The contribution of earthworms to soil aggregate formation

Anecic and endogeic earthworm species are known as “ecosystem engineers” that significantly contribute to the porosity and coherence of soil as well as soil water infiltration and the turnover rates of organic matter (OM). Additionally, earthworms actively excrete nutrient rich mucus, release bacteria within casts and translocate litter into the subsurface. In this way, earthworms not only shape the structure of soils but also the chemical milieu of the drilosphere where mucus forms a prominent fraction of OM. Furthermore, other biogenic extracellular polymeric substances (EPS) are known to form organo-mineral associations, which suggests that earthworms also facilitate their further attachment into soil aggregates.

With this study, we investigated how earthworms contribute to soil aggregate formation and impact aggregate properties by OM translocation and incorporation in the drilosphere. At vertically sampled burrow walls predominantly formed by Lumbricus terrestris, a patchy and depth dependent distribution of hydrophobic and hydrophilic regions was found with the water drop penetration time (WDPT) test. In the hydrophobic regions, we identified an enrichment of carbon and aggregate surface coatings containing plant residuals, bacteria, OM screenings and enmeshments by scanning electron microscopy (SEM) and elemental analysis. These structures were further investigated by factor analysis of Fourier transform infrared (FTIR) spectra that permitted the FTIR-band extraction of earthworm typical aggregation agents as, e.g., bacterial EPS, earthworm mucus and plant components (leaves, roots and sprout). Furthermore, with sorption experiments to typical minerals of temperate soils (e.g. illite and goethite), we found a mineral-specific adsorption of earthworm cutaneous mucus (of Lumbricus terrestris and Aporrectodea caliginosa) and bacterial EPS (of Bacillus subtilis). Specifically, a preferential adsorption of phosphorus containing constituents of mucus and bacterial EPS to goethite has been observed. The resulting formation of organo-mineral associations characterized by screened mineral surface charges was shown by zeta potential measurements.

We show that besides the active incorporation of particulate OM, as e.g. plant residuals and microorganisms, the mineral specific adsorption of EPS formed by earthworms and bacteria induce the formation of organo-mineral associations and alteration of the physico-chemical properties of earthworm-formed structures and soil aggregates.