Changes in soil microbial community structure in the presence of microbial-feeding nematodes and protozoa

The effects of microfaunal grazing on the composition of the microbial comm unity were studied in a soil microcosm experiment. Soil and chopped grass r esidues, both defaunated by chloroform fumigation, were mixed together and inoculated with: a bacterial suspension (control treatment - Ctrl); control plus nematodes (Nema:), control plus protozoa (Prot), or control plus both nematodes and protozoa (NxP). Replicates of each treatment were planted wi th a single, sterile ryegrass seedling. After six weeks, plant growth had b een significantly enhanced in treatments with protozoa (Prot and NxP) compa red to the Ctrl and Nema treatments. The microbial community in the microco sms was then analysed metabolically (by phospholipid fatty acid analysis of lipids directly extracted from soil), physiologically (from a community-le vel physiological profile using Biolog plates) and genetically (from analys is of DNA directly extracted from soil). There were distinct shifts in the microbial community as a result of grazing pressure, although only PLFA ana lysis allowed any interpretation of the changes. There was a greater propor tion of Gram-positive bacteria in the presence of protozoa.