Phosphatase activity in the drilosphere and its link to phosphorus uptake by grass

Earthworms can increase the solubility of phosphorus (P) in soil, an effect which to a large extent is controlled by mineralisation of organic P. Phosphatases, a class of hydrolytic enzymes, catalyse this mineralisation process. However, a consistent comparison of their activity among earthworm species and for different soil spheres is still missing. Here we aim to better understand the activity of phosphatases in relation to earthworm-enhanced P-availability, and specifically that of phosphomonoesterases (PME) which directly liberate orthophosphate. We conducted a greenhouse pot experiment with five different earthworm species. The PME activity was assessed in earthworm casts, burrows and bulk soil for both single species and mixed communities. Analyses were performed at both the pH-H₂O of the bulk soil (6.5) and of the casts (7.5). The PME activity measured at both pH values was highly correlated (R² = 0.98; p = 2.2x10^-16) and was strongly elevated by earthworm activity in the order cast > burrows > bulk soil. The PME activity was particularly high in the casts of A. longa relative to casts of other species. Large variation in PME activity in the drilosphere was observed among earthworm species, but this variation was not related to earthworm ecological categories. Our data also indicate that an elevated P concentration in grass shoots could result from increased PME activity via greater hydrolysis of organic P.