Impact of biochar amendment on the growth, physiology and fruit of a young commercial apple orchard

Mechanistic understanding of how biochar affects tree physiology is deficient. We determined the effects of biochar amendment on crop yields, growth and tree physiology of a high-input perennial horticultural system. The biochar was acacia whole tree green waste that had undergone pyrolysis in a continuous flow kiln at temperatures up to 550 °C for 30–40 minutes. Tree growth, crop yield efficiency and fruit quality were assessed to investigate the effects of biochar, compost and combined biochar and compost (B+C) treatments on the productivity of a newly planted apple orchard over a four-year period. The site was characterised by an A1 horizon 38 cm deep with a CEC of 35.15 cmol kg⁻¹, pH_cacl2 of 5.7 and an organic carbon of 2.42%. All treatments received approximately 42.5, 5.98, 131.1 and 12 kg ha^-1 year^-1 of N, P, K and Ca via fertiliser and green fowl manure inputs, respectively. Leaf gas exchange, leaf nutrient concentration and water status were recorded during the second cropping season in the biochar and control treatments only. Crop yield and fruit quality parameters were unaffected by the soil amendment treatments. Trunk girth was significantly higher than the control in the B+C and biochar treatments, in the first year and fourth year, respectively, while compost had no effect in any year. Neither photosynthetic capacity nor leaf nutrient concentration was influenced by treatment. Seasonal daily tree water use was similar between biochar and control treatments. The general lack of difference between treatments suggests that perennial horticultural systems characterised by high inputs of nutrients and water, may not respond to biochar. This is the first report investigating the whole-plant physiology of apple trees with biochar amendment.