Preferential flow paths: biological 'hot spots' in soils

Preferential flow of water in soils is a common phenomenon. Our objective w as to investigate whether preferential flow paths have higher microbial bio mass and different microbial community structures than the rest of the soil . We stained the preferential flow paths in a forest soil with a food dye a nd sampled soil material from preferential Row paths and from the soil matr ix at four depths down to 1 m. Distinct differences in physico-chemical pro perties between preferential Row paths and soil matrix existed and, thus, d ifferent environmental living conditions for microorganisms. The experiment al addition of wood ash increased pH and base saturation in the preferentia l flow paths to a higher extent than in the soil matrix, highlighting the i mportance of preferential Row paths for solute input into the mineral soil. The organic C concentrations were approximately 10-70% higher in the prefe rential flow paths than in the matrix. The organic N concentrations were al so enriched in the preferential flow paths, as well as the effective cation exchange capacity and the base saturation. Microbial biomass determined wi th the fumigation-extraction method was 9-92% higher in the preferential fl ow paths than in the soil matrix, probably due to the better nutrient and s ubstrate supply. The DNA concentrations and direct cell counts showed a sim ilar pattern, while domain-specific genetic fingerprints based on small sub unit ribosomal RNA genes did not reflect the differences between preferenti al flow paths and soil matrix. Eukarya and Archaea only showed a depth-depe ndence and Bacteria showed no changes with flow region or with depths. Howe ver, Pseudomonas displayed different community structures between preferent ial flow paths and soil matrix. This indicated that possibly only few commu nities with a broad acceptance for substrates and aerobic as well as anaero bic growth specifically profit from the favourable conditions in the prefer ential Row paths. (C) 2001 Elsevier Science Ltd. All rights reserved.