Changes in soil quality under different agricultural management in Chinese Mollisols

Mollisols (called Black soils) are of major agricultural importance in China. Most of these soils have been used agriculturally for the last 50 years or so. Due to intensive cultivation and improper soil management, loss of organic matter and yield suppression resulting from soil erosion are serious problems in the region. The current research studied the physical and chemical properties of a typical Mollisol in China, characterized the changes of soil organic carbon (SOC) with cultivation, investigated the effects of agricultural management systems on SOC and total organic nitrogen (TON) contents and their vertical distribution, and examined the negative impacts of continuous soybean on crop and soil productivity. The soil is characterized with a thick (60 cm) mollic epipedon, higher organic carbon (5.8%), CEC (43.7 cmol(+)/kg) and macro-aggregate (\u3e0.25 mm) stability, and greater macronutrients and water availability in the upper epipedon. Bulk density increases with depth, and total porosity declines with depth. Soil texture is clay loam. Overall characteristics make this soil fertile and productive. The SOC content declined rapidly at early years of cultivation and gradually afterwards. Compared with organic matter in the uncultivated soil, total SOC loss was 17%, 28%, and 55% in 5-, 14- and 50-year cultivation, respectively. Wheat-soybean rotation with addition wheat straw or pig manure resulted in a substantial increase in SOC content in 9 years. Compared with a wheat-corn-soybean rotation, continuous cropping reduced SOC and N contents in the profile, particularly SOC content. Moldboard plowing significantly reduced SOC and N contents whereas integrated tillage increased SOC and N relative to conventional tillage. Use of chemical fertilizers (N, P, and K) along with return of crop residues resulted in a substantial increase in SOC and N in top layers of the soil. Continuous soybean results in unbalanced soil enzymes activities, declines of SOC, total K, Zn, available K and N contents, soil pH and bacteria/fungi ratio. It is proposed that the best management for maintaining soil productivity in the area would be crop rotation along with the integrated tillage and addition of crop residues and chemical fertilizers