Standardised protocols for soil fauna extraction: crafting manuals, wet and dry extractors

Understanding the status and global trends of soil invertebrate diversity requires accurate and comparable data across geographical regions. However, soil animal extraction approaches still vary among laboratories, and no commonly accepted, openly available and well-documented protocols exist across taxa. Here, we present harmonised methodologies, assembled by an international group of experts, for extracting soil- and litter-inhabiting nematodes, enchytraeids, microarthropods, and larger invertebrates. Illustrated with images and videos, the protocols include advice for overcoming the most frequently encountered issues (‘expert tips’) for maximising extraction efficiency and reproducibility. In addition, we provide results from two pilot experiments that further improve nematode and large invertebrate extractions. We show that using two layers of milk filters instead of one in wet extraction of nematodes yields very similar extraction efficiency and has little effect on the sample cleanliness. Further, we demonstrate that on average 31.3% of large soil animals (body length 3 mm and longer) are overlooked during hand sorting but were captured by heat extraction of the sorted samples across three geographically distinct laboratories. Observed differences among the laboratories call for standardized tests of extraction efficiency of large soil animals across regions and research groups. Overall, we provide openly available expert protocols for assessing soil animal taxa in ecological studies worldwide, ultimately facilitating comparisons for a better understanding of the distribution and dynamics of soil biodiversity.