A hidden world beneath our feet

Everyone is familiar with earthworms. After a heavy rain, you might see them lying on the ground. Earthworms need moisture to move above ground and find a mate. In the soil, they play a crucial role as detritivores (waste eaters). They consume dead organic material and, through their digestion, release nutrients for plants.

Different types of earthworms live in different soil layers and feed on various types of organic matter. Some emerge from their vertical burrows at night, searching for a suitable leaf, which they drag back underground—all without eyes! The Netherlands has 25 species of earthworms, which can be divided into three groups:

• Litter-dwelling worms (Epigeic): Live in horizontal burrows within the litter layer and are often dark-colored.
• Soil-dwelling worms (Endogeic): Found deeper in the soil, consuming organic material and bacteria along with soil particles. They remain underground their entire lives.
• Deep burrowers (Anecic): Live in vertical burrows that can extend meters deep. They feed above ground, dragging leaves into their burrows, mixing organic material from the surface with deeper soil layers. This speeds up decomposition and increases soil permeability, which is crucial for rainwater infiltration.

The ability to shape soil and their role in nutrient release make earthworms true ‘ecosystem engineers’. They manipulate their environment and create opportunities for other species.

^{Figure 1: Different types of earthworms. Source: www.sciencelearn.org.nz}

Another key player in the soil food web is nematodes. They are primarily known for their plant-parasitic lifestyle and the damage they can cause in agriculture. This perception is only partially justified: of the more than 1,200 nematode species in the Netherlands, only about 100 cause harm to plants.

Nematodes are not only one of the most species-rich groups in the soil food web but also one of the most abundant. Researchers often classify nematodes based on their mouthparts, as these reveal their dietary preferences. Nematodes are an interesting group to study, due to their vast variation in size and lifestyle. The presence of different types of nematodes provides insight into how well-developed the soil food web is. If only detritivores and bacteria-feeders are found (at the bottom of the food chain), the food web is less developed than when predatory nematodes are also present.

^{Figure 2: Different mouth parts reflecting the feeding groups of nematodes.
Source: Ed Zaborski, University of Illinois.}

A lesser-known but remarkable group. These tiny creatures are six-legged arthropods, but not insects—they belong to their own group. They owe their name to the furcula: a fork-shaped appendage beneath their abdomen that acts as a spring to escape predators.

^{Figure 3: Springtail 'A' lives in the upper soil layer, springtail 'B' lives in deeper soil layers.
Source: University of Waikato.}

In addition to this unique organ, springtails also possess a collophore: an organ that allows them to absorb moisture and regulate their metabolism. Springtails come in various shapes and sizes, from tiny spheres to elongated bodies, colorful or pale, hairy or smooth, with or without eyes. These physical traits indicate their lifestyle. Species lacking hair, pigment, a furcula, or eyes are adapted to life in deeper soil layers.

^{Figure 4: A springtail unfolding its collophore to protrude from its body. Source: Alexis Tinker-Tsavalas.}

An essential group in the soil food web. In soil, mites can occur in high densities and fulfill various roles. Soil-dwelling mites mainly belong to the groups Oribatida, Prostigmata, Mesostigmata, and Astigmata.

• Oribatida are recognizable by their spherical shape.
• Prostigmata and Mesostigmata can be distinguished based on the position of their stigmata (respiratory openings). Prostigmata have these openings mainly at the front of their body, while Mesostigmata have them halfway along their body.
• Astigmata completely lack stigmata! Instead, they absorb oxygen directly through their skin. 

Mites are an incredibly diverse group of arachnids, with over 2,400 species recorded in the Netherlands. Not all species live in soil—mites can be found in nearly every habitat, from Antarctica to the ocean, and from plants to animals (including humans). In soil, some mites feed on fungi or bacteria, while predatory mites (mainly Mesostigmata) consume other invertebrates, such as parasitic nematodes. As a result, they play a crucial role in regulating populations and maintaining a healthy soil food web.

^{Figure 5: Top left: Oribatida. Top right: Astigmata. Bottom left: Mesostigmata. Bottom right: a velvet mite (belongs to Prostigmata). Source: S.E. Thorpe; Daktaridudu; www.chaosofdelight.org; André Karwath.}