Increasing Endomycorrhizal Fungus Inoculum in Forest Nursery Soil with Cover Crops

Corn, millet, sudex, and sorghum were all effective cover crops for increasing inoculum density of vesiculararbuscular fungi (Glomus spp.) in nursery soils. Spore production was increased approximately 7 to 12 times, depending on the cover crop used. Sweetgum seedlings did not differ significantly in size on plots previously planted with any of the four cover crops. Eighty-nine percent of the sweetgum seedlings grown after cover cropping had root-collar diameters exceeding the minimum (0.25 inch) recommended for outplanting of this species. A reliable supply of high-quality seedlings is a prerequisite for successful artificial hardwood regeneration. Unfortunately, forest tree nurseries have not consistently produced seedlings of the commercially important hardwood species that form vesicular-arbuscular (VA) mycorrhizae. Development of such mycorrhizae early in the growing season is beneficial to seedling development and can result in consistent production of highquality seedlings (Bryan and Kormanik 1977, Kormanik et al. 1976, South 1977). One method of increasing nursery soil inoculum levels is to plant cover crops having fibrous roots that are hosts for VA mycorrhizal fungi. Of course, crops which are preferred hosts for commonly occurring pathogenic fungi in nursery soils should be avoided. Periodic fumigation is necessary in most nurseries; however, it simultaneously eliminates or reduces beneficial mycorrhizal fungi, as well as the root pathogens and weed seeds which are the targets of fumigation. It takes at least one growing season after fumigation to build up the inoculum potential of the VA fungi to effective levels. In an extensive study of various forest tree nurseries in the SOUTHERN JOURNAL OF APPLIED FORESTRY southeastern United States, Barnard ( 1977) reported that nurseries employing soil fumigation were characterized by low VA inoculum densities. The purpose of this study was to determine the capacity of some common cover crops for increasing VA inoculum density in nursery soils and to determine how inoculum densities might affect the subsequent crop of hardwood seedlings.