How to find the organisms living in soils?
Well, this depends on their size. Some are easily visible and can just be collected to then count and identify them. The earthworms for example.
To find macro- and mesofauna, we dig a 25cm x 25cm square to a depth of 20cm. This block of soil is lifted onto a plastic sheet with a shovel. To search for life, we kneel over the soil block and carefully crumble it apart. We collect all the life that can be seen with the naked eye – unless it is faster than us and quickly escapes: earthworms go into one plastic container, microarthropods and all other crawling life go into another container. Afterwards, we take our findings back to the lab. We often also take soil samples on-site for chemical analyses of the soil, such as pH value or available nutrients in the soil.
Earthworms
When it comes to their identification, it gets a bit more difficult. There are 10400-11200 species of earthworms of which 7000 are approximated to be true earthworms. However, those numbers might even be much higher since a lot of species are yet to be discovered. It is estimated that the real number ranges at 30 000 true earthworms (Soil biodiversity atlas, 2016). That is a crazy amount if you think about it. They all look more or less wormy, some a bit longer, some a bit thicker, some a bit darker but all in all not at all that easy to identify them. A microscope, a lot of experience and knowledge and some patience is needed. Most of them can be identified through their ‘belt’, the clitella, giving some hints. The clitella is used in reproduction for the formation of cocoons during mating. Further identificiation help is provided by the segmentation, their colour and size and the gut anatomy.
Sampling the soil here at our university, which is situated on top of this hill, close to the forests, we encountered some looong earthworms.
In the university campus in Pontevedra, we encountered very chubby worms. While the worms in the field are stored with some soil, in the laboratory they need to be “fixed” in alcool to keep their tissue intact. Otherwise, being invertebrates, their body would decompose very rapidly.
Earthworms are often killed for identification because a detailed examination of their anatomy is necessary to determine certain species. This may involve dissecting the worm’s body and examining organs such as the reproductive system and other internal structures. Some characteristics important for earthworm identification may not be externally visible or may vary depending on the developmental stage or gender. While there are alternative methods for identifying earthworms, such as DNA analysis or non-invasive imaging techniques, these may not always be as accurate or practical as traditional anatomical examination. Hopefully, in the future, less invasive or non-lethal methods will be developed to identify earthworms.
A little collection of earthworms per site before we are identifying them. It was creazy how those numbers vary between sites. So many more earthworms were found in organic croplands compared to conventionally managed croplands. Unfortunately, we still need to kill the organisms for their identificiation. I hope those methods will change one day.
Small insects and other arthropods
Microarthropods include, among others, small (often difficult to see with the naked eye) mites, springtails, larvae, and beetles. They are extracted using the so-called Berlese funnel, a dry extraction method. For this, heat from a light bulb is projected onto a soil sample for a week. We take the sample with a cylinder that is 10cm deep and 5cm wide. LED bulbs are not helpful here – what we need are the inefficient, old, and often hard-to-find incandescent bulbs. The arthropods flee from the heat and drying soil into deeper zones until they fall into a jar with 70% alcohol.
Podworms/Enchytraedeas
This is how we extract the enchytraideas of the soil. This is also called a wet extraction. They are kind of fleeing from the warmth created by the bulb so they move downwards. There, gravety collects them in a little Eppendorf glass.
This is how the extraction looks like from below. On the right, too much soil went through, probably a site with very fine, silty soil. This is not the ideal, here I would need to conduct an extra filter step to get rid of all the soil.
In a next step, I count them under the microscope.
Then they are “fixed” by adding some alcohol. This kills them but also allows that they stay ‘intact’ so they can be identified, e.g. by DNA analysis.
This is how the identification with the microscope and some identification books look like.
The labaratory situation at times
Our laboratory got a bit chaotic for a short moment of time filled with a looot of soil arriving from our partner universities all over Europe.
A little pile of earthworm samples waiting to be identified…
