Plant interspecific interactions can be influenced by common mycorrhizal networks (CMNs), which mediate nutrient trade (rarely focused on K and such fungal benefits), but the involved mechanisms remain unclear. To test whether and how CMNs mediate nutrient trade to benefit individuals during interspecific interactions, the growth advantage and K distribution in tomato/potato-onion system were studied using root exclusion compartments and ⁴¹K labeling in multi-compartment systems. Intercropping increased plant dry weight, percentage root length colonized by hyphae and arbuscules and K content of tomato, but decreased those of potato-onion. Damaging the CMNs decreased the effects of such positive interspecific interactions on growth and nutrition of tomato and re-established CMNs connection could partially recover the positive effects. A strong asymmetric transfer of K between plants occurred as potato-onion transferred more labeled ⁴¹K to tomato than that transferred from tomato to potato-onion.