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[SANET-MG] On-farm production of AM fungus inoculum

Hello folks,

I just ran across an article about "On-farm production of AM fungus inoculum..." that may be of interest to some of you...


Bioresource Technology
Volume 97, Issue 6 , April 2006, Pages 809-818

On-farm production of AM fungus inoculum in mixtures of compost and vermiculite D.D. Douds, Jr.a, , , G. Nagahashia, P.E. Pfeffera, C. Reiderb and W.M. Kayserc

On-farm production of arbuscular mycorrhizal (AM) fungus inoculum can reduce the cost of the inoculum and increase utilization of this symbiosis in plant production. Bahiagrass (Paspalum notatum Flugge) seedlings, colonized by AM fungi, were transplanted into raised bed enclosures. Media within the enclosures was vermiculite mixed with either field soil or yard clippings compost in Experiment I and vermiculite mixed with yard clippings compost or dairy manure/leaf compost in Experiment II. Compost and vermiculite mixtures yielded more propagules of AM fungi than soil-based mixtures in Experiment I. Growth of plants in a 1:4 (v/v) mixture of yard clippings compost and vermiculite produced more inoculum (503 propagules cm−3) than growth in 1:9 and 1:99 (v/v) mixtures (240 and 42 propagules cm−3, respectively). Water, inorganic nutrient solution minus P, and fish protein digest were added to inoculum production enclosures in Experiment II. Results indicated that supplemental nutrient addition was unnecessary. This method produces a concentrated inoculum of AM fungi in a form readily used as an amendment to horticultural potting media for the production of vegetable seedlings.

Keywords: Sustainable agriculture; Organic agriculture; Biofertilizer


4. Discussion
Inoculum of AM fungi was successfully produced in compost diluted with vermiculite. Overall averages of 530 and 505 propagules cm&#8722;3 were produced in Experiments I and II, respectively, and up to 80 spores cm&#8722;3, in 1:4 (v/v) yard clippings compost and vermiculite. This greatly surpasses the target inoculum density of 80?100 propagules cm&#8722;3 to qualify as mass production (Feldmann and Grotkass, 2002). The lower average of 505 propagules cm&#8722;3 yields 37.57 × 106 propagules per 0.61 × 0.61 m enclosure section. A target inoculation of 100 propagules per vegetable seedling to ensure colonization during growth in the greenhouse prior to outplanting means one enclosure section can be used to inoculate 375,700 plants. This translates to 7.6 ha of tomatoes at a planting density of 49,400 plants ha&#8722;1, or five times that with use of the entire enclosure. Uniformly mixing such a small amount of inoculum into the potting media is difficult, so a more generous inoculation may be more practical.

On-farm production of AM fungus inoculum via different techniques has been described previously. Sieverding, 1987 and Sieverding, 1991 developed a procedure in Colombia whereby Glomus manihotis was introduced into fumigated soil. Direct comparison with the method reported here is difficult because total propagules were not estimated. After 4 months of growth of host plants, 42?96 spores cm&#8722;3 were produced (assuming an average bulk density of soil of 1.2 (Thompson and Troeh, 1973)). Only 9 spores cm&#8722;3 were produced without fumigation. Dodd et al. (1990) used this method to produce inocula containing three AM fungi, including approximately 208 spores cm&#8722;3 of G. manihotis.

Another method was developed at the Tata Energy Research Institute in India (Douds et al., 2000 and Gaur, 1997). One of a number of AM fungi was inoculated into furrows in raised beds of fumigated soil. Three crops, e.g., sorghum, corn, and carrot, were grown successively in the bed, each for 3 months. The soil and roots were chopped and remixed prior to planting the next nurse host. This method produced an average of 22.7 infectious propagules cm&#8722;3 after three 1-year cycles. A modification of this method using one of three vegetable crops as a host plant produced approximately 1 propagule cm&#8722;3 in soil amended 1:1 or 1:2 (v/v) with compost (Gaur et al., 2000 and Gaur and Adholeya, 2002).

