Estimating the growth potential of the soil protozoan community

We have developed a method for determining the potential abundance of free-living protozoa in soil. The method permits enumeration of four major functional groups (flagellates, naked amoebae, testate amoebae, and ciliates) and it overcomes some limitations and problems of the usual 'direct' and 'most probable number' methods. Potential abundance is determined using light microscopy, at specific time intervals, after quantitative re-wetting of air-dried soil with rain water. No exogenous carbon substrates or mineral nutrients are employed, so the protozoan community that develops is a function of the resources and inhibitors present in the original field sample. The method was applied to 100 soil samples (25 plots x 4 seasons) from an upland grassland (Sourhope, Southern Scotland) in the UK. Median abundances for all four functional groups lie close to those derived from the literature on protozoa living in diverse soil types. Flagellates are the most abundant group in soil, followed by the naked amoebae, then the testate amoebae and ciliates. This order is inversely related to typical organism size in each group. Moreover, preliminary evidence indicates that each functional group contains roughly the same number of species. All of these observations would be consistent with soil having fractal structure across the size-scale perceived by protozoa. The method described will be useful for comparing the effects on the soil protozoan community of different soil treatments (e.g. liming and biocides).