Effect of Slope Forms on Soil Organic Matter Fractions and Carbon Stocks of Soils From the Brazilian Semi-Arid Region

Assessing soil organic matter (SOM) dynamics in the semi-arid region must consider the heterogeneity of the environment. Despite numerous studies exploring the role of landform on SOM, the micro-topographic features have been overlooked. This study aimed to quantify the carbon stock and humic fractions in associated carbon of soil profiles in different microrelief (straight, convex, and concave) in Brazilian semi-arid region. Three sequences of soils on slopes with different forms were studied: T1 (straight), T2 (convex), and T3 (concave). Soil chemical and physical attributes, and humic fractions-associated in total organic carbon (TOC) were determined. The increase of TOC from the upper to the lower slope in T1 confirms that erosion processes can be considered efficient for carbon distribution in semiarid region. The degree of humification of organic matter was similar in all slopes, with predominance of humic acid and humin fractions in soil. The predominance of humin in the surface horizons indicates a strong interaction between organic matter and the mineral matrix, being these organic mineral complexes facilitated by the presence of exchangeable cations. Our study confirms that change in the humic fractions occur on slopes with low topographic features, demonstrating that SOM fractions are influenced by microrelief.