Survival and growth of Pseudotsuga menziesii seedlings inoculated with biocide-treated soils at planting in a degraded clearcut

To determine the component(s) of transfer soils responsible for increased seedling survival and growth, we inoculated planting holes with forest, plantation, and clear-cut soils that were (i) treated with fertilizer to test for effects of nutrients; treated with biocides to test tar effects of (ii) microarthropods or nematodes, (iii) protozoa, (iv) fungi, and (v) bacteria: (vi) pasteurized; (vii) Tyndallized: and (viii) untreated. Odds of survival were increased by inoculation with untreated plantation soils, but not if they were fertilized or treated with dimethoate + carbofuran (grazercide), fumagillin (protozoacide), or oxytetracycline + penicillin (bactericide). Addition of untreated forest soil did not increase survival. For all soils, survival odds were increased by captan (fungicide), pasteurization, and Tyndallization. Treatments affected seedling dry weights differently than survival. Untreated plantation and forest soil transfers increased dry weights whereas neither did when treated with dimethoate + carbofuran. Dry weights of seedlings given clear-cut soil were increased by fertilization, pasteurization, and Tyndallization of the soil; the latter two treatments also increased the number of short roots. We hypothesize that stimulation of seedling growth by soil transfers was related to an increased rate of nutrient mineralization due to microbivorous soil animals contained within the transfer soils. Soil transfers may have enhanced seedling survival by at least two mechanisms: (i) by providing a safe site for beneficial rhizosphere organisms to proliferate, free from competing organisms that have proliferated in the clear-cut soil; (ii) through volatile organic compounds that stimulated seedling root growth, especially ethylene, which previous studies have shown to be produced at significantly lower rates in soils of the clear cut compared with adjacent forest soils. Further research is needed to distinguish between these possibilities.