Effect of Organic Acids on Aluminum Toxicity in Subsoils

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Abstract
Aluminum has long been recognized as a major limiting factor for root growth in acid subsoils, but little has been done to delineate toxic and nontoxic forms of soil‐solution Al. In an effort to determine if the presence of organic acids in soil solutions affected Al phytotoxicity, short‐term, split‐root experiments were conducted with cotton (Gossypium hirsutum L.) taproots as the growth indicator. Based on pure solution experiments, short‐chain, carboxylic acids can be divided into three groups as Al detoxifiers: (i) strong (citric, oxalic, tartaric), (ii) moderate (malic, malonic, salicylic), and (iii) weak (succinic, lactic, formic, acetic, phthalic). The Al detoxifying capacities of these acids were positively correlated with the relative position of OH/COOH groups on their main C chain, positions that favored the formation of stable 5‐ or 6‐bond ring structures with Al. In addition, analyses of soil solutions from several eluviated acid horizons (E, EB, BE) revealed the presence of several organic acids whose concentrations were generally higher in forested than in cultivated soils. Based on these concentrations, total solution Al (as measured by ICAP) was partitioned into monomeric Al (Al3+ + hydroxy‐Al species) and complexed Al (Al‐organic acid complexes). The latter accounted for 93 and 76% of the total solution Al concentrations of the two acid subsoils into which elongation rate of cotton taproot was studied. Root growth was significantly correlated with monomeric Al but not with total Al in soil solutions.