The role of common mycorrhizal networks in mediating cadmium accumulation, glomalin production, and soil enzyme activity in co-cultures of poplars and leeks

Cadmium (Cd(II) ion) is a mobile metal, that can be toxic to plants and microorganisms, yet its effects on common mycorrhizal networks (CMNs) are seldom explored. Arbuscular mycorrhizal fungi (AMF) can improve Cd tolerance in plants by mediating uptake, sequestering Cd into hyphae and/or in exogenous glomalin proteins. Here, we examined how the AM fungus Rhizophagus irregularis affects Cd accumulation, glomalin production and microbial enzyme activity in single plant cultures and in interspecific co-culture conditions (CMN). A glasshouse experiment with Populus trichocarpa (poplars) and Allium porrum (leeks) was conducted including three factors: Contamination (Control vs Cd(II), 27 mg kg⁻¹), Mycorrhization (non-mycorrhizal vs AM) and Culture type (single vs co-culture, 1 poplar + 10 leek seedlings). We assessed biomass, Cd(II) uptake, glomalin concentration and carbon cycling enzyme activities. Poplar biomass was unaffected by Cd, and mycorrhization reduced foliar Cd by 34 %. Root Cd accumulation was highest in non-mycorrhizal poplars when co-cultured with leeks (102 mg kg⁻¹), suggesting increased Cd mobility from root exudation and acidification due to having multiple plants. However, root Cd decreased by 64 % under CMN, possibly because of hyphal binding and glomalin production, but we also propose this could be due to reduced exudation caused by AMF. Cd stimulated glomalin production, sometimes by 25-fold compared to controls, but had little effect on the studied soil enzymes. We conclude that i) Cd stimulates glomalin production in mycorrhizal plants, and ii) Non-mycorrhizal co-culture between leeks and poplars enhances Cd accumulation in poplar roots, but this effect is mitigated by CMNs.