Com base em composições de ácidos orgânicos de exsudatos radiculares obtidos da rizosfera de plantas de milho cultivadas assepticamente, pH da rizosfera e matriz química da solução do solo, extratores sintéticos desses exsudatos foram formulados. Esses extratores, que imitam o ambiente da rizosfera, onde os elementos-traço do solo são dissolvidos e absorvidos pelas plantas, foram usados para extrair elementos-traço de solos 16 vezes sucessivas. A concentração de Zn, Cd, Ni, Cr e Cu do solo tratado com lodo de esgoto era de 71,74; 0,21; 15,90; 58,12; e 37,44 mg kg-1, respectivamente. Os exsudatos radiculares sintéticos foram constituídos pelos ácidos orgânicos acético, butírico, glutárico, lático, maleico, propiônico, pirúvico, succínico, tartárico e valérico. A concentração das misturas de ácidos orgânicos foi de 0,05 e de 0,1 mol L-1 de -COOH. Os elementos-traço removidos pelas extrações sucessivas podem ser usados para representar a disponibilidade desses elementos para as plantas nesses solos. A especiação química dos elementos-traço na fase líquida foi calculada. Os resultados mostraram que os elementos-traço em solos tratados com lodo de esgoto foram dissolvidos e formaram complexos solúveis com os diferentes exsudatos radiculares baseados em ácidos orgânicos. Os ligantes orgânicos mais reativos foram lactato, maleato, tartarato e acetato. Os ligantes inorgânicos cloreto e sulfato tiveram papel insignificante na dissolução dos elementos-traço. Com exceção de Cd, íons livres não foram importantes espécies químicas de elementos-traço na rizosfera do solo. Como diferentes elementos-traço formaram complexos solúveis com diferentes ligantes na rizosfera, nenhum extrator, baseado em apenas um reagente, poderia ser capaz de recuperar todos os elementos-traço potencialmente disponíveis do solo; as misturas de ácidos orgânicos baseados em exsudatos testados neste estudo podem ser mais adequadas para recuperar os elementos-traço do solo potencialmente disponíveis para as plantas do que os extratores convencionais.

Synthetic root exudates were formulated based on the organic acid composition of root exudates derived from the rhizosphere of aseptically grown corn plants, pH of the rhizosphere, and the background chemical matrices of the soil solutions. The synthetic root exudates, which mimic the chemical conditions of the rhizosphere environment where soil-borne metals are dissolved and absorbed by plants, were used to extract metals from sewage-sludge treated soils 16 successive times. The concentrations of Zn, Cd, Ni, Cr, and Cu of the sludge-treated soil were 71.74, 0.21, 15.90, 58.12, and 37.44 mg kg-1, respectively. The composition of synthetic root exudates consisted of acetic, butyric, glutaric, lactic, maleic, propionic, pyruvic, succinic, tartaric, and valeric acids. The organic acid mixtures had concentrations of 0.05 and 0.1 mol L-1 -COOH. The trace elements removed by successive extractions may be considered representative for the availability of these metals to plants in these soils. The chemical speciation of the metals in the liquid phase was calculated; results showed that metals in sludge-treated soils were dissolved and formed soluble complexes with the different organic acid-based root exudates. The most reactive organic acid ligands were lactate, maleate, tartarate, and acetate. The inorganic ligands of chloride and sulfate played insignificant roles in metal dissolution. Except for Cd, free ions did not represent an important chemical species of the metals in the soil rhizosphere. As different metals formed soluble complexes with different ligands in the rhizosphere, no extractor, based on a single reagent would be able to recover all of the potentially plant-available metals from soils; the root exudate-derived organic acid mixtures tested in this study may be better suited to recover potentially plant-available metals from soils than the conventional extractors.

