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 funct ional groups (flagellates, naked amoebae, testate amoebae, and ciliates) an d it overcomes some limitations and problems of the usual 'direct' and 'mos t probable number' methods. Potential abundance is determined using light m icroscopy, at specific time intervals, after quantitative re-wetting of air -dried soil with rain water. No exogenous carbon substrates or mineral nutr ients are employed, so the protozoan community that develops is a function of the resources and inhibitors present in the original field sample. The m ethod was applied to 100 soil samples (25 plots x 4 seasons) from an upland grassland (Sourhope, Southern Scotland) in the UK. Median abundances for a ll four functional groups lie close to those derived from the literature on protozoa living in diverse soil types. Flagellates are the most abundant g roup in soil, followed by the naked amoebae, then the testate amoebae and c iliates. This order is inversely related to typical organism size in each g roup. Moreover, preliminary evidence indicates that each functional group c ontains roughly the same number of species. All of these observations would be consistent with soil having fractal structure across the size-scale per ceived by protozoa. The method described will be useful for comparing the e ffects on the soil protozoan community of different soil treatments (e.g. l iming and biocides).