Growing Australian Rice in Non-Flooded Soil Increases Water Use Efficiency and Mycorrhizal Colonisation, but Reduces Grain Micronutrient Concentrations

Climate change is driving a global shift from flooded (anaerobic) to dryland/rainfed (aerobic) rice production. While aerobic systems reduce water use and methane emissions, they can exacerbate soil zinc (Zn) and iron (Fe) deficiencies due to altered redox conditions. Arbuscular mycorrhizal (AM) fungi are more effective at colonising and functioning in aerobic soil and may enhance host plant water use efficiency (WUE) and micronutrient uptake in nutrient-limited soils. We assessed the potential for AM fungi to support aerobic rice growth compared to flooded conditions, in combination with Zn fertiliser treatments. Two Australian rice cultivars, Topaz and Viand, were grown under three watering conditions (flooded, 60% or 80% of soil field capacity [FC]) using a precision irrigation platform. Plants were inoculated with AM fungi (Rhizophagus irregularis) or not inoculated, and Zn was applied at 0 or 5 mg Zn kg⁻¹ soil. At plant maturity, the grain yield, water use, WUE and concentrations of grain Zn were measured. Both rice cultivars produced significantly more grain under aerobic soil conditions than flooded, with improved WUE. AM fungal inoculation led to reduced water use in Viand under 60% and 80% FC. However, reduced water use in mycorrhizal Viand plants was in line with lower grain yield in those plants, so WUE did not increase. Zn fertilisation enhanced grain Zn concentrations of Topaz and Viand grown in both aerobic treatments, but not under flooded conditions. However, the combination of Zn fertilisation and AM fungi reduced grain Fe concentration in all watering conditions, indicating an antagonistic Zn–Fe relationship. These findings support the potential adaptability of Topaz and Viand to aerobic production. Future research should explore integrated nutrient strategies to mitigate the trade-off between Zn and Fe in rice grain, and the potential of AM fungi to contribute to WUE.