Matéria orgânica de Latossolos Húmicos: análises térmica e espectroscópica, efeito do uso e correção química / Organic matter of humic Latosol (Oxisol): thermal and spectroscopic analyses, land use and chemical correction

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The objective of this study was to contribute to the clarification of several aspects related to the behavior of organic matter (OM) in highly weathered tropical soils. Six samples of humic A horizon Latosol were collected from three sites in Minas Gerais: i) Sericita, (under coffee crop, pasture, ferns and Atlantic Forest fragment); ii) Mutum (under Atlantic Forest fragment); and iii) Araçuaí (under Savanna). Composed samples were obtained for the 0-10 and 60-100 cm deep layers in the A horizon. At the Sericita site, samples of litter under coffee crop, pasture and Atlantic Forest fragment were collected for comparative analyses of lipid composition. Lipids (from litter and soil) were Soxhlet-extracted with dichloromethane/methanol (3:1 v/v) and analyzed by gas chromatography-mass spectrometry. Soil samples collected from all the sites were processed for extraction and purification of humic (HA) and fulvic (FA) acids, which were characterized by elemental analysis; UVvisible and infrared spectroscopy (FTIR), nuclear magnetic resonance (13C-NMR) and thermal desorption (280 C) and true pyrolysis (610 C). In Sericita, the soil under the forest, in the 0-10 cm layer, showed higher lipid content than the soil under cultivation. There was selective preservation of alyphatic biopolymers from the litter, accumulating in the 60-100 cm layer. It was also detected a greater preservation of short chain alkanes in soils under forest. The lipid composition under different agroecosystems allowed the evaluation of changes related to land use. The techniques used for analyzing HA and FA were efficient to elucidate the structural differences between these frations, as well as the changes in the soil profile. The UV-visible spectroscopy showed the contribution of fungal melanin pigments as the most probable cause for the darker color of the humic subsurface A horizon. FTIR spectra revealed that HA extracted from the Savanna site showed less decarboxylation, dehydration, condensation and aromatization than the ones from Atlantic Forest sites. HA characterization through 13C - RMN indicated a humification pathway tending to decrease alkyl groups and increase the C-aromatic signal region. Thermodegradation showed that HA and FA extracted from topsoil were structurally more diverse than those from the subsurface and under natural vegetation there was more N-compounds in the humic structure. The results also indicated that the increase in the C-aromatic signal was caused by benzenes and toluenes. Higher proportion of polysaccharides was detected in FA from the 60-100 cm layer, revealing important movements of these acids in the soil profile. In the topsoil, the agricultural use of the soil caused reduction in aliphatics and N-compounds at the HA structure, and phenolic, lignin, fatty acids and N- compounds at the FA structure. Thermodegradation by desorption and pyrolysis showed that the increase in aromatic level (~20%) verified by 13C-NMR for HA extracted between 60-100 cm corresponded mainly to the structural incorporation of heterocyclic N-compounds. The structural stabilization involved accumulation of Ncompounds, aromatic and aliphatic compounds in HA, and aromatic and phenolic compounds in FA. The carboxylic and phenolic attributes of FA are key components in complexation and translocation of Fe and Al in tropical humic horizons. Because the intense use of humic Latosols with coffee crop and pasture in Sericita, the impact of agricultural practices such as liming, fertilization and incorporation of labile carbon sources (C) on OM decomposition was evaluated in parallel. The experiment arranged in a complete randomized design, in a 5x2x2 factorial scheme, with five liming levels (0; 0.5; 1; 2 and 3 times the amount needed to increase base saturation to 60%), two levels of phosphate (with and without) and two levels of labile C source as sucrose (with and without). Respiration curves were established from the accumulated CO2 liberated during 136 days and then fitted to the first order kinetic model. Following the incubation period, C total in the soil and humic fractions (fulvic, humic and humin) were evaluated. The total C-CO2 production accumulated over time was positively affected by the sucrose. The fulvic acid fraction in the soil tended to increase with sucrose addition, but reduced in presence of phosphate. Liming caused reduction in C of the humic acid fraction but increased C of the humin fraction. Results indicated that agricultural practices such as liming, phosphate fertilizations and addition of residues in the form of labile C changed the C dynamics in organic matter of humic Latosols.

ASSUNTO(S)

soil use humificação espectroscópio ciencia do solo humification spectroscope uso do solo

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