Comparison of methods to investigate microbial populations in soils under different agricultural management

The microbial community in sludge-treated and nearby untreated soils was investigated using direct microscopic counting, plate culture, carbon substrate utilisation (Biolog), and fatty acid methyl esters. Long-term sewage sludge applications had resulted in higher concentrations of organic carbon and had altered other factors in the soil, including the concentration of metals. Bioluminescence was inhibited in assays of the sludge-treated soil, although microbial counts were similar in all soils. A detailed analysis of carbon substrate utilisation patterns and fatty acid methyl esters showed qualitative differences in the microbial populations. This work shows that a variety of approaches are required to assess microbial communities in soil where, despite large differences in land management, the populations are similar in size and overall composition.