Mineralização de nitrogênio e enxofre e caracterização de substâncias húmicas em solos brasileiros por 13C-RMN e cromatografia por exclusão de tamanho

Abstract

With the purpose of evaluating the dynamic of organic matter and nitrogen and sulfur mineralization in soils under natural vegetation and cultivated, with and without liming and phosphorus, four experiments were conducted under laboratory conditions. In the first experiment, it was evaluated the effect of soil acidity and phosphorus application on nitrogen and sulfur mineralization in seven soils with a large variability in chemical and physical properties. In the second experiment, the content and distribution of carbon and nitrogen in whole soil and particle-size fractions and their effect on cation exchange capacity (CEC) were analyzed in a very clayey textured red latosol (LE1) under forest and cropped, with and without correction of soil acidity, and in a clayey textured red latosol (LE2) under cerrado and cropped, with or without phosphorus. In the third experiment it was evaluated the molecular weight (mw) as well as the proportions of humic and fulvic acids in soil humic substances extracted with NaOH and Chelex resin, via High Pressure Size Exclusion Chromatography (HPSEC). Finally, in the four experiment soil organic matter composition as a consequence of cropping was investigated using a combination of natural 13C abundance technique and Liquid State 13C Nuclear Magnetic Resonance (NMR) in a forested and cultivated latosol (LE1). Mineralized nitrogen amounts were not affected by soil acidity, although liming accelerated nitrogen mineralization in HGH and LV2 soils. Liming the soils to a pH around 6.0 resulted in increased net sulfur mineralization in HGP and LV2 soils. Addition of superphosphate did not influence nitrogen and sulfur mineralization processes. The greater prevalence of nitrate and sulphate in some of the limed soils suggests that the criterion used to choose time of lime application needs to be changed. Chromatographic analyzes of humic substances revealed the usefulness of HPSEC in determining the molecular weight and proportions of humic and fulvic acids from soil humic material. Organic matter in the LE1 is composed majority by low molecular weight and active fractions, the fulvic acids (mw=1375 daltons). The molecular weight of the organic fractions in lower proportions, the humic acids, was 31500 daltons. 13C NMR spectra revealed the presence of extensively decomposed organic fractions, rich in aromatic-C and COOH. The drastic reduction of carbon and nitrogen in the cultivated areas of the LE1 was followed by intensive depletion in CEC pH 7,0 and in an ihcrease of aromaticity and decreased proportions of alkyl C on whole soil and particle size fractions in limed and control plots, stressing the importance of management and conservation of organic matter in this soil. Increased addition of fresh and active organic matter,by the adoption of more sustainable management practices, is proposed in order to increase the fertility and sustainability of the tropical soils studied.