Earthworms affect reactive surface area and thereby phosphate solubility in iron-(hydr)oxide dominated soils

Summary

Sustainability of agricultural systems is at stake, as phosphorus (P) is a non-renewable resource while its global reserves are limited. Stimulating earthworm activity can be a technology to increase the level of readily plant-available phosphate (PO₄). However, conclusive evidence on the mechanisms underlying an earthworm-enhanced PO₄ solubility is yet missing. This study aimed to reveal possibly overlooked pathways by which earthworms affect PO₄ solubility, and quantify the relative importance of all contributing mechanisms. Therefore, we set up a greenhouse pot experiment in which we investigated the large increase in water-extractable PO₄ in casts of three earthworm species (Lumbricus rubellus, Aporrectodea caliginosa, Lumbricus terrestris) in soils with either predominantly Fe- or Al-(hydr)oxides. Oxalate-extractable PO₄ was increased in earthworm casts compared to bulk soil which can be attributed to the mineralisation of natural organic matter (NOM). Surface complexation modelling was used to elucidate the mechanisms that control earthworm-enhanced PO₄ solubility. The results of our modelling showed that the increase in pH in earthworm casts relative to bulk soil affects PO₄ solubility only to a minor extent. Besides NOM mineralisation, two major mechanisms contributing to earthworm-enhanced PO₄ solubility are (i) a decrease in the reactive surface area (RSA) of the metal-(hydr)oxide fraction; and (ii) a decrease in the competition between NOM and PO₄ for binding sites of the metal-(hydr)oxides. As the newly discovered decrease of the RSA was only found for Fe-(hydr)oxide-dominated soils, earthworms have the largest potential to enhance PO₄ solubility in those soils.