Ecological functions of earthworms in soil
Walter S. Andriuzzi
Earthworms are known to play an important role in soil structure and fertility, but there are still big knowledge gaps on the functional ecology of distinct earthworm species, on their own and in interaction with other species. This thesis investigated how earthworms affect soil biochemical and biophysical functioning, and other organisms such as plants and smaller soil organisms.
Two field experiments with stable isotope tracers were performed to investigate how anecic earthworms (which feed on organic matter at the soil surface and dig deep burrows) transfer carbon and nitrogen from fresh plant litter into soil, and how this in turn affects soil organic matter composition, protists and nematodes. Another field experiment tested whether the anecic earthworm Lumbricus terrestris can counteract negative effects of intense rainfall on soil and plants (ryegrass). A greenhouse experiment was carried out to study how co-occurring earthworm species – two anecic and one endogeic (smaller, soil-feeding) – affect transfer of nitrogen from dung to soil and plants, nitrogen retention in soil, and plant growth. For the latter experiment, a method to produce herbivore (rabbit) dung triple-labelled with carbon, nitrogen and sulphur stable isotopes was developed.
Overall, the findings highlight important functions of earthworms in carbon and nitrogen cycling, soil biophysical structure maintenance due to burrow formation, and resulting biotic interactions. A novel finding was that the sphere of influence of anecic earthworms in soil (the ‘drilosphere’) is a much larger biochemical and biological hotspot than hitherto assumed. Rapid movement of carbon and nitrogen from surface to soil thanks to anecic earthworm activity resulted in spatial heterogeneity in soil carbon content, organic matter composition, and density of smaller eukaryotes (e.g. bacterial-feeding protists). Evidence was found that distinct earthworm anecic species may have dissimilar effects on soil biochemistry and plant growth, and that both anecic and endogeic earthworms may feed on surface organic matter (dung). This shows that the validity of earthworm ecological groups depends on the function under study, and suggests that, for some research questions, species identity should not be neglected; other approaches to quantify ecological differences between species (e.g. functional traits) are appraised. Finally, L. terrestris was found to ameliorate the disturbance of intense rain on plants, giving evidence to the idea that some components of soil biodiversity may contribute to ecosystem stability in the face of disturbance.