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The role of agricultural practices on humification patterns in virgin and cultivated soils from South Africa, Tanzania and Zimbabwe

AutorAlmendros Martín, Gonzalo ; González-Vila, Francisco Javier ; Zancada Fernández, M. Cristina ; Pardo, Mª Teresa
Fecha de publicación18-may-2008
Citación18th Int. Symp.Annal. Appl. Pyr. 2008
ResumenCultivation of Southern African soils often led to considerable depletion of soil organic matter, the qualitative and quantitative effects of which were studied by standard agronomical analyses, isolation and quantification of the different amounts of soil humic fractions, and analytical pyrolysis of whole soil material. Pyrolysis was performed using a 2020 Frontier Laboratories unit directly connected to a GC/MS system Agilent 6890 with a 30-m fused silica column HP 5MS. Oven temperature was at 50 ºC for 1 min, then increased up to 100 ºC at 30 ºC min-1, from 100 to 300 ºC at 10 ºC min-1 and isothermal at 300 ºC for 10 min at a heating rate of 20 ºC min-1. The carrier gas used was helium with a flow of 1 ml min-1. In six experimental farms from three Southern African countries a regular trend attributable to the effects of cultivation consisting of a relative increase of organic fractions tightly bonded to the mineral fraction (i.e., humic acid and insolubilized humin) was often observed. In some cases (e.g. Domboshawa and Guquka fields), a significant decrease of the humic acid-to-fulvic acid ratio as a result of cultivation was also observed, indicating enhanced oxidative processes leading to the breakdown of humic acid molecules into oligomer fractions, whereas in other soils the relative depletion of the humic acid fraction could point out to enhanced leaching processes in cultivated soils. The results obtained by analytical pyrolysis showed that under tropical conditions no large changes were observed in pyrolytic descriptors of the alteration degree of plant lignins, as reflected on changes in the methoxyphenolic patterns, whereas the most significant changes were observed in the structures derived from the alkyl domain. In fact, in Chiqwaka and Domboshawa soils a cultivation-induced ¿alkyl depletion¿ was observed.In Guquka, Hertzog and Mafiga soils there were small changes and lower amounts of aromatic products whereas in Mkindo soils a cultivation-induced ¿alkyl enhancement¿ was observed. These differences are being interpreted as regards to the extent to which plant-derived organic matter is substituted by microbial products. Preliminary conclusions, supported by the distribution of the major C-forms in soil humic acids studied by 13C-NMR suggested: a) alkyl pyrolysis compounds (except fatty acids) are important indicators of different biogeochemical trends in the sites studied, b) mainly ¿,¿-alkanedienes and alkanes were found to be responsive for microbial metabolism and selective preservation of alkyl biomacromolecules (cutans, suberans, etc.), c) due to the intense lignin degradation in the sites studied, the methoxyphenolic patterns had a limited value to illustrate the extent to which lignin is transformed into humic acid-like macromolecules, d) as a difference to cultivation effects often described in soils from countries under wet and/or temperate climate, a possibility observed in the South African experimental farms studied is that strongly aromatic and condensed organic matter in some uncultivated ecosystems may turn into less transformed, comparatively aliphatic, young organic matter upon cultivation, as a possible result of the interaction of herbaceous crop wastes and intense microbial reworking in wet sites with fallowed soils.
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