Propriedades redox de ácidos húmicos isolados de um solo cultivado com cana-de-açúcar por longo tempo

Abstract

RESUMOA matéria orgânica do solo é o maior reservatório de C nos sistemas naturais. Em tais sistemas a qualidade e a estabilidade do C podem ser estimadas pelo aumento da concentração das frações humificadas que, dentre outros fatores, está condicionada ao balanço entre as perdas e os ganhos que envolvem as reações de oxidação e de redução da matéria orgânica do solo. O objetivo deste estudo foi avaliar a eletroquímica, usando titulações redox iodimétricas, de ácidos húmicos isolados de solos cultivados continuamente com cana-de-açúcar submetida ou não à queima da palha para a colheita ou à adição anual de vinhaça. Os ácidos húmicos apresentaram valores do potencial formal padrão do eletrodo entre 0,760 e 0,779 V a 25 °C. A capacidade de oxidação dos ácidos húmicos variou de 1,01 a 3,44 mol c kg -1 a pH 5,0 e de 1,64 a 6,44 mol c kg -1 a pH 7,0. Observou-se correlação positiva e significativa entre a capacidade de oxidação dos ácidos húmicos e suas concentrações de grupos funcionais fenólicos, quinonas e semiquinonas.Termos de indexação: química do solo, substâncias húmicas, potencial redox, capacidade de oxidação, grau de humificação.

Redox index of soil carbon stability

“…Positive correlations between redox capacity and this aromaticity were reported by Scott et al (1998) and Baldotto et al (2007Baldotto et al ( , 2008 and support the hypothesis that the quinone moieties are significant redox groups in HA. Baldotto et al (2007Baldotto et al ( , 2008 used redox titrations (I 2 ) and the techniques of methylation or acetylation to evaluate the functional groups involved in redox properties of HA and showed that a greater and similar decrease of oxidation capacity (~ 91 %) was verified in both the methylated and acetylated derivatives in comparison with the original HA, suggesting that the free quinones and phenolic groups, that can be blocked by derivatization, were the principal participants in the redox reactions. Baldotto et al (2007) also verified by infrared spectroscopy of HA samples that the absorption attributed to C-H stretching (2.927-2.942 cm -1 ) increased with HA methylation or acetylation.…”

“…These sites were described and characterized (Table 1) by Baldotto et al (2007Baldotto et al ( , 2008, Canellas et al (2008) and Dobbss et al (2008Dobbss et al ( , 2009). The study also included an Entisol under continuous sugarcane cultivation over the course of 55 years (Canellas et al, 2003Busato et al, 2005a,b;Baldotto et al, 2008). In this case the soil plots differ only in the practice of burning or not burning sugarcane at harvest or in vinasse applications (with crop burning).…”

“…The samples were air-dried, ground, sieved (2 mm) and stored for characterization in the laboratory. Details of soil organic matter fractionation, HA isolation, purification and elementary composition, functional group contents, oxidation capacity, as well as ultraviolet-visible (E 4 /E 6 ), fluorescence (FI), nuclear magnetic resonance (NMR 13 C) and electron spin resonance (ESR) spectroscopy for the HA have previously been presented and discussed by Baldotto et al (2007Baldotto et al ( , 2008, Canellas et al (2008) and Dobbss et al (2008Dobbss et al ( , 2009). Some of the soils and HA properties were selected and are shown in the tables 2 and 3, respectively.…”

“…The humus reactivity is determined by the functional groups found in humic substances (HS) (Schnitzer & Gupta, 1965;Schnitzer & Riffaldi, 1972;Baldotto et al, 2007Baldotto et al, , 2008Sposito, 2008).…”

“…This index of stability, in a first approach, would be defined combining the oxidation capacity and the stock of soil HA. The oxidation capacity (mol c kg -1 ) of humic acids is defined as the amount of charge that compost can transfer to an oxidant (e.g., I 2 ) and can be determined by redox titrations (Struyk & Sposito, 2001;Baldotto et al, 2007Baldotto et al, , 2008. The humic acid pool (Mg ha -1 ) is obtained based on the HA concentration in a sampled soil layer with determined density .…”