Large Variation in Cast Properties of Earthworm Species: The Link to Soil Phosphorus.

Phosphorus (P) is an important nutrient for plant growth, but is often difficult to access by roots because of its strong adsorption to the soil mineral phase. Recent research has shown that earthworms can temporally and locally make adsorbed P available to plants. However, it is not clear to what extent this capacity varies among earthworm species. Here we study in a greenhouse pot experiment on a soil with a low P status differences in the amounts of readily available P and other P-related properties of casts of eight commonly occurring earthworm species in the Netherlands. The pH of earthworm casts (7.4-8.2) was significantly higher (p<0.001) than the control soil (6.6) and differed significantly (p=0.003) among earthworm species. Similarly, the dissolved organic carbon (DOC) concentration was an order of magnitude larger in earthworm casts compared to the control soil, and varied among species (p<0.001). Despite selective feeding on P-rich constituents, no significant differences in the size of the adsorbed P pool were found among earthworm species. Differences among species became more pronounced for the readily available P pools modified P-Olsen and water-extractable ortho-P. Ortho-P concentrations were dramatically higher in casts of all species as compared to the control soil (0.9-6.8 vs 0.06 mg l^-1; p=0.006). A positive correlation between the concentrations of DOC and ortho-P was observed (r²=0.72), which is in agreement with our previous modelling study that suggested competitive desorption of DOC and ortho-P as an important pathway for earthworm-induced increased P-availability. Multivariate statistics indicated that the variation in P-relevant cast properties among earthworm species could not be explained by conventional ecological earthworm classifications. Our results suggest that the nature and magnitude of earthworm-induced P-availability differs dramatically among earthworm species, indicating that functional diversity rather than only size of the earthworm community is key to optimizing P-availability to plants.