EFFECTS OF ROOT-DERIVED ORGANIC ACIDS ON METAL SPECIATION IN SOIL SOLUTION AND BIOAVAILABILITY

AUTOR(ES)
DATA DE PUBLICAÇÃO

2000

RESUMO

Several plant species can exude relative large amounts of low-molecular-weight organic acids (LMWOAs) into the hizosphere. Limited information is available about the effects of these compounds on the mobilization and bioavailability of trace metals for plants. The objective of this study was to measure the impact of in situ-produced and added LMWOAs on metal speciation in soil solution and metal uptake by plants. A series of three pot experiments used samples from a Plano silt loam soil (fine-silty, mixed, mesic Typic Argiudolls). Results from rhizosphere and bulk soil evaluations for a low-phosphorus (P) soil showed that the content of Co, Cr, Cu, Zn, Fe, and Mn in white lupin (Lupinus albus L.) shoots and the isotopically exchangeable fractions (E-value and L-value) of Mn and Zn were inversely related to P availability. The differences were attributed to the effect of P in decreasing root exudation of LMWOAs or other complexing agents. The content of Cd, Mn, and Zn in plant shoots was directly related to total dissolved concentration and free ion activity in soil extracts obtained by the saturation paste method and analyzed by the Donnan equilibrium technique. Manganese was the element most affected by treatments and it competitively influenced Cd and Zn uptake by plants. The effects of sterilizing soil by gamma radiation on metal reactions were inconclusive. This treatment increased dissolved concentrations of metals and organic carbon. Using a metal-contaminated soil, the addition of LMWOAs (citric, fumaric and L-malic acids) and ethyleneglycoltetraacetic acid (EGTA) at several rates increased the total dissolved concentration of several elements and organic carbon. However, the LMWOAs had slight effects on Cd and Cu content in soybean [Glycine max (L.) Merr.] while decreasing Zn content. This behavior could be explained by the decrease of (free ion / total concentration) ratio of these elements with increasing rates and by increasing soil pH. No consistent relationship between free ion activity in solution and Cd, Cu, and Zn content in plants was observed for the LMWOA treatments.

ASSUNTO(S)

organic acids (lmwoas) rizosfera Ácidos orgânicos absorção complexação bioavailability of trace metals for plants metais pesados especiação química ciencia do solo

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