The method developed here is competitive with existing methods and, in addition to not requiring fumigation, offers several advantages to farmers in temperate climates. First, bahiagrass is a proven host plant that supports abundant colonization and sporulation by most, if not all, AM fungi. In addition, as a tropical C4 grass it is frost killed. This minimizes its potential to become a weed pest when the inoculum is used in the field. Also, bahiagrass is taxonomically unrelated to most crops and, therefore, is unlikely to allow the proliferation of crop pathogens. Second, all materials for the construction of the enclosures are readily available to farmers. The precolonized host plants could be purchased or grown on-site using commercially-available inocula. Another strength lies in the benefits of compost. Compost provides all the nutrients necessary for the growth of bahiagrass. The P concentration of compost, however, requires that it be diluted with a relatively inert material, such as vermiculite, prior to use because high levels of available P suppress the colonization of roots by AM fungi (Smith and Read, 1997). Compost also provides a broad spectrum of microbes to benefit the biological health of the soil and is known to suppress plant diseases (Hoitink and Fahy, 1986 and Kim et al., 1997). Lastly, a taxonomically diverse mixed inoculum can be produced when different AM fungi are inoculated into separate enclosure sections. Recent research indicates that there is functional diversity among AM fungi (Smith et al., 2000 and Stampe and Daehler, 2003). Though some AM fungi are beneficial in a wide variety of soil?plant combinations (Sylvia et al., 1993), some data suggest that host benefits are greater with certain AM fungi than others (Blaszkowksi, 1993 and Pope et al., 1983). A diverse inocula increases the opportunity for an optimal match with the eventual crop host, and it also allows for the inclusion of Gigaspora spp. to aide soil aggregation (Miller and Jastrow, 1992 and Wright and Upadhyaya, 1996). Gigaspora spp. are typically absent from commercial formulations.

This inoculum production system targets vegetable producers who grow their own seedlings for transplant to the field. The most practical use for this inoculum is mixing it into horticultural potting media in which the seedlings are grown. Inocula produced by all currently available on-farm methods are not amenable to large scale direct application to fields via farm machinery. Direct application to the field is possible in labor intensive farming systems in the tropics (Gaur et al., 2000 and Sieverding, 1987) or in small scale organic or community supported agriculture (CSA) farms.

Though plant response to AM fungus inoculation may vary from positive to negative in the presence of different AM fungi (Klironomos, 2003 and Marin et al., 2003), there are management situations in which the response to AM fungus inoculation can be more reliably predicted. Certain crops such as Cruciferae, spinach, and sugar beet do not become colonized by these fungi and will not respond to inoculation. Soil solarization and fumigation treatments adversely affect indigenous AM fungi, and inoculation with AM fungi has been shown to enhance plant growth and yield following these treatments (Wininger et al., 2003). Other situations in which AM fungus inoculation would be beneficial are after a long period of bare fallow (Thompson, 1987), following a nonmycotrophic crop such as rapeseed, or in soils with low available P.

The inoculum production method described here also may be used to proliferate indigenous AM fungi. Therefore, farmers may be able to re-introduce to their production fields the AM fungi adapted to their soils. Doing so may only require adding soil from an uncultivated part of the farm (e.g., fence row or natural wooded area), to the compost. No pre-inoculated plants would be required. Further, a contributing factor to the high yields found when native soils are first put under cultivation is the soil biological community (Aleman, personal communication), important members of which are the AM fungi.

We have developed a method for on-farm production of hundreds of millions of propagules of AM fungi within raised bed enclosures containing compost and vermiculite. More research is needed to develop formulae to predict the proper dilution ratio for optimum inoculum production given the nutrient analysis of specific composts and explore the production of indigenous AM fungi in similar enclosures.

Joel Gruver
Dept of Soil Science
NC State University

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