Biochar interferes with kiwifruit Fe-nutrition in calcareous soil

• In biochar-amended soils Fe-chlorosis symptoms of kiwifruit vines were pronounced.
• Increasing biochar rates progressively removed dissolved micronutrients.
• XPS analyses revealed Fe2O3 on the biochar surface when exposed to a Fe source.
• A redox reaction occurred on the Fe-exposed biochar surface.

We combined lab experiments, a 2-year pot experiment, light microscopy and X-ray photoelectron spectroscopic measurements to understand the fate of Fe in biochar-amended soils with, and without, three sustainable Fe-fertilization strategies (siderite, aqueous extract of Amaranthus retroflexus either alone or enriched with FeSO4). In addition, we examined biochar effects on plant-available micronutrients using metal Fe. We also assessed nonlinear interactions between biochar and Fe management strategies, evaluating the effectiveness of sustainable fertilization strategies in improving Fe-nutrition of kiwifruit vines under Fe-stressed conditions. Fe chlorosis symptoms of pot-cropped kiwifruit vines were pronounced in the biochar-amended soils, especially during the first season. However, the biochar rate adopted in this study was relatively high (5 g kg− 1 w/w) while the real impact of this strategy may change at lower rates.In the lab, increasing biochar rates progressively removed dissolved micronutrients from solution. Light microscopy revealed rust-colored patches on the biochar surface exposed to a Fe source in solution and XPS analyses confirmed that the biochar surface had become coated with Fe and O in ratios consistent with the formation of Fe-(hydr)oxides. Although we investigated one specific biochar, our results suggest that some redox reactions can act to form a micronutrient sink on the biochar surface. This mechanism likely contributed to binding soil Fe (through reactive functional groups on its surface) into plant-unavailable forms, promoting the occurrence of Fe chlorosis symptoms of kiwifruit vines grown in soils amended with biochar. We also confirmed the effectiveness of siderite and A. retroflexus extract enriched with FeSO4 in alleviating the occurrence of Fe-deficiency symptoms in kiwifruit vines.