Soil solarization reduces arbuscular mycorrhizal fungi as a consequence of weed suppression

Soil solarization, the process of heating soil by covering fields with clear plastic, is a promising method to reduce populations of soilborne pests and weeds without the use of pesticides. However, the destruction of beneficial organisms such as arbuscular mycorrhizal (AM) fungi also may occur, thereby reducing positive effects of solarization. We compared the effects of solarization and chemical fumigants on the survival of indigenous AM fungi in 1995 and 1996. The infectivity of AM fungi was monitored before and after solarization using a greenhouse bioassay with Sorghum bicolor L. for both years. AM colonization of roots was also monitored in the field 8 months after solarization in 1995. Weed densities were measured 8 months after treatment in 1996. Solarization increased the average daily soil temperature 6-10°C and the maximum soil temperature reached by 10-16°C (5-20 cm depth). Solarization did not reduce the infectivity of AM fungi immediately after the solarization period in either year, as determined by the greenhouse bioassay. Infectivity was greatly reduced in solarized plots 8 months after solarization (over winter) in both years as assessed in the field (1995) or with the greenhouse bioassay (1996). Fumigation with metam sodium at 930 1 ha-1 (350 kg active ingredient ha-l) reduced the infectivity of AM fungi in both years, and fumigation with methyl bromide at 800 kg ha-1 eliminated infection by AM fungi. Solarization was as effective as methyl bromide and metam sodium at 930 1 ha-l in controlling winter annual weeds measured 8 months after treatment. Solarization apparently reduced AM fungi in soil indirectly by reducing weed populations that maintained infective propagules over the winter. Fumigation with metam sodium or methyl bromide directly reduced AM fungi in soil.