CHANGES IN RHIZOSPHERE MICROFLORA AND MICROFAUNA 10 YEARS FOLLOWING DOUGLAS-FIR LIVE TREE INJECTION WITH CHLOROPICRIN OR METHYLISOTHIOCYANATE

Live Douglas-fir (Pseudotsuga menziesii (Mirb.) France) trees were inj ected in 1981 with chloropicrin or methylisothiocyanate (MITC) in an e ffort to control laminated root rot caused by Phellinus weirii (Murr.) Gilb. Soil samples were collected in 1991 from around structural root s 0.5, 1, and 2 m from the base of injected or noninjected trees. The activity of fungi and bacteria, total fungal and bacterial biomass, th e number of protozoa (flagellates, ciliates, and amoebae), and the num ber and types of nematodes were evaluated, Active fungal biomass was r educed by both chemicals as compared with noninjected trees 10 years a fter application Both active and total bacterial biomass were also sig nificantly lower around roots of chloropicrin- and MITC-treated trees, as were flagellate numbers. The number of bacterial-feeding nematodes was decreased around roots of chloropicrin-treated trees, while other nematode-feeding groups were not changed. The number of root-feeding, bacterial-feeding, and fungal-feeding nematodes were significantly gr eater around MITC-treated roots, while predatory nematode numbers were lower, than around control roots. Reduced bacterial and fungal biomas s around MITC-treated trees may be the result, therefore, of increased feeding by nematodes, and thus MITC-treated trees may benefit from mo re rapid nutrient-cycling rates but may suffer more pest damage from r oot-feeding nematodes as a result of the chemical application. Since m any organism groups were reduced around roots of chloropicrin- and MIT C-treated trees, this suggests possible impacts related to reductions in nutrient cycling rates and production of plant-available N around t hese trees.