Free-living Soil Nematodes

Nematode worms are among the most numerous of all multi-cellular soil animals and live in all soils on Earth, even in hot and cold deserts. In size, they lie between the single-celled Protozoa and the springtails and mites - that is between 0.1 to 2 mm long. You count them with a stereo microscope but to see them in detail, you need a compound microscope. They are quite beautiful creatures as they are glassy, transparent worms, often spindle-shaped with a variety of different mouthparts depending on their diet. They are soft-bodied and need water to live in and to swim through. They are too small to push soil particles aside and must make use of the water films in existing spaces between soil particles.

Although some soil nematodes are pests and feed on plant roots, many soil nematodes are "goodies" and help cycle nutrients in terrestrial ecosystems. Although they don't feed directly on soil organic matter particles, they do feed on soil microbes (bacteria and fungi) to such an extent that they directly affect nutrient cycling rates. Like Protozoa, nematodes help provide nutrients to plants by feeding on bacteria and fungi whose tissues are high in nitrogen. Nematodes excrete nitrogen in excess of their requirements, into the soil solution where plant roots can use it.

Some nematodes can feed on 5,000 bacterial cells a day or 6.5 times their own bodyweight. The fungal-feeding nematodes have piercing mouthparts through which they can suck up the cell contents of the microscopic filaments of soil fungi. Soil nematodes can be predators, feeding on Protozoa and other nematodes. A single predatory nematode can have a daily diet of over 80 juvenile nematodes.

Being soft-bodied and needing a water-film in which to live, you'd think that nematodes would be vulnerable to drying out. But they are tough critters and survive dry periods in soil by coiling up, encysting and becoming dormant until rain falls again. In this dormant state, they can lose over 95% of their body-water. This is how they can live in deserts. It takes only 24 hours for them to come out of this encysted state when placed in water.

You may wonder how we extract such delicate animals from a messy substance like soil. We crumble soil onto a paper tissue lying on a mesh tray inside another container. We add just enough water so that it moistens the soil and leave for 6 hours. The nematode worms move from the soil into the water. We carefully lift off the mesh and soil. We then tip the water into a tall container and concentrate the nematodes by decanting off the top water layer several times and let them settle to the bottom in between times. By using smaller containers for each decantation, we get a rich soup of nematodes.

Revive and survive! Encysted soil nematodes have hatched out of dryness-induced encystment after being 40 years in a dormant state when re-wetted. They can live in the soil of one of the most extreme of Earth's environments - the McMurdo Dry Valleys in Antarctica - an area with low soil moisture levels as it has surprisingly low snowfalls and is permanently free of ice. The nematodes spend most of their time in the coiled, encysted state.