Ectomycorrhizas and rhizosphere microorganisms of seedlings of Pseudotsuga menziesii (Mirb.) Franco planted on a degraded site and inoculated with forest soils pretreated with selective biocides

ABSTRACT

Inoculation of planting holes with small amounts of soil from a mature forest or a plantation can improve formation of ectomycorrhizas on Pseudotsuga mensiesii (Mirb.) Franco seedlings in degraded eleareuts in southwestern Oregon. To determine the component (s) of transferred soil responsible for increased ectomycorrhiza formation, we treated soil from a clearcut, a mature forest, and a plantation with one of the following: fertilizer to test for the effect of nutrients, dimethoate and carbufuran to test for the effect on microarthropods or nematodes, fumagillin to test for the effect on protozoa, captan to test for the effect on fungi, penicillin and oxytetracycline to test for the effect on bacteria, pasteurization to test for the effect of active forms of organisms. Tyndallization to test for the effect of resting forms of organisms, or water as a control. We studied the effect of inoculation with soil subjected to these treatments on number and types of ectomycorrhizas, and length of active mycelium, and number of active bacteria in the rhizosphere. Inoculation with untreated forest or plantation soils increased the number of ectomyeorrhizas but did not change the mycorrhizal types present. Most agents had different effects in different soils, inoculation with pasteurized and Tvndallized clearcut and plantation soils increased the number of Rhizopagon- and Thelephora-type ectomyeorrhizas and decreased the number of active bacteria, as did untreated forest soil. We hypothesize that the role of the soil transfer is to provide a rhizosphere environment free from a deleterious organism present in the clearcut. In this environment, beneficial organisms present in the clearcut or brought in with the seedling from the nursery can proliferate.