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Good news for earthworm numbers in grazed pastures - the role of dung pats

30 June 2020
Type Media Article

Grazed grassland management regimes can have various positive and negative effects on soil fauna such as earthworms. Teagasc Johnstown Castle and UCD researchers Owen Fenton, Giulia Bondi and Olaf Schmidt tell us more about this supervised two year field study which examined the relationship between earthworms and dung pats.

Grazed grassland management regimes can have various positive and negative effects on soil fauna such as earthworms. For example, dung deposition adds a food source for earthworms, but livestock trampling can compact wet soil. In order to examine such relationships properly the sampling and classification of earthworms found in grazed fields must be accurate. A key knowledge gap was the role dung pats played in this accountancy exercise. 

The study

Dung pats on two permanent, intensively grazed pastures were monitored for up to 27 weeks and the earthworms around and underneath these pats were extracted, counted and classified by a PhD student Matthias Bacher. Specifically, the study concentrated on the behaviour of different earthworm species and their movement towards and aggregation under dung pats. Results showed that earthworms congregate in huge numbers under dung pats, with about 3 to 6 times more earthworms being present under dung pats than in control grass areas without dung. Earthworm densities directly beneath dung pats were equivalent to 4,000 worms per square metre – a huge number of active earthworms that feed on the dung, decompose it and incorporate it into the underlying soil. Not all earthworm species were attracted equally, the surface-dwelling red species were always attracted, while some species that dwell deeper in the mineral soil layers did not take much notice. Interestingly, it was estimated that earthworms travelled up to 1.1 m towards dung pats, in other words they will move to the spots where they are needed!

Take home message

This study showed that the presence of dung pats in pastures creates highly attractive temporary ‘hot spots’ in spatial earthworm species distribution, which changes over time. From a practical perspective the findings highlight the importance of earthworms in dung decomposition and incorporation. Furthermore, the observed intense aggregation of earthworms beneath dung pats suggests that earthworm populations and their soil functions need to be assessed separately at these hot spots in scientific studies of grazed systems.

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One of the experiments with simulated dung pats at Johnstown Castle. The dung was placed on mesh to allow researchers to measure how fast it was incorporated into the soil (Photograph: M. Bacher) and an earthworm found within the study area.

More details can be found at: https://www.teagasc.ie/crops/soil--soil-fertility/soil-quality/

Open access scientific article: https://bmcecol.biomedcentral.com/articles/10.1186/s12898-018-0216-6