Nursery inoculation with arbuscular mycorrhizal fungi improves yield of <i>Allium fistulosum</i> during a 3-year field trial

Using arbuscular mycorrhizal (AM) fungi in crop production for sustainable agriculture has attracted attention. The phosphorus (P) utilization efficiency of crops in Japan is approximately 20%, and the amount of P fertilizer applied is high. The purpose of this study was to determine the effect of nursery inoculation with AM fungi on the yield response of Welsh onion (<i>Allium fistulosum</i> L.) and to determine the effect of environmental factors on the yield response to AM fungal inoculation. Superphosphate was applied at rates of 0(P0), 500(P500) or 1,000 (P1000) P₂O₅ kg ha⁻¹) in 13 fields in Japan from 2015 to 2017, and <i>A. fistulosum</i> seedlings inoculated with or without <i>Rhizophagus</i> sp. strain <i>R</i>-10 were transplanted. Yield and soil chemical characteristics (pH, NH₄-N, NO₃-N, Truog-P, exchangeable Ca, Mg, K, and cation exchange capacity) were measured, and multiple regression analysis was performed. The relative field mycorrhizal dependency (RFMD) was 15%, and inoculation significantly increased the yield of Welsh onion. The RFMD was positively affected by year, pH, NH₄-N, and exchangeable Ca, and negatively affected by Truog-P, exchangeable Mg and K. These results indicate that it may be possible to replace P fertilization with AM fungi in Welsh onion production, regardless of the amount of available P in the soil, and that NH₄-N may be an indicator of the use of AM fungi.