Direct and residual effect of biochar application on mycorrhizal root colonisation, growth and nutrition of wheat

The influence of biochar (biomass-derived black carbon) on crop growth and nutrient uptake varies based on the rate of biochar applied with fertilisers. We investigated the effect of deep-banded oil mallee biochar at different rates (0, 1.5, 3.0, and 6 t/ha) with 2 types of fertiliser (non-inoculated MultiMAPS® at 30 or 55 kg/ha; inoculated Western Mineral Fertiliser at 100 kg/ha) on wheat growth at a farmer’s field in a low rainfall area of Western Australia. Wheat yield increased significantly when biochar was applied with inoculated fertiliser and 30 kg/ha non-inoculated fertiliser. Mycorrhizal colonisation in wheat roots increased significantly with biochar application with inoculated mineral fertiliser. Mycorrhizal hyphae may have improved water supply to reduce drought stress in these treatments by extending crop exploration of water from the wide inter-rows. Grain yield increases were due to better grain survival and grain fill with reduced drought stress. Early stage phosphorus uptake was not improved by mycorrhizal colonisation—phosphorus supply from the soil and applied fertiliser was already adequate. The residual effect of biochar and mineral fertilisers was assessed using a mycorrhizal bioassay for soil collected from the field trial 2 years after application of biochar. Biochar and both fertilisers increased mycorrhizal colonisation in clover bioassay plants. Deep-banded biochar provided suitable conditions for mycorrhizal fungi to colonise plant roots.