Clay minerals transformation in soil particles of Udic Ferrisols under 36-year Chinese milk vetch application in southern China

To elucidate the impact of green manure application on soil clay minerals, surface soil samples (0–20 cm) were collected from Udic Ferrisols developed from Quaternary clay sediments, under no fertilization (CK) and Chinese milk vetch (MV) application treatments, based on a 36-year long-term experiment in southern China. Synchrotron X-ray diffraction (XRD), thermogravimetric analysis (TG), ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and high-resolution transmission electron microscopy (HRTEM) were employed to investigate the composition and structural characteristics of clay minerals in soil particles (100–2000 nm and 25–100 nm). The clay minerals in the 100–2000 nm and 25–100 nm particles were mainly kaolinite and illite, containing a minor amount of vermiculite. As the particle size decreased to the nanoscale (25–100 nm), the kaolinite content increased, the vermiculite content decreased, and the crystallinity of the clay minerals weakened. Aluminum (Al) in the structure of the clay minerals mainly existed as octahedral coordinated Al (^VIAl), with a small proportion of tetrahedral coordinated Al (^IVAl) (5.8 %–18.4 %) predominantly as Q3(1Al) (where silicon (Si) in the tetrahedron was connected to three neighboring Si atoms through oxygen to form a framework structure). The abundance of ^IVAl decreased significantly with decreasing particle size, and the structure of the clay minerals gradually shifted from a 2:1 to a 1:1 type. In the 100–2000 nm particles, MV application reduced illite content and increased ^IVAl abundance. HRTEM observation revealed a transition of lattice fringes from 1.0 nm to 1.38 nm. Conversely, in the 25–100 nm particles, MV application decreased ^IVAl abundance and increased kaolinite content, with the 1.38 nm lattice fringes gradually transformed into 0.71 nm. These findings indicate that long-term MV incorporation promoted the transformation of illite to vermiculite in the 100–2000 nm particles and vermiculite to kaolinite in the 25–100 nm particles to a certain extent.