Agroecology niche for New Zealand's native earthworms

Exotic lumbricid earthworms have had some limited success colonising productive agricultural pastures in New Zealand, in place of native megascolecid species that did not adapt to the conversion from native vegetation cover. Native earthworms in lowland intensively-farmed landscapes are now almost entirely restricted to small fragments of native vegetation on marginal land where they coexist with adventive lumbricids. In recent years, regular pasture replacement using cultivation has been largely replaced by herbicide spray-out followed by direct drilling; much less soil disturbance potentially creates a new opportunity for native earthworms to colonise pastures. Reclaiming native earthworm habitat is critical not only to protecting diversity, but also to sustaining New Zealand's unique soil ecosystem services strongly associated to role of native species; however, there have been no studies related to this. Using a mesocosm experiment, we investigated how native and exotic earthworms modify physicochemical properties of a sheep-grazed pasture soil. The pasture soil was found to be equally suitable for the growth and survival of native and exotic species. Most burrowing earthworms increased the more soluble forms of macronutrients (N, P, S, K and Mg), but differences between native and exotic species were found to be larger than between functional groups. Two species of native earthworms, Octochaetus multiporus and Maoridrilus transalpinus, modified some soil properties in similar way to exotic species, but had significantly different impacts on pH, mineralisation of nutrients, and plant availability of trace elements. Compared to control, plant available N (PAN) and S concentrations were higher by more than 100% and 40%, respectively, in soils with presence of burrowing activity of the two native earthworms. Future co-existence of communities of native and exotic earthworms in agricultural pastures appears realistic, and this would benefit conservation of native species as well as dairy production. Further research may also reveal additional and unique benefits to soil quality that could be attributable to this formerly unavailable niche for native earthworms.