Soil Mineralogy Changes With Different Agricultural Practices During 8-Year Soil Development From the Parent Material of a Mollisol

Clay minerals are involved in the formation of soil structure and soil cation exchange capacity through interaction with soil organic matter. Through these interactions, clay minerals contribute to soil functions and the delivery of soil ecosystem services. Although clay minerals are considered stable over long periods of time, recent studies have demonstrated that the crystallographic structures and types of clay minerals can undergo spontaneous modification and transformation with changes in environmental conditions. However, little is known whether and how soil organic matter affects the evolution of clay minerals during these transitions. We examined changes in soil mineralogy during the 8 years of soil development from the parent material (PM) of a Mollisol under different agricultural practices in Northeastern China. Mineralogical changes were determined by X-ray diffraction and data analysis through spectra decomposition. The mineral composition varied with initial particle size classes and showed rapid changes, including transformation between illite and vermiculite or illite and smectite, depending on K+ availability and transformations between smectite and vermiculite under acidic conditions. The mineral composition changed with soil depth due to transport of vermiculite and well-crystallized illite as fine particles. Soil organic matter was more strongly bound to vermiculite and smectite than to kaolinite and illite and therefore influenced the transport of these mineral particles. However, the soil organic matter–mineral association did not affect transformation between illite and vermiculite or smectite. Therefore, the effects of particle size, organic matter, and soil depth on soil mineral composition were exhibited to different extents and interacted differently under different agricultural practices. These results of the 8-year field experiment suggest that soil mineral composition of the more intensely weathered PM of the studied Mollisols can change after agricultural restoration more quickly than previously known. The rate and extent of these transformations must be considered in agricultural practices that are used to manage soil functions and associated ecosystem services of soil systems.