Physicochemical properties and micromorphology of degraded alpine meadow soils in the Eastern Qinghai-Tibet Plateau

Abstract

The Qinghai-Tibet Plateau is currently suffering from severe ecological problems. Soil degradation is a major environmental problem, and there is strong evidence that such processes present an immediate threat to both biomass and economic yields. The aim of the present study is to clarify the response mechanism of mattic epipedon during soil degradation process in alpine meadow, we investigated (1) morphological characteristics and physicochemical properties of soils, (2) soil micromorphology under different degradation stage. According to plant and land use coverage, three sites were chosen from the Hequ horse farm: lightly degraded meadow (HQ1-L); moderately degraded meadow (HQ2-M); and heavily degraded meadow (HQ3-H). A vegetation survey and soil profile morphology showed that Kobresia kansuensis was the dominant species in HQ1-L and HQ2-M, featuring in topsoil horizons rich in rhizogenic organic matter which creates turfs. There are strong relationships between the presence of the mattic epipedon (OA horizon) and soil physicochemical properties. Thus, exchangeable cations, cation exchangeable capacity (CEC), total nitrogen (TN), and organic carbon (OC) decrease during the degradation of grassland; however, the pH value shows an opposite trend. Exchangeable cations, CEC, TN, and OC provide an index of soil fertility to measure the degree of grassland degeneration. Soils were classified as Luvic Phaeozems, Haplic Phaeozems, and Calcaric Phaeozems. From the micromorphology of soils in thin section, considering soil degradation, the porosity and fractal dimension decrease, and the soil microstructure evolves to an intergrain micro-aggregate structure. Also, by counting the number of excrements in the soil thin sections, and combining this with the fractal dimension of the porosity, we found the HQ1-L is more conducive to soil fauna survival, by comparison to HQ2-M, and HQ3-H which follow HQ1-L in condicivity. These results suggest that the existence of the OA horizon (mattic epipedon) is directly related to soil degradation.