Disentangling the role of earthworms in soil phosphorus cycling

Earthworms play a crucial role in enhancing phosphorus (P) availability in soils by processing organic matter as well as mineral soil particles and associated P. This role could represent a lever to increase agronomic P use efficiency. However, if earthworms are employed to unlock soil P, soils will still need to be replenished with nutrients. This could be accomplished through the application of circular fertilisers. In this context, earthworms could help to mobilize P from emerging mineral fertilisers recovered from waste streams, such as struvite. Despite these potential benefits, the biotic influence of earthworms on P cycling remains poorly understood and the interactions between earthworms and emerging fertilisers are unknown. Here, we present results of two studies aiming at (1) testing an isotopic approach based on the oxygen isotopes ratio in phosphate (PO₄) to study the biotic effect of earthworms on soil P cycling in arable soils; and (2) investigating the role of earthworms in mobilizing P from poorly soluble struvite. In a mesocosm experiment using straw and ¹⁸O-enriched water in the presence of soil-dwelling earthworms, we found that earthworms have a significant effect on the mineralization of P from organic residues in litter-amended soils with a low PO₄ availability. We demonstrated that the ¹⁸O-isotopic approach provides a promising method to study the influence of earthworms on PO₄ cycling. In a field study using struvite and ryegrass in the presence of multiple combinations of earthworm species, we showed that struvite performs comparably to conventional mineral P fertiliser in terms of plant P uptake, highlighting that struvite could be an efficient P fertiliser. The effect of earthworms on plant P uptake was significant but relatively small. These two studies emphasize that the effect of earthworms on P cycling is highly context dependent (e.g., soil P status and organic matter), with optimal efficiency observed in P-poor soils when a suitable food source is provided.