Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/15/2010
Publication Date: 1/14/2011
Citation: Douds, D.D., Nagahashi, G., Wilson, D.O., Moyer, J. 2011. Monitoring the decline in AM fungus populations and efficacy during a long term bare fallow. Plant and Soil. 342:319-326. Interpretive Summary: Arbuscular mycorrhizal [AM] fungi are beneficial soil fungi that form a symbiosis with the roots of crop plants. Among the benefits to the plant attributed to AM fungi are increased nutrient uptake, drought resistance, and disease resistance. This makes the utilization of the symbiosis very important if we are to increase the sustainability of US agriculture. However, since their primary mode of action is through increased uptake of the plant mineral nutrient phosphorus [P], their role in the high P soils of the mid-Atlantic region is of interest. Determining this role requires that we grow plants in soils free of AM fungi to serve as controls. We have approached this by covering a field plot with ground cover fabric to prohibit plant growth and, in so doing, selectively starve the AM fungi in the soil. Analysis of soil collected from under the cover over a three year period indicated a sharp decline in numbers of AM fungi during the second growing season of coverage. Growth of leek seedlings in this soil showed a progression from no response to added AM fungi in soil collected early in the experiment, to a significant growth increase in response to added AM fungi later in the experiment, indicating an ineffective background native population. The results indicate that we have developed a method that will allow the future testing of the functioning of AM fungi in high P soils.
Technical Abstract: Producing nonmycorrhizal plants in the field is a challenge due to the ubiquitous distribution of arbuscular mycorrhizal [AM] fungi and impacts of chemical treatments upon nontarget organisms. A field plot was covered with ground cover fabric to prohibit plant growth and take advantage of the obligate symbiotic nature of AM fungi to selectively starve and remove them from the soil microbiological community. The decline in the AM fungus population was monitored through spore counts and most probable number bioassays. Response to inoculation experiments were conducted to contrast the response of Allium porrum L. to inoculation with in vitro produced spores of Glomus intraradices Schenck and Smith when grown in the AM fungus-depleted soil vs soil from an adjacent, cropped plot. Data indicated a strongly diminished, yet still viable population of AM fungi after 39 months. Plants grown in cropped soil showed no growth response nor increase in percentage root length colonized as a result of inoculation, while the response to inoculation of plants grown in the covered soil increased as the population of AM fungi declined below one propagule cm**3.