The effect of earthworms and arbuscular mycorrhizal fungi on growth of and 32P transfer between Allium porrum plants

To measure P transfers, two small pots, placed 1 cm apart and each containing one leek (Allium porrum L.) plant, were embedded within a larger sand volume. Mesh windows on the facing pot sides allowed the penetration of hyphae but not roots. The effect of earthworms (Aporrectodea caliginosa Sav.) in the hyphal compartment and of arbuscular mycorrhizal fungi (AMF) on plant growth and 32P transfer between plants was investigated. In addition, the effect of mechanical disruption of the hyphal compartment on mycorrhizal plants was studied. AMF did not affect plant growth, although infection levels were high. Earthworms generally increased shoot and root growth; this effect did not appear to be related to location of earthworm activity. 32P transfer from the dying root system of labelled ‘donor’ plants to unlabelled ‘receiver’ plants was increased where earthworm activity was evident between pots, whereas mechanical disruption eliminated the effect of AMF on 32P transfer between mycorrhizal plants. Magnitude of transfer was only related to AMF colonisation levels in the donor plant in the absence of earthworms or mechanical disruption. These results suggest that movement of 32P into the hyphal compartment via the donor AMF mycelium was more important in influencing transfer magnitude than direct transfer via AMF hyphal interconnections between the two plants. Uptake of 32P by the receiver AMF hyphae appears to have been increased by earthworm activity in the hyphal compartment; it is proposed that earthworm-mediated mobilisation of 32P, partly contained within the donor AMF mycelium, led to enhanced 32P availability in the hyphal compartment.