Fulvic and humic acids extracted from Eucalyptus and hay ameliorate Al toxicity effects on plant root growth through multiple mechanisms.

Abstract Background and Aims In acidic soils aluminium (Al) toxicity, remains a critical crop limitation. Organic amendments can ameliorate Al toxicity due to the production of complex high molecular weight carbon compounds. This study evaluated the role of fulvic and humic acids (FA and HA) from Eucalyptus and hay, in ameliorating effects of Al toxicity. Methods The interaction between Eucalyptus and hay FA and HA, and Al on root growth was investigated using dilute nutrient solution. Five Al concentrations (0-270 µM) were combined with four organic acid (OA) treatments; including Nil-OA, FA40 and HA40 (each at 40 mg C L‑1) and a combined treatment FA40HA40. Results Eucalyptus FA and HA stimulated root growth. In the absence of Al, hay FA and HA inhibited root growth but addition of Al alleviated this effect. In the presence of FA and HA root growth was not inhibited by nominally toxic monomeric Al concentrations. When root growth was expressed relative to the OA treatment (0 µM Al) the response was consistent with Al toxicity. Conclusion Differential effects on root growth of Al toxicity and the FA and HA were identified. FA and HA effects were either inhibitory or stimulatory depending on the source. FA and HA directly complexed with Al to reduce Al toxicity. The direct effect of organic ligands should be considered in root bioassay studies that evaluate Al toxicity in soils. The study identifies both complexation and precipitation as important mechanisms for detoxification of Al.