Texture and land-use effects on soil organic matter in Cerrado Oxisols, Central Brazil

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Due to the continuous agricultural expansion in the Brazilian Cerrados and the known adverse effects of agriculture on soil organic matter (SOM), SOM dynamics of typical Cerrado land-use systems were studied. Bulk soil samples and particle-size separates from clayey and loamy Oxisols under crop, pasture, and reforestation sites were compared with the natural Cerrado savanna in order to characterise texture and treatment effects on SOM and to follow the alterations of organic compounds in the different particle-size fractions. The analyses included an assessment of SOM, particulate organic matter (POM), polysaccharides, lignin oxidation products (VSC-lignin), and scanning electron microscopy (SEM) of different particle-size fractions. The amounts of SOM, polysaccharides, and VSC-lignin were clearly higher in the clayey soils due to the increasing stabilisation of organic substances to the clay fraction. Conversely, the amounts of POM were comparable between the substrates resulting in a higher proportion of labile organic matter, and thus an overall greater lability of the loamy soils. Continuous cropping and reforestation with pine led to a clear reduction of SOM contents, whereas pasture and eucalyptus reforestation might have increased both the amount and quality of SOM in relation to the Cerrado control. The analysis of particle-size separates showed the decline of lignin and plant-derived polysaccharides from the sand to the clay fraction, and an accumulation of microbially metabolised polysaccharides in the clay fraction, suggesting that humification and mineralisation of organic matter in Cerrado soils follow along a biogeochemical gradient similar to that of temperate soils and climates.