Exploring Mechanisms behind Earthworm-Induced Phosphorus Availability.

Phosphorus (P) is an essential nutrient for crop growth. As many soils are unable to supply it sufficiently, P availability often is a growth-limiting factor to plants. Fertilization practices over the past decades have resulted in large amounts of "residual" soil P that are difficult to access. While economically viable global P fertilizer reserves are being depleted, new strategies to enhance the availability of this residual soil P become increasingly important. In earlier studies, we found that P availability can be considerably higher in earthworm casts than in the surrounding soil, which can result in increased plant P uptake. The underlying mechanisms through which this increase of available P occurs are unclear. In a greenhouse experiment with Italian ryegrass (Lolium multiflorum) on a soil with low P availability we tested whether the presence of the anecic earthworm Lumbricus terrestris resulted in an increased growth and P uptake. The presence of L. terrestris significantly increased aboveground biomass from 164 to 188 g dry matter m^-2 (p=0.044) and P uptake from 0.21 to 0.27 g m^-2 (p=0.002). In the experiment earthworm casts were collected and analysed for water extractable P as well as for most other common ions. Concentrations of total dissolved P and dissolved inorganic P in earthworm casts were 7-9 times higher than in bulk soil. In a separate batch experiment we found that the increased pH in the casts (from 5.9 to 8.1) could have contributed to, but cannot solely explain this increase in P availability. We hypothesize that the main driving mechanism is competition for surface adsorption sites between orthophosphate and natural organic matter in the casts. We tested this hypothesis using advanced surface complexation modelling (the CD model), and results will be discussed.