Soil microbial community structure: Effects of substrate loading rates

A fuller understanding of the interactions which affect rhizosphere microbial community structure requires experimental manipulation of the individual components of that interaction (e.g. amount and composition of exudate, soil moisture and soil nutrient status). We describe an experiment where a synthetic root exudate was applied continuously to a soil held at constant water potential. The solution contained compounds characteristic of root exudates (fructose, glucose, sucrose, succinic acid, malic acid, arginine, serine and cysteine), which were added at a range of concentrations. After 14 d of such substrate addition, a central portion of soil, known to be influenced by the added substrate, was removed for analysis. Microbial community structure of this central core was determined by the broad-scale measurements; community DNA hybridisation and %G+C profiling, and phospholipid–fatty acid analysis (PLFA). The trend was that microbial community structure changed consistently as substrate loading increased, and that fungi dominated over bacteria at high substrate loading rates. The DNA and the PLFA analyses both indicated that there was a coherent gradient of changes with increased substrate loading. This may have arisen as a consequence of the competitive ability of soil microorganisms being dependent on the quantity of available substrate.