Testing soil nematode extraction efficiency using different variations of the Baermann-funnel method

Nematodes are increasingly used as powerful bioindicators of soil food web composition and functioning in ecological studies. Todays' ecological research aims to investigate not only local relationships but global patterns, which requires consistent methodology across locations. Thus, a common and easy extraction protocol of soil nematodes is needed. In this study, we present a detailed protocol of the Baermann-funnel method and highlight how different soil pre-treatments and equipment (soil type, soil height, sieving, and filter type) can affect extraction efficiency and community composition by using natural nematode communities. We found that highest nematode extraction efficiency was achieved using lowest soil height as indicated by the thickness of the soil sample in the extractor (1, 2, or 4 cm soil height) in combination with soil sieving (instead of no sieving), and by using milk filters (instead of paper towels). PCA at the family level revealed that different pre-treatments significantly affected nematode community composition. Increasing the height of the soil sample by adding more soil increased the proportion of larger-sized nematodes likely because those are able to overcome long distances but selected against small nematodes. Sieving is suggested to break up soil aggregates and, therefore, facilitate moving in general. Interestingly, sieving did not negatively affect larger nematodes that are supposed to have a higher probability of getting bruised during sieving but, even if not significant, tended to yield more extracted nematodes than no sieving. We therefore recommend to use small heights of sieved soil with milk filter to extract free-living soil nematodes with the Baermann-funnel method. The present study shows that variations in the extraction protocol can alter the total density and community composition of extracted nematodes and provides recommendations for an efficient and standardized approach in future studies. Having a simple, cheap, and standardized extraction protocol can facilitate the assessment of soil biodiversity in global contexts.