Shifting precipitation patterns pose a threat to pastures. Prolonged droughts halt grass growth and can damage the sod, whereas peak rains can lead to inundation and the leakage of nutrients. As agricultural grasslands make up 23% of the European surface area (24% in the Netherlands), improving the water regulation capacity of grasslands could have a major impact. Because of their burrowing behaviour and its effects on soil drainage, earthworms may play an important role.

The anecic earthworm L. terrestris

The anecic earthworm species L. terrestris is referred to as an ‘ecosystem engineer’ because it creates vertical burrows that can be over two meters deep and can remain intact for years. Population densities are highest in grassland and can reach abundances of >100 earthworms per m². With this set of traits, L. terrestris is able to contribute substantially to climate adaptation by improving water infiltration rate and capacity, while also facilitating deeper plant rooting. L.terrestris is endemic to temperate Europe and widespread. Nevertheless, at the level of the individual pasture the species was found on only 23% of Dutch dairy farms during a large-scale earthworm inventory. This low prevalence could root in slow dispersal. It may well be that part of the pastures where L. terrestris is currently absent, is actually suitable for the species, but barriers like roads and water ways keep it out of reach for colonization. We think that through a better understanding of what makes a suitable habitat for L. terrestris and by exploring the possibility of inoculating pastures with this species, we can help farmers to improve the water regulation capacity of their pastures. Two research projects were set out to gather more insights.

The Louis Bolk Institute work

An L. terrestris inventory on 32 pastures of dairy farms on sandy soil was executed to look for correlations between L. terrestris abundance and various soil factors (texture, water table and possible interaction with earthworm species Lumbricus rubellus).

“Our data showed that L. terrestris seemed to display a preference for loamy soil, although it was also able to survive in very sandy soil”.

Results slightly indicated a preference towards pastures with a lower water table and a possible competitive interaction with L. rubellus (which feeds on the same resources). A 15-month field experiment on two dairy farms was executed to test whether pastures could be inoculated with L. terrestris. It was found that some of the earthworms were able to survive, create >60 cm deep burrows and produce offspring, showing potential for establishment of a population. However, survival rate was low, meaning that we cannot yet be certain that L. terrestris inoculation is feasible.

“Currently, we are working on a follow-up experiment. A better understanding of interactions with pathogens, the influence of the life history of the inoculum on its viability and long-term experiments in non-enclosed plots are needed to evaluate the potential of inoculating pastures with L. terrestris as an ecological innovation towards climate adaptation.”