Earthworms in the soil ecosystem

In a living soil, there are between 100 and 1,000 earthworms per m², which represents between 2 to 5 tons per hectare. What do they do in the soil and why is it so important to preserve them?

Different species and categories

There are several species of earthworms (already 150 in France) all having different behavior. Depending on this behavior and the role they occupy in the ecosystem, it is possible to classify earthworms into three ecological categories :

Endogeics

Living in the deep mineral soil, they have no skin pigments. They feed on soil that is poor in organic matter and have thus developed a digestive system with a very large absorption surface. They do not form large galleries since they can move by ingesting soil, horizontally.

Epigeics

They live mostly on the surface, in the most organic part of the soil where they work on twigs, debris, and dead leaves. Quite small and very prolific, their dark pigmentation allows them to hide from predators to which they are very exposed.

Anecics

They are the most numerous (80% of earthworms) and the largest. Having lost their clitellum, they work the soil more

vertically. They feed on debris present on the surface which they pull inside the soil and then use to line the walls of their tunnels before consuming them. Finally, they come back to the surface to defecate, leaving small heaps called casts. They are true architects, or soil engineers.

Role of earthworms

Natural soil work

In a fertile meadow, the galleries of anecics, lined up end to end, can represent 400 m of galleries per square meter. They aerate the soil, facilitate infiltration and good water dispersion as well as the penetration and distribution of roots. Where there are earthworm galleries, plants exploit the environment better, developing more deeply. Moreover, the thickness of the topsoil

increases while its humus content increases. In the vertical galleries, earthworms mix mineral matter with organic matter which allows stable aggregates and thus good soil structure.

Soil mixing

If earthworms are very active, they can move an impressive amount of soil : 270 kg of dry soil per 1 kg of fresh anecics each year according to Marcel Bouché. Having no teeth, earthworms simultaneously ingest OM and some soil to grind the plants. They digest this soil then eject it on the surface, enriched with undigested organic matter in the form of casts. These can represent 400 to 1200 kg of soil per hectare (Bertrand and Renaud). This intimate mixing optimizes

the degradation of organic matter and allows the enrichment of the soil in elements assimilable by plants.

Soil enrichment

Earthworms have a biochemical role since the mixing of organic and mineral matter will allow significant enrichment of the different soil layers in mineral elements (nitrogen, phosphorus, potash, magnesium, calcium,…) and mineral trace elements which contribute to the nutritional balance of plants.

Nitrogen cycle with the earthworm

The plant, by returning its OM to the soil, produces nitrogen and makes it available to earthworms.

The earthworm, feeds on this OM (and its own excrements), assimilates nitrogen and transfers it through its metabolism.

To move in the gallery, it lines it with mucus, rich in nitrogen which it assimilates.

Microorganisms concentrate on the mucus and decompose it : nitrogen becomes assimilable by plants.

Plants send their roots into earthworm galleries, rich in assimilable nitrogen.

They bring this nitrogen up from their root system to their aerial system, creating organic biomass, which will fall to the soil, be decomposed, assimilated, etc. The cycle is thus closed and we face a nitrogen distribution to plants in a just-in-time flow.

1 ton of earthworms, per ha per year:

Consumes 2.3 tons of nitrogen.

Defecates 1.7 tons of nitrogen (partly re-ingested by earthworms, partly made available to plants).

Excretes 600 kg of nitrogen assimilable by plants (in the form of mucus (95%) and urine (5%, in the form of ammonia thus directly assimilable by plants)).

Indicatively, it takes 14 days for the nitrogen in the mucus to reach the plants, so it is a very efficient model. Indeed, an earthworm renews the nitrogen present in its body in 40 days and this nitrogen is 100% found in plants, so there is no loss, no nitrogen leaching. The balance of intestinal and metabolic earthworm transits is therefore more than interesting for the soil and plants. Indeed, a farmer applies on average 250 kg/ha/year of nitrogen.

How to attract earthworms

What agroecosystem satisfies them?

Only no-till techniques and simplified soil work, coupled with regular input of fresh OM, maintain a high earthworm population level. Indeed, a plow strike means 80% fewer earthworms in the soil, and the rest is undeniably weakened. In a completely bare soil, poor in organic matter, which will be worked, earthworm losses can be close to 100%.

In well-managed conservation agriculture, levels are higher thanks to more OM input as well as cessation of insecticides and fungicides.

The presence of dense and almost permanent cover crops protects earthworms and feeds them effectively
Reduction, or even absence, of soil work ensures conservation of higher population levels than in other systems.
It is especially in summer that conservation agriculture is relevant since it guarantees earthworms can spend this season in good conditions, thus survive and continue their activity. Conservation techniques indeed allow the soil to store more water and better resist lack of rainfall and high temperatures.