Protist, Vol. 151, 69–80, May 2000 © Urban & Fischer Verlag http://www.urbanfischer.de/journals/protist Introduction The protozoa that are adapted for life in soil share two important, if paradoxical characteristics. They are be- lieved to be important grazers of bacteria and other micro-organisms, and this activity appears to stimu- late the rates of carbon and nitrogen cycling in soil (Alphei et al. 1996; Bonkowski and Schaefer 1997; Clarholm 1989; Ekelund and Rønn 1994; Griffiths et al. 1999; Strauss and Dodds 1997; Verhagen et al. 1993; Wright et al. 1995). As soil protozoa are so small (5–200 μm), this would imply that they are nu- merically abundant; but this is not usually obvious to the investigator who may, by direct observation with a microscope, find some testate amoebae, but little else. The paradox arises from the fact that protozoa in the trophic stage are very vulnerable to desicca- tion; they are separated from their surrounding envi- Estimating the Growth Potential of the Soil Protozoan Community Bland J. Finlay a,1 , Helaina I. J. Black b , Susan Brown a , Ken J. Clarke a , Genoveva F. Esteban a , Ruth M. Hindle a , José L. Olmo a , Alison Rollett b , and Keith Vickerman c a Institute of Freshwater Ecology, Windermere Laboratory, The Ferry House, Ambleside, Cumbria LA22 0LP, UK b Institute of Terrestrial Ecology, Merlewood Research Station, Grange-over-Sands, Cumbria LA11 6JU, UK c Division of Environmental and Evolutionary Biology, Graham Kerr Building, The University of Glasgow, Glasgow G12 8QQ, UK Submitted January 27, 2000; Accepted February 28, 2000 Monitoring Editor: Michael Melkonian 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, tes- tate 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 spe- cific time intervals, after quantitative re-wetting of air-dried soil with rain water. No exogenous car- bon 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 × 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). ORIGINAL PAPER 1 Corresponding author; fax 44-15394-46914; e-mail bjf@ceh.ac.uk Protist 1434-4610/00/151/01-069 $ 12.00/0