Poucos trabalhos têm sido realizados sobre a comparação da taxa de dessorção e da quantidade de metais extraídos sucessivamente por misturas de ácidos orgânicos que imitam a composição da rizosfera e por extratores usados na rotina em amostras de solo tratado com lodo de esgoto. Extrações de Zn, Cd, Ni, Cr e Cu foram realizadas em amostras de um solo de carga variável tratado com lodo de esgoto usando-se sete extratores: quatro misturas com ácidos orgânicos e três extratores usados na rotina (DTPA, Mehlich-I e acetato de amônio). Os resultados para uma única extração, em que o conteúdo de metais extraíveis foi determinado simplesmente extraindo-os do solo uma única vez, bem como após 15 extrações sucessivas, em que o resíduo sólido da primeira extração foi extraído sucessivamente por mais 14 vezes, foram analisados. A extractabilidade dos metais em uma extração simples foi, em geral, como segue: Mehlich-I > DTPA > ácidos orgânicos > NH4OAc. As mais elevadas taxas de extração seguiram a ordem: DTPA > Mehlich-I > ácidos orgânicos > NH4OAc. A maior extração de metais foi obtida com Mehlich-I, enquanto, o DTPA também extraiu alta quantidade de Zn, Cd, Ni e Cu em uma extração simples, bem como obteve as mais altas taxas de extração entre os extratores estudados. A transferência de metais do solo para as soluções de ácidos orgânicos foi mais lenta que para os extratores DTPA e Mehlich-I.

Few investigations have been carried out about the comparison of desorption rate and amount of heavy metals extracted successively by organic acid mixtures mimicking the rhizosphere and routine extractants in sewage sludge-amended soils. Extractions of Zn, Cd, Ni, Cr, and Cu were performed in samples of a sewage sludge-amended soil using seven extractants: four organic acid mixtures and three routine extractants (DTPA, Mehlich-I, and ammonium acetate). Results from single pass extractions, in which the extractable metal contents were determined by simply extracting the soil a single time, as well as from 15 successive extractions, in which the solid residues of the first extraction was successively extracted 14 additional times, of heavy metals were analyzed. The extractability of heavy metals in a single pass extraction was, in general, as follows: Mehlich-I > DTPA > organic acids > NH4OAc. The highest rates of extraction followed the general order: DTPA > Mehlich-I > organic acids > NH4OAc. While Mehlich-I presented the highest extractability of heavy metals among studied extractants, DTPA showed a high extractability of Zn, Cd, Ni, and Cu in a single extraction as well as the highest rates of extraction among the studied extractants. The transfer of heavy metals from soil to organic acid solutions is slower than to DTPA and Mehlich-I extractants.

The disposal of sewage sludge from domestic wastewater treatment plants is a growing problem worldwide. Because of the total P content and other characteristics, it is highly probable that in Chile important amounts of the sewage sludge will be disposed of on agricultural lands. The distribution of P forms in Chilean soils and sewage sludge was studied by chemical fractionation and 31P-NMR spectroscopy. The 0-15 cm depth of soils derived from volcanic materials (one Ultisol and two Andisols), alluvial materials (Mollisol), and sewage sludge from a domestic wastewater treatment plant located in Santiago were considered. The total P concentration is 6 to 18 times higher in the sewage sludge than in soils, but a similar 31P-NMR pattern was found in all samples. The most important signals correspond to inorganic orthophosphate and monoester P, with small signals assigned to pyrophosphate and diester P. The disposal of sewage sludge might not succeed as soil P fertilizer, contributing to an increase in the accumulation of P in volcanic soils, and it might be a potential pollutant in soils and water

The phosphorus distribution in soil (<2 mm) and in clay size separates (<2 mu m) of two uncultivated volcanic soils from southern Chile (one Ultisol and one Andisol) was studied by phosphorus-31 nuclear magnetic resonance spectroscopy (31P-NMR). The total P content ranges of the Ultisol samples were 733-1108 mg kg-1 (soil) and 746-896 mg kg-1 (clay), lower than 2163-2216 mg kg-1 (soil) and 2683-3362 mg kg-1 (clay) obtained for Andisol samples. In both fractions the 31P-NMR spectra of a single alkaline extraction, using 0.25 M NaOH and Chelex 100, showed the presence of inorganic orthophosphate, monoester-P, diester-P and pyrophosphate. Phospholipids-teichoic acids P were observed, but only in the clay size separates of the Andisol. Important differences in spectra of soils and clay size separates were observed only in samples with high organic matter content. The clay size separates enriched with organic matter have more and better defined signals of organic P forms. This allows the identification of P forms that might not be detected by 31P-NMR in the whole soil extracts