Earthworm mucus contributes to the formation of organo-mineral associations in soil

Earthworms are considered as “ecosystem engineers” impacting soil properties as well as nutrient and element cycles. As they move through soil, earthworms secrete cutaneous mucus which is metabolized by soil microorganisms and a source of plant-available nutrients. Earthworm-processed soil contains carbon enriched, earthworm-specific soil aggregates (e.g. casts and middens) in comparison to earthworm unaffected soil. The reason could be that organic polymeric substances in earthworm mucus bind to soil minerals. The objective of this study was to investigate the small-scale interactions between earthworm mucus and secondary soil minerals, e.g. goethite and illite, leading to the formation of organo-mineral associations. We characterized the chemical composition of earthworm mucus by FTIR and 13C-NMR spectroscopy and compared spectra of mucus to microbial extracellular polymeric substances (EPS), an abundant and well-known type of organic matter that binds with soil minerals. Mucus from anecic (Lumbricus terrestris L.) and endogeic (Aporrectodea caliginosa Sav.) earthworm species was dominated by proteins and carbohydrates. Between 21 and 36% of the total organic carbon in the mucus containing treatments adsorbed to illite and goethite, and most of the binding with goethite was associated with phosphorus containing mucus compounds. The surface charge of newly-formed organo-mineral associations was determined by measuring the isoelectric point (IEP). The IEP of mucus-goethite associations was 6.8, which was lower than the bare goethite IEP of 8.4. The zeta potential of mucus-illite associations was greater than bare illite. We conclude that the specific adsorption of earthworm mucus constituents to soil minerals leads to the formation of mucus-mineral associations. These associations contribute to retention of organic substances from earthworm mucus in soil (micro-)aggregates and explain the altered physicochemical properties of earthworm-formed aggregates in comparison to the earthworm unaffected “bulk” soil material.