Improved quantification of active soil microfauna by a "counting crew"

Recently, a comparative study revealed that the predominately used most probable number (MPN) method yields seriously biased estimates for active soil ciliates (Protozoa). The direct counting method, in contrast, gave reliable and reproducible data with a high level of discrimination between the invstigated sites between the investigated sites. However, this method requires that fresh soil samples are used, limiting its versatility. In order to overcome this problem a team ("counting crew") of 13 people was trained to carry out direct counts of soil microfauna simultaneously. The densities of active ciliates, rotifers, nematodes and tardigrades in the litter layer of a spruce stand were assessed by the direct counting technique. It could be shown that there was no significant influence of individual crew members on ciliate, rotifer or nematode numbers. The ciliate abundance did not vary significantly in the bulk sample with time of day. A significant increase in active ciliates was observed when counts took longer than 90 min. Comparing our results to literature data obtained by diverse extraction methods revealed that the direct counting method is very efficient for microscopic soil metazoa as well. If a counting crew is available, more precise and realistic data on active soil microfauna can be obtained.