Measuring changes in soil microbial populations by analysis of their phospholipid signatures
Microorganisms are the most abundant life form in soil. This soil microbial community consists of a large number of types of organisms - the actual number is so large that it will probably never be known. The biomass of each type present, the microbial community structure, is affected by many factors such as nutrient level and source, temperature, moisture, available oxygen and the presence of other types. These organisms affect many aspects of agriculture-related activities from being essential for plant growth as providers of necessary nutrients, through such processes as organic matter decomposition and nitrogen fixation, to being unwanted pests as the agents of plant diseases. Their universal presence and the sensitivity of their community structure to changes in their environment also makes them good candidates for indicators of changes in soil quality. To use them as indicators, simple, reliable and sensitive methods of measuring changes in the soil microbial community structure must be available. One method under development is based on the chemical composition of the phospholipids present in the cell walls of the microorganisms. Phospholipids are a major cell wall component and their chemical composition differs for different types of organisms thereby providing a link between chemical composition and community structure. In this study we examined the merits of two different measures of phospholipid composition, fatty acid composition and class composition, for detecting changes induced by differences in tillage practice at four paired field sites. Fatty acid composition was determined by gas chromatograph and differences were observed between the paired sites, between the till and no-till treatments and between high and low plot elevations. Class composition was determined by 31P NMR spectroscopy and consistent differences between tillage treatments were also found. The complexity of the differences in the fatty acid composition and the limited state of our knowledge of the composition of most soil organisms restricted the conclusions the could be drawn about the changes in community structure involved.