Protist, Vol. 152, 301–314, December 2001 © Urban & Fischer Verlag http://www.urbanfischer.de/journals/protist Introduction Most of the studies including soil protozoa have fo- cused on either ecology and quantification (Ekelund and Rønn 1994) or diversity and taxonomy (Ekelund and Patterson 1997; Foissner 1993). One major rea- son for this is probably that the quantitative methods used for ecological studies (often the so-called most probable number (MPN) method (Darbyshire et al. 1974; Rønn et al. 1995)), do not allow species dis- tinction, whereas the methods used for qualitative work (the non-flooded petri dish method (Foissner 1987) and the coverslip method (Ekelund and Patter- son 1997)) do not allow proper quantification. Recent work has combined the ecology and tax- onomy of ciliates and testate amoebae (Foissner 1987, 1997, 1999a), probably because these organ- isms are large enough to allow a contemporary and Quantitative Estimation of Flagellate Community Structure and Diversity in Soil Samples Flemming Ekelund a,1 , Regin Rønn a , and Bryan S. Griffiths b a Department of Terrestrial Ecology, Zoological Institute, Universitetsparken 15, DK-2100 Copenhagen Ø, Denmark, b Soil Plant Dynamics Unit, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK Submitted May 22, 2001; Accepted July 9, 2001 Monitoring Editor: Michael Melkonian Heterotrophic flagellates occur in nearly all soils and, in most cases, many different species are pre- sent. Nevertheless, quantitative data on their community structure and diversity are sparse, possibly due to a lack of suitable techniques. Previous studies have tended to focus on either total flagellate numbers and biomass, or the identification and description of flagellate species present. With the in- creased awareness of the role of biodiversity and of food web interactions, the quantification of species within the community and their response to environmental change is likely to become more important. The present paper describes a modification of the most probable number method that al- lows such a quantification of individual flagellate morphotypes in soil samples. Observations were also made on the biomass of flagellate morphotypes in soil. 20 to 25 morphotypes of heterotrophic flagellates were detectable per gram of two different arable soils, which were treated experimentally to test the technique. One of the soils was fumigated with chloroform vapour for different lengths of time (0, 0.5, 2 or 24 hours); this led to a reduction in the number of morphotypes, in the Shannon diversity index and in the evenness. The other soil was planted with wheat, and while rhizosphere soils con- tained the same morphotypes as bulk soil, the abundance of individual morphotypes was significantly different and the Shannon diversity index in rhizosphere soils was significantly higher. Soil influenced by an elevated CO 2 level likewise differed significantly in morphotype abundance when compared to soil exposed to ambient levels of CO 2 . The technique recovered more than 80% of the discernible mor- photypes and could also be used to quantify amoebal and ciliate communities in a similar way. Protist 1434-4610/01/152/04-301 $ 15.00/0 ORIGINAL PAPER 1 Corresponding author; fax 45 3532 1250 e-mail fekelund@zi.ku.dk