Optimum range of soil phosphorus fertility needed for effective arbuscular mycorrhizal inoculation of Welsh onions in a non-allophanic Andosol

It has been demonstrated that inoculation of Welsh onions (Allium fistulosum) with arbuscular mycorrhizal (AM) fungi during the nursery period followed by transplantation was effective in improving growth and reducing phosphorus fertilizer application. However, what range of soil phosphorus fertility is suitable for AM fungal inoculation remains unclear. To elucidate the optimal level of soil phosphorus fertility in a non-allophanic Andosol, we conducted a series of Welsh onion inoculation experiments under field conditions. Experiments were conducted in 2009–2010 and 2015–2016 in experimental plots at Tohoku University, northern Japan. Soil phosphorus fertility levels varied as follows: very high, high, medium, and low at 750–1,000, 150–250, 70–150, and 30–70 mg P₂O₅ kg soil⁻¹, respectively (Truog method). Welsh onions were inoculated with a commercial Rhizophagus sp. R10 inoculum and grown for 7–9 weeks in a greenhouse nursery bed. The seedlings were transplanted into plots under the application of one, two or three levels of phosphorus fertilization. In the medium and high plots, inoculation increased marketable yields of Welsh onions irrespective of phosphorus fertilization, while no effect was found in the very high plot. In the low plot, inoculation did not clearly improve the yield, which was low irrespective of the treatments. These results indicate that AM fungal inoculation of Welsh onions in this soil was effective when available phosphorus ranged from 70 to 200 mg P₂O₅ kg soil⁻¹.