Earthworms in New Zealand pastures

Dr Nicole Schon handsorting a soil sample for earthworms on a dairy farm

Earthworms burrow through the soil and feed on organic matter, consequently improving the movement of air, water and nutrients through the soil. Considering that the live weight of earthworms living in the soil is similar to the live weight of stock above ground, their contribution to these ecosystem services (movement of air, water and nutrients) are not to be ignored.

Not all earthworms are the same, with different species having different burrowing and feeding behaviours in the soil. Three main functional groups can be distinguished, these include the epigeic and endogeic species living in the topsoil and the deep-burrowing anecic species (see figure below).

earthworms-in-NZ-pastures

Earthworm functional groups. Epigeic earthworms (i.e. Lumbricus rubellus) feed on organic matter on the soil surface and do not form permanent burrows. Endogeic earthworms (i.e. Aporrectodea caliginosa) ingest topsoil and its associated organic matter, forming semi permanent burrows. Anecic earthworms (i.e. Aporrectodea longa) draw organic matter from the soil surface into their deep, permanent burrows to feed on. Figure adapted from Fraser and Boag, photos of common earthworms courtesy of R. Gray.

Deep-burrowing anecic earthworms, such as Aporrectodea longa, are larger than topsoil species, feeding on larger quantities of organic matter on the soil surface, having the potential to incorporate this deeper in the soil profile.

Earthworms in New Zealand pastoral systems arrived accidentally with European settlers and hence species have a patchy distribution. Introductions of earthworms into soils where they were absent have been successful in the past, being observed to increase pasture production by 10-30%. Despite these introductions, their patchy distribution has again been highlighted in a recent on-farm survey in the Central North Island where only 14% of paddocks sampled contained species from all three earthworm functional groups.

Deep-burrowing anecic earthworms have a particularly patchy distribution in New Zealand pastures, being absent from large areas of grassland. However, where they are present they are associated with increased pasture productivity and stocking rates. In these more productive pastures their ability to move deeper in the soil to escape treading pressures in combination with increased food inputs suggests they may be able to support or even substitute the actions of epigeic earthworms.

We are interested in determining the potential of the deep burrowing anecic earthworm A. longa in carbon storage by quantifying the amount of organic matter they can incorporate into the soil and determining the land area which may benefit from their introduction.