No abstract

Converting natural ecosystems into agro-ecosystems often reduces soil organic carbon content by decreasing carbon input, as well as by increasing erosion losses and organic matter decomposition rate. This study aimed at evaluating carbon stocks, soil aggregate stability, and spatial variability of some other attributes in soils of Southern Amazonas state. The study was carried out on areas with archaeological dark earth (ADE), under rainforest, pasture, agroforestry environments, sugarcane, and cassava. We collected disturbed and undisturbed soil samples from 64 points in a regular spacing of 10 m at 0.0-10 cm depth. From these samples, we determined the stock of organic carbon (STOC), organic carbon (OC), organic matter (OM) content, soil aggregate stability (SAS), and soil bulk density (SBD). Data analysis included univariate, multivariate, and descriptive statistics. The STOC was higher in ADEs and the adjusted semivariograms pointed out a greater spatial variability for soils under pasture and cassava crop. Kriging maps of principal component analysis scores proved a positive correlation between the studied variables and terrain slope, with higher values for lower lands.

Abstract. Soil position in the landscape reveals its history of formation and genesis. Therefore, the landscape is the combination of features of the surface of the earth with subsurface components (parent material), while the soil is a three-dimensional, dynamic natural body inserted in the landscape. This research aimed to study the soil-landscape relationship in a sandstone-gneiss topolithosequence in Amazonas, Brazil. The study was carried out along a 9.253-meter transect from the top downwards the softer slope. Soil profiles were selected in five landscape compartments (top, upper third, lower third, transport foothill, and deposition foothill). Morphological, mineralogical, physical, chemical, and ray diffraction characterizations were performed. Soils had different morphological, physical, chemical, and mineralogical attributes due to the variations of the geological substrate and landscape position. The mineralogy of the clay fraction is composed of kaolinite, goethite, hematite, and gibbsite, with goethite being the predominant iron oxide. A sand fraction dominance was observed in relation to the other fractions in all the profiles, being related to the alluvial nature of the parent material, with the highest values occurring in the lower third. The separation of the landscape into geomorphic surfaces and identification of the parent material were effective for understanding the variation of soil attributes along the landscape.

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