BIOCIDE MANIPULATION OF N FLOW TO INVESTIGATE ROOT MICROBE COMPETITION IN FOREST SOIL

Root/microbe competition was investigated as a mechanism controlling f ertilizer N uptake by coniferous forest trees. Combinations of biocide s both with and without N-15 labelled urea, were applied to microcosms containing Sitka spruce seedlings, to selectively inhibit target micr obial groups which may be competing with roots for N. After 1 growing season, concentrations of fertilizer N and total N in the trees, as we ll as populations of microbes and animals, were determined. Biocidal i nhibition of microbial populations, particularly of fungi, was associa ted with significant increases in concentrations of fertilizer N and t otal N in Sitka spruce seedlings. Application of the fungicide benlate , for example, increased the concentration of fertilizer-derived N in spruce needles by one order of magnitude, and was associated with sign ificant reductions in FDA-active hyphal lengths of fungi. This approac h to investigating N-flow offers considerable potential for short term experiments involving competition for fertilizer/available N, where t he microbial biomass represents the major 'sink' for N in competition with roots.