Rheological evaluation of soil aggregate microstructure and stability across a forested catena
Rheological characteristics of soils, including their deformation and flow behaviors when subjected to external stress, can provide important information on microstructural stability. In this study we used rheological measurements to examine the soil aggregate microstructure and stability of four different soil orders – Alfisol, Mollisol, Inceptisol, and Entisol – along a forested catena in Mazandaran Province, northern Iran. Amplitude sweep tests were used to quantify the initial values of the storage and loss moduli, deformation limit (when the material begins to transition from reversible to irreversible deformation), deformation at flow point (when the material becomes viscous), and integral z (which summarizes the overall visco-elasticity of the material). The deformation limit was significantly higher in subsoil layers than topsoil layers, and was also higher in the Mollisol than the other pedons. The flow point and integral z values, which relate to the structural stiffness of soil matrices, were largest in the Btg horizons of the Alfisol and Mollisol, implying that these soils had more rigid microstructures. In contrast, the Entisol Ckg horizon, which had high sand content and little soil development, had the lowest values for all properties, thus indicating a lack of micro-aggregate stability. Regression analyses revealed that integral z was influenced by soil physicochemical properties, and was higher in soils whose clay fraction was dominated by expansive clay minerals and pedogenic iron and aluminum sesquioxides. Altogether, the rheological parameters indicated that older, more developed soils had greater microstructural stability than their less developed counterparts. As a result, rheological measurements may be useful for identifying the major factors that affect soil aggregation, and can indicate the relative amount of soil development along gradients such as the studied forest catena.