Preferential flow paths: biological ‘hot spots’ in soils

Preferential flow of water in soils is a common phenomenon. Our objective was to investigate whether preferential flow paths have higher microbial biomass 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 and sampled soil material from preferential flow paths and from the soil matrix at four depths down to 1 m. Distinct differences in physico–chemical properties between preferential flow paths and soil matrix existed and, thus, different environmental living conditions for microorganisms. The experimental addition of wood ash increased pH and base saturation in the preferential flow paths to a higher extent than in the soil matrix, highlighting the importance of preferential flow paths for solute input into the mineral soil. The organic C concentrations were approximately 10–70% higher in the preferential flow paths than in the matrix. The organic N concentrations were also enriched in the preferential flow paths, as well as the effective cation exchange capacity and the base saturation. Microbial biomass determined with the fumigation-extraction method was 9–92% higher in the preferential flow paths than in the soil matrix, probably due to the better nutrient and substrate supply. The DNA concentrations and direct cell counts showed a similar pattern, while domain-specific genetic fingerprints based on small subunit ribosomal RNA genes did not reflect the differences between preferential flow paths and soil matrix. Eukarya and Archaea only showed a depth-dependence and Bacteria showed no changes with flow region or with depths. However, Pseudomonas displayed different community structures between preferential flow paths and soil matrix. This indicated that possibly only few communities with a broad acceptance for substrates and aerobic as well as anaerobic growth specifically profit from the favourable conditions in the preferential flow paths.