in agricultural systems, in Biodiversity and Ecosystem Function, Ecological Studies 99, Schulze, E.D and Mooney, H.A (Eds.), Springer-Verlag, Berlin, Germany, 15 –41 Sylvia, D.M and Williams, S.E., 1992 Vesicular-arbuscular mycorrhizae and environmental stress, in Mycorrhizae in Sustainable Agriculture, Bethlenfalvay, G.J and Lindermann,... incorporating AM fungi within an annual cropping system that are cost effective over the long term 920103_CRC20_0904_CH 07 160 1/13/01 10:54 AM Page 160 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT We have found that for cotton production systems in west Texas, cotton planted into terminated winter wheat (Zak et al., 1998) is one approach that can be economically used to maintain AM inoculum... 920103_CRC20_0904_CH 07 164 1/13/01 10:54 AM Page 164 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT McGonigle, T.P and Miller, M.H., 1996 Development of fungi below ground in association with plants growing in disturbed and undisturbed soils Soil Biol Biochem., 28:263 –269 Menge, J.A., 1984 Inoculum production, in V Mycorrhiza, Powell, C.L and Bagyaraj A D.J (Eds.), CRC Press, Boca Raton FL, 1 87 –204... STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Cooper, A.J., 1 973 Root Temperature and Plant Growth Commonwealth Agricultural Bureaux Slough, England, 73 Daniels-Hetrick, BA., 1984 Ecology of VA mycorrhizal fungi, in V Mycorrhiza, A Powell C.L and Bagyaraj, D.J (Eds.), CRC Press, Boca Raton FL, 35 –55 Douds, D.D., Jr, Galvez, L., Janke R.R., and Wagoner P., 1995 Effect of tillage and farming... yield decline with continuous corn and soybean Agron J., 84:3 87 390 Johnson, N.C and Pfleger, F.L., 1992 Vesicular-arbuscular mycorrhizae and cultural stress, in Mycorrhizae in Sustainable Agriculture, Bethlenfalvay, G.J and Lindermann, R.G (Eds.), ASA Special Publication Number 54, Madison, WI, 71 –100 Kabir, Z., O’Halloran, I.P., and Hamel, C., 1999 Combined effects of soil disturbance and fallowing on... (Wright and Upadhyaya, 1998) Importantly, Wright et al (1996) found that all AM fungi tested produced glomalin In a recent survey of aggregate stability and glomalin from 37 sites across the U.S and from Scotland subjected to various land-use practices and cropping sequences, Wright and Upadhyaya (1998) found that aggregate stability was 920103_CRC20_0904_CH 07 158 1/13/01 10:54 AM Page 158 STRUCTURE AND FUNCTION. .. fertilizer continues to increase, the global climate changes, and water becomes more costly, there will be greater need to begin to reexamine our reliance on high input agriculture methods (Swift and Anderson 1993) and to expand upon our understanding of the roles and ecology of AM fungi in a greater variety of agriculture systems To incorporate effectively AM fungi within an annual cropping system, there... fraction of root infected New Phytol., 96:55 –69 Wien, H.W., Minott, P.L., and Grubinger, V.P., 1993 Polyethylene mulch stimulater early root growth and nutrient uptake of transplanted tomatoes J Amer Soc For Hort Sci., 118:2 87 311 920103_CRC20_0904_CH 07 166 1/13/01 10:54 AM Page 166 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Wright, S.F., Frank-Snyder, M., Morton, J.B., and Upadhyaya,... 78 Abbas Al-Ani, N.K and Hay, P.K.M., 1983 The influence of growing temperature on the growth and morphology of cereal seedling root systems J Exp Bot., 34: 172 0 – 173 0 Addy, H.D., Miller, M.H., and Peterson, R.L., 19 97 Infectivity of the propagules associated with extraradical mycelia of two AM fungi following winter freezing New Phytol., 135 :74 5 75 3 Addy, A.D., Boswell, E.P., and Koide, R.T., 1998... Early maturation Inclusion of Bacillus thuringiensis toxin gene Herbicide resistance Increased cold tolerance Increased drought tolerance Decreased seed gossypol content Increased seed size Oil quality Increased yields and decreased water use Increased insect resistance Direct application of herbicide for weed control Increased growth and yield Increased stand establishment and yield Increased usage . was 920103_CRC20_0904_CH 07 1/13/01 10:54 AM Page 1 57 158 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT linearly and significantly correlated with two components of total soil glom- alin. Therefore,. Page 155 156 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Varietal Responses and Breeding Programs To date, there have been no breeding programs established whose spe- cific aims. root 920103_CRC20_0904_CH 07 1/13/01 10:54 AM Page 1 47 148 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT elongation rates may significantly decrease (Klepper, et al., 1 973 ), affecting colonization