Interactions between soil biogeochemistry and native earthworms in New Zealand

Abstract

Despite apparently similar burrowing and feeding behaviours to introduced Lumbricidae earthworms, native Megascolecidae, with more than 179 recognised species, have become isolated in natural vegetation remnants on the margins of agricultural land. Long-term geographic isolation has provided high endemic earthworm diversity in New Zealand, but they appear to have a poor ability to adapt to anthropogenic disturbance. Although earthworms are well known as ‘soil engineers’, there is lack of knowledge of the role of endemic earthworms in New Zealand’s soil ecosystems. The aims of the present PhD study were to identify endemic earthworm preferences for and influences on soil biogeochemistry, and to investigate interactions between the drilosphere of native earthworms and the rhizosphere of native plants. Species of earthworm, collected from native vegetation, natural remnants and restoration sites in Canterbury and on the West Coast of South Island, were identified using DNA barcoding with 16S and COI primers. Thirteen endemic and nine exotic species were identified and, of these, eight abundant earthworms were selected for this study: 5 endemic taxa identified as Deinodrilus sp.1 (epigeic), Maoridrilus transalpinus and Maoridrilus sp.2 (anecic), Megascolecidae sp.1 and Octochaetus multiporus (endogeic), and 3 exotic species: Eisenia fetida (epigeic), Octolasion cyaneum and O. lacteum (endogeic). Both endemic and exotic earthworms preferred agricultural soils to a native forest soil. Ryegrass litter was preferred to litter of native plants, although Coprosma robusta was also favoured by endemic earthworms. There was more preference for less acid soil than for high organic matter soil. Earthworm species could also be separated on the basis of their effects on soil biogeochemistry, in terms of organic matter consumption, nutrient mineralisation, soil microbial biomass and greenhouse gas emissions from the soil. Earthworm inoculation of soils increased more mobile forms of the key nutrients N and P, and emissions of N2O and CO2 from an agricultural soil. Lesser differences were found between native and exotic earthworms than between functional (burrowing) groups. Native earthworms increased growth of plants, including L. perenne, and had a marked interaction with root morphology of two native species of tea tree (Leptospermum scoparium and Kunzea robusta). They also stimulated microbial activity in rhizosphere soil. An anecic species, M. transalpinus, enhanced rates of root nodulation of a native leguminous shrub (Sophora microphylla), enhancing critical concentrations of nitrate, but also reducing nitrous oxide emissions. Maoridrilus spp. enhanced plant productivity in biosolids-amended soils, but raised some potential environmental concerns through increased N2O emissions in <50 % biosolid treatments. They also significantly increased ammonium and nitrate in soil, microbial activity and soil concentrations of soluble copper. The results showed that endemic earthworms could play a critical role providing soil ecosystem services in New Zealand’s production landscapes. Novel habitats within agricultural management systems provide an important refuge for threatened species conservation. Enhanced restoration of native vegetation into agricultural landscapes will enhance the dispersion and sustainability of communities of native earthworms. An integrated understanding of plant growth and microbial communities with earthworm functionality is essential for effective management of soil biogeochemistry and to inform ecological restoration on former agricultural